| 1. |
- Isaeva, Leyla, et al.
(författare)
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Dynamic stabilization of cubic AuZn
- 2015
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Ingår i: Materials today: Proceedings. - : Elsevier BV. ; , s. S569-S572
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Konferensbidrag (refereegranskat)abstract
- A recently developed temperature-dependent effective potential method is employed to study the martensitic phase transformation in AuZn. This method is based on ab initio molecular dynamics and allows to obtain finite-temperature lattice vibrational properties. We show that the transversal acoustic TA(2)[110] mode associated with the phase transition is stabilized at 300 K. Temperature evolution of single-phonon dynamic structure factor at the wave vector q= 1/3[1,1,0], associated with phonon softening and Fermi surface nesting, was also studied.
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| 2. |
- Abrikosov, Igor A., et al.
(författare)
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Finite Temperature, Magnetic, and Many-Body Effects in Ab Initio Simulations of Alloy Thermodynamics
- 2013
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Ingår i: TMS2013 Supplemental Proceedings. - Hoboken, NJ, USA : John Wiley & Sons. - 9781118605813 - 9781118663547 ; , s. 617-626
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Bokkapitel (refereegranskat)abstract
- Ab initio electronic structure theory is known as a useful tool for prediction of materials properties. However, majority of simulations still deal with calculations in the framework of density functional theory with local or semi-local functionals carried out at zero temperature. We present new methodological solution.s, which go beyond this approach and explicitly take finite temperature, magnetic, and many-body effects into account. Considering Ti-based alloys, we discuss !imitations of the quasiharmonic approximation for the treatment of lattice vibrations, and present an accurate and easily extendable method to calculate free ,energies of strongly anharmonic solids. We underline the necessity to going beyond the state-of-the-art techniques for the determination of effective cluster interactions in systems exhibiting mctal-to-insulator transition, and describe a unified cluster expansion approach developed for this class of materials. Finally, we outline a first-principles method, disordered local moments molecular dynamics, for calculations of thermodynamic properties of magnetic alloys, like Cr1-x,.AlxN, in their high-temperature paramagnetic state. Our results unambiguously demonstrate importance of finite temperature effects in theoretical calculations ofthermodynamic properties ofmaterials.
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| 3. |
- Abrikosov, Igor, et al.
(författare)
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Theoretical description of pressure-induced phase transitions: a case study of Ti-V alloys
- 2015
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Ingår i: High Pressure Research. - : Taylor andamp; Francis: STM, Behavioural Science and Public Health Titles. - 0895-7959 .- 1477-2299. ; 35:1, s. 42-48
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Tidskriftsartikel (refereegranskat)abstract
- We discuss theoretical description of pressure-induced phase transitions by means of first-principles calculations in the framework of density functional theory. We illustrate applications of theoretical tools that allow one to take into account configurational and vibrational disorders, considering Ti-V alloys as a model system. The universality of the first-principles theory allows us to apply it in studies of different phenomena that occur in the Ti-V system upon compression. Besides the transitions between different crystal structures, we discuss isostructural transitions in bcc Ti-V alloys. Moreover, we present arguments for possible electronic transitions in this system, which may explain peculiar behaviour of elastic properties of V upon compression.
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| 4. |
- Alling, Björn, et al.
(författare)
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Dynamic and structural stability of cubic vanadium nitride
- 2015
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X .- 2469-9950 .- 2469-9969. ; 91:5, s. 054101-
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Tidskriftsartikel (refereegranskat)abstract
- Structural phase transitions in epitaxial stoichiometric VN/MgO(011) thin films are investigated using temperature-dependent synchrotron x-ray diffraction (XRD), selected-area electron diffraction (SAED), resistivity measurements, high-resolution cross-sectional transmission electron microscopy, and ab initio molecular dynamics (AIMD). At room temperature, VN has the B1 NaCl structure. However, below T-c = 250 K, XRD and SAED results reveal forbidden (00l) reflections of mixed parity associated with a noncentrosymmetric tetragonal structure. The intensities of the forbidden reflections increase with decreasing temperature following the scaling behavior I proportional to (T-c - T)(1/2). Resistivity measurements between 300 and 4 K consist of two linear regimes resulting from different electron/phonon coupling strengths in the cubic and tetragonal-VN phases. The VN transport Eliashberg spectral function alpha F-2(tr)(h omega), the product of the phonon density of states F(h omega) and the transport electron/phonon coupling strength alpha(2)(tr)(h omega), is determined and used in combination with AIMD renormalized phonon dispersion relations to show that anharmonic vibrations stabilize the NaCl structure at T greater than T-c. Free-energy contributions due to vibrational entropy, often neglected in theoretical modeling, are essential for understanding the room-temperature stability of NaCl-structure VN, and of strongly anharmonic systems in general.
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| 5. |
- Belonoshko, Anatoly B., et al.
(författare)
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Quenching of bcc-Fe from high to room temperature at high-pressure conditions : a molecular dynamics simulation
- 2009
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 11
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Tidskriftsartikel (refereegranskat)abstract
- The new high-temperature (T), high-pressure (P), body-centered cubic (bcc) phase of iron has probably already been synthesized in recent diamond anvil cell (DAC) experiments (Mikhaylushkin et al 2007 Phys. Rev. Lett. 99 165505). These DAC experiments on iron revealed that the high-PT phase on quenching transforms into a mixture of close-packed phases. Our molecular dynamics simulation and structural analysis allow us to provide a probable interpretation of the experiments. We show that quenching of the high-PT bcc phase simulated with the embedded-atom model also leads to the formation of the mixture of close-packed phases. Therefore, the assumption of the stability of the high-PT bcc iron phase is consistent with experimental observation.
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| 6. |
- Benshalom, Nimrod, et al.
(författare)
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Dielectric response of rock-salt crystals at finite temperatures from first principles
- 2022
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Ingår i: Physical Review Materials. - : American Physical Society. - 2475-9953. ; 6:3
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Tidskriftsartikel (refereegranskat)abstract
- We combine ab initio simulations and Raman scattering measurements to demonstrate explicit anharmonic effects in the temperature-dependent dielectric response of a NaCl single crystal. We measure the temperature evolution of its Raman spectrum and compare it to both a quasiharmonic and anharmonic model. Results demonstrate the necessity of including anharmonic lattice dynamics to explain the dielectric response of NaCl, as it is manifested in Raman scattering. Our model fully captures the linear dielectric response of a crystal at finite temperatures and may therefore be used to calculate the temperature dependence of other material properties governed by it.
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| 7. |
- Benshalom, Nimrod, et al.
(författare)
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Phonon-Phonon Interactions in the Polarization Dependence of Raman Scattering
- 2023
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Ingår i: The Journal of Physical Chemistry C. - : AMER CHEMICAL SOC. - 1932-7447 .- 1932-7455. ; 127:36, s. 18099-18106
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Tidskriftsartikel (refereegranskat)abstract
- We have found that the polarization dependence of Raman scattering in organic crystals at finite temperatures can only be described by a fourth-rank tensor formalism. This generalization of the second-rank Raman tensor stems from the effect of off diagonal components in the crystal self-energy on the light scattering mechanism. We thus establish a novel manifestation of phonon-phonon interaction in inelastic light scattering, markedly separate from the better-known phonon lifetime.
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| 8. |
- Bykova, E., et al.
(författare)
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Metastable silica high pressure polymorphs as structural proxies of deep Earth silicate melts
- 2018
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Ingår i: Nature Communications. - : NATURE PUBLISHING GROUP. - 2041-1723. ; 9
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Tidskriftsartikel (refereegranskat)abstract
- Modelling of processes involving deep Earth liquids requires information on their structures and compression mechanisms. However, knowledge of the local structures of silicates and silica (SiO2) melts at deep mantle conditions and of their densification mechanisms is still limited. Here we report the synthesis and characterization of metastable high-pressure silica phases, coesite-IV and coesite-V, using in situ single-crystal X-ray diffraction and ab initio simulations. Their crystal structures are drastically different from any previously considered models, but explain well features of pair-distribution functions of highly densified silica glass and molten basalt at high pressure. Built of four, five-, and six-coordinated silicon, coesite-IV and coesite-V contain SiO6 octahedra, which, at odds with 3rd Paulings rule, are connected through common faces. Our results suggest that possible silicate liquids in Earths lower mantle may have complex structures making them more compressible than previously supposed.
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| 9. |
- Cohen, Adi, et al.
(författare)
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Diverging Expressions of Anharmonicity in Halide Perovskites
- 2022
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Ingår i: Advanced Materials. - : Wiley-V C H Verlag GMBH. - 0935-9648 .- 1521-4095. ; 34:14
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Tidskriftsartikel (refereegranskat)abstract
- Lead-based halide perovskite crystals are shown to have strongly anharmonic structural dynamics. This behavior is important because it may be the origin of their exceptional photovoltaic properties. The double perovskite, Cs2AgBiBr6, has been recently studied as a lead-free alternative for optoelectronic applications. However, it does not exhibit the excellent photovoltaic activity of the lead-based halide perovskites. Therefore, to explore the correlation between the anharmonic structural dynamics and optoelectronic properties in lead-based halide perovskites, the structural dynamics of Cs2AgBiBr6 are investigated and are compared to its lead-based analog, CsPbBr3. Using temperature-dependent Raman measurements, it is found that both materials are indeed strongly anharmonic. Nonetheless, the expression of their anharmonic behavior is markedly different. Cs2AgBiBr6 has well-defined normal modes throughout the measured temperature range, while CsPbBr3 exhibits a complete breakdown of the normal-mode picture above 200 K. It is suggested that the breakdown of the normal-mode picture implies that the average crystal structure may not be a proper starting point to understand the electronic properties of the crystal. In addition to our main findings, an unreported phase of Cs2AgBiBr6 is also discovered below approximate to 37 K.
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| 10. |
- Dangic, Dorde, et al.
(författare)
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The origin of the lattice thermal conductivity enhancement at the ferroelectric phase transition in GeTe
- 2021
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Ingår i: npj Computational Materials. - : Nature Research. - 2057-3960. ; 7:1
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Tidskriftsartikel (refereegranskat)abstract
- The proximity to structural phase transitions in IV-VI thermoelectric materials is one of the main reasons for their large phonon anharmonicity and intrinsically low lattice thermal conductivity kappa. However, the kappa of GeTe increases at the ferroelectric phase transition near 700 K. Using first-principles calculations with the temperature dependent effective potential method, we show that this rise in kappa is the consequence of negative thermal expansion in the rhombohedral phase and increase in the phonon lifetimes in the high-symmetry phase. Strong anharmonicity near the phase transition induces non-Lorentzian shapes of the phonon power spectra. To account for these effects, we implement a method of calculating kappa based on the Green-Kubo approach and find that the Boltzmann transport equation underestimates kappa near the phase transition. Our findings elucidate the influence of structural phase transitions on kappa and provide guidance for design of better thermoelectric materials.
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| 11. |
- Dewandre, Antoine, et al.
(författare)
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Two-Step Phase Transition in SnSe and the Origins of its High Power Factor from First Principles
- 2016
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Ingår i: PHYSICAL REVIEW LETTERS. - : AMER PHYSICAL SOC. - 0031-9007. ; 117:27
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Tidskriftsartikel (refereegranskat)abstract
- The interest in improving the thermoelectric response of bulk materials has received a boost after it has been recognized that layered materials, in particular SnSe, show a very large thermoelectric figure of merit. This result has received great attention while it is now possible to conceive other similar materials or experimental methods to improve this value. Before we can now think of engineering this material it is important we understand the basic mechanism that explains this unusual behavior, where very low thermal conductivity and a high thermopower result from a delicate balance between the crystal and electronic structure. In this Letter, we present a complete temperature evolution of the Seebeck coefficient as the material undergoes a soft crystal transformation and its consequences on other properties within SnSe by means of first-principles calculations. Our results are able to explain the full range of considered experimental temperatures.
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| 12. |
- Heine, Matthew, et al.
(författare)
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Effect of thermal lattice and magnetic disorder on phonons in bcc Fe: A first-principles study
- 2019
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Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 100:10
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Tidskriftsartikel (refereegranskat)abstract
- We present a first-principles theoretical approach to calculate temperature dependent phonon dispersions in bcc Fe, which captures finite temperature spin-lattice coupling by treating thermal disorder in both the spin and lattice systems simultaneously. With increasing temperature, thermal atomic displacements are found to induce increasingly large fluctuations in local magnetic moment magnitudes. The calculated phonon dispersions of bcc Fe show excellent agreement with measured data over a wide range of temperatures both above and below the magnetic and structural transition temperatures, suggesting the applicability of the developed approach to other magnetic materials.
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| 13. |
- Heine, Matthew, et al.
(författare)
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Temperature-dependent renormalization of magnetic interactions by thermal, magnetic, and lattice disorder from first principles
- 2021
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Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 103:18
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Tidskriftsartikel (refereegranskat)abstract
- We put forth an ab initio framework to calculate local moment magnetic interaction parameters, renormalized to treat both the lattice and magnetic systems as a function of temperature T. For bcc Fe, magnetic and lattice thermal disorders act in opposition, the former strengthening the Heisenberg-like interactions, while the latter decreasing them. Below T-C, J stays nearly independent of T, while around and above T-C, it exhibits a sharp decrease. This remarkable behavior reflects an intricate spin-lattice coupling and its evolution with T, in which magnetic interactions and interatomic bonds are each renormalized by the other. This finding is consistent with magnetization data and with the observed softening of magnon and phonon modes at high temperatures. Magnetization as well as magnon and phonon mode softening are discussed.
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| 14. |
- Hellman, Olle, et al.
(författare)
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Charge Redistribution Mechanisms of Ceria Reduction
- 2012
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 108:13, s. 135504-
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Tidskriftsartikel (refereegranskat)abstract
- Charge redistribution at low oxygen vacancy concentrations in ceria have been studied in the framework of the density functional theory. We propose a model to approach the dilute limit using the results of supercell calculations. It allows one to reproduce the characteristic experimentally observed behavior of composition versus oxygen pressure dependency. We show that in the dilute limit the charge redistribution is likely to be driven by a mechanism different from the one involving electron localization on cerium atoms. We demonstrate that it can involve charge localization on light element impurities.
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| 15. |
- Hellman, Olle, et al.
(författare)
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Lattice dynamics of anharmonic solids from first principles
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 84:18, s. 180301-
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Tidskriftsartikel (refereegranskat)abstract
- An accurate and easily extendable method to deal with lattice dynamics of solids is offered. It is based on first-principles molecular dynamics simulations and provides a consistent way to extract the best possible harmonic-or higher order-potential energy surface at finite temperatures. It is designed to work even for strongly anharmonic systems where the traditional quasiharmonic approximation fails. The accuracy and convergence of the method are controlled in a straightforward way. Excellent agreement of the calculated phonon dispersion relations at finite temperature with experimental results for bcc Li and bcc Zr is demonstrated.
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| 16. |
- Hellman, Olle, et al.
(författare)
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Phonon thermal transport in Bi2Te3 from first principles
- 2014
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 90:13, s. 134309-
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Tidskriftsartikel (refereegranskat)abstract
- We present first-principles calculations of the thermal and thermal transport properties of Bi2Te3 that combine an ab initio molecular dynamics (AIMD) approach to calculate interatomic force constants (IFCs) along with a full iterative solution of the Peierls-Boltzmann transport equation for phonons. The newly developed AIMD approach allows determination of harmonic and anharmonic interatomic forces at each temperature, which is particularly appropriate for highly anharmonic materials such as Bi2Te3. The calculated phonon dispersions, heat capacity, and thermal expansion coefficient are found to be in good agreement with measured data. The lattice thermal conductivity, kappa(l), calculated using the AIMD approach nicely matches measured values, showing better agreement than the kappa(l) obtained using temperature-independent IFCs. A significant contribution to kappa(l) from optic phonon modes is found. Already at room temperature, the phonon line shapes show a notable broadening and onset of satellite peaks reflecting the underlying strong anharmonicity.
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| 17. |
- Hellman, Olle, et al.
(författare)
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Temperature dependent effective potential method for accurate free energy calculation of solids
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We have developed a thorough and accurate method of determining anharmonic free energies. The technique is based in ab initio molecular dynamics and map a model Hamiltonian to the fully anharmonic system. We can accurately deal with low-symmetry systems, such as random alloys. The formalism and the numerics are described in great detail. A number of practical examples are given, and favourable results are presented, both with respect to experiment and established techniques.
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| 18. |
- Hellman, Olle, et al.
(författare)
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Temperature dependent effective potential method for accurate free energy calculations of solids
- 2013
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 87:10
-
Tidskriftsartikel (refereegranskat)abstract
- We have developed a thorough and accurate method of determining anharmonic free energies, the temperature dependent effective potential technique (TDEP). It is based on ab initio molecular dynamics followed by a mapping onto a model Hamiltonian that describes the lattice dynamics. The formalism and the numerical aspects of the technique are described in detail. A number of practical examples are given, and results are presented, which confirm the usefulness of TDEP within ab initio and classical molecular dynamics frameworks. In particular, we examine from first principles the behavior of force constants upon the dynamical stabilization of the body centered phase of Zr, and show that they become more localized. We also calculate the phase diagram for 4He modeled with the Aziz et al. potential and obtain results which are in favorable agreement both with respect to experiment and established techniques.
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| 19. |
- Hellman, Olle, et al.
(författare)
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Temperature-dependent effective third-order interatomic force constants from first principles
- 2013
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 88:14
-
Tidskriftsartikel (refereegranskat)abstract
- The temperature-dependent effective potential (TDEP) method is generalized beyond pair interactions. The second- and third-order force constants are determined consistently from ab initio molecular dynamics simulations at finite temperature. The reliability of the approach is demonstrated by calculations of the mode Grüneisen parameters for Si. We show that the extension of TDEP to a higher order allows for an efficient calculation of the phonon life time, in Si as well as in ε-FeSi; a system that exhibits anomalous softening with temperature.
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| 20. |
- Hellman, Olle, 1981-
(författare)
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Thermal properties of materials from first principles
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the search of clean and efficient energy sources intermediate temperature solid oxide fuel cells are among the prime candidates. What sets the limit of their efficiency is the solid electrolyte. A promising material for the electrolyte is ceria. This thesis aims to improve the characteristics of these electrolytes and help provide thorough physical understanding of the processes involved. This is realised using first principles calculations.The class of methods based on density functional theory generally ignores temperature effects. To accurately describe the intermediate temperature characteristics I have made adjustments to existing frameworks and developed a qualitatively new method. The new technique, the high temperature effective potential method, is a general theory. The validity is proven on a number of model systems.Other subprojects include low-dimensional segregation effects, adjustments to defect concentration formalism and optimisations of ionic conductivity.
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| 21. |
- Hellman, Olle, et al.
(författare)
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Tuning ionic conductivity in ceria by volume optimization
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Effect of volume change on the ionic conductivity in ceria has been studied in the framework of the density functional theory. We show that properly controlling external conditions one can treat the lattice constant of ceria as an adjustable parameter and change the topology of the energy landscape for the oxygen ion diffusion. We reveal the existence of the narrow range of lattice parameters, which optimize the ionic conductivity in ceria.
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| 22. |
- Herriman, Jane E., et al.
(författare)
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Phonon thermodynamics and elastic behavior of GaN at high temperatures and pressures
- 2018
-
Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 98:21
-
Tidskriftsartikel (refereegranskat)abstract
- The effects of temperature and pressure on the phonons of GaN were calculated for both the wurtzite and zinc-blende structures. The quasiharmonic approximation (QHA) gave reasonable results for the temperature dependence of the phonon DOS at zero pressure but unreliably predicted the combined effects of temperature and pressure. Pressure was found to change the explicit anharmonicity, altering the thermal shifts of phonons and more notably qualitatively changing the evolution of phonon lifetimes with increasing temperature. These effects were largest for the optical modes, and phonon frequencies below approximately 5 THz were adequately predicted with the QHA. The elastic anisotropies of GaN in both wurtzite and zinc-blende structures were calculated from the elastic constants as a function of pressure at 0 K. The elastic anisotropy increased with pressure until reaching elastic instabilities at 40 GPa (zinc blende) and 65 GPa (wurtzite). The calculated instabilities are consistent with proposed transformation pathways to rocksalt GaN and place upper bounds on the pressures at which wurtzite and zinc-blende GaN can be metastable.
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| 23. |
- Kerdsongpanya, Sit, et al.
(författare)
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Phonon Thermal Conductivity of Scandium Nitride for Thermoelectric Applications from First-Principles Calculations
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The knowledge of lattice thermal conductivity of materials under realistic conditions is vitally important since most technologies either require either high or low thermal conductivity. Here, we propose a theoretical model for determining lattice thermal conductivity with the effect of microstructure. This is based on ab initio description that includes the temperature dependence of the interatomic force constants, and treats anharmonic lattice vibrations. We choose ScN as a model system, comparing the computational predictions with the experimental data by Time Domain Thermoreflectance (TDTR). Our results show a trend of reduction in lattice thermal conductivity with decreasing grain size, with good agreement between the theoretical model and experimental data. There results suggest a possibility to control thermal conductivity by tailoring the microstructure of ScN. More importantly, we provide a predictive tool for the effect of the microstructure on the lattice thermal conductivity of materials based on first-principles calculations.
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| 24. |
- Kerdsongpanya, Sit, et al.
(författare)
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Phonon thermal conductivity of scandium nitride for thermoelectrics from first-principles calculations and thin-film growth
- 2017
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 96:19
-
Tidskriftsartikel (refereegranskat)abstract
- The knowledge of lattice thermal conductivity of materials under realistic conditions is vitally important since many modern technologies require either high or low thermal conductivity. Here, we propose a theoretical model for determining lattice thermal conductivity, which takes into account the effect of microstructure. It is based on ab initio description that includes the temperature dependence of the interatomic force constants and treats anharmonic lattice vibrations. We choose ScN as a model system, comparing the computational predictions to the experimental data by time-domain thermoreflectance. Our experimental results show a trend of reduction in lattice thermal conductivity with decreasing domain size predicted by the theoretical model. These results suggest a possibility to control thermal conductivity by microstructural tailoring and provide a predictive tool for the effect of the microstructure on the lattice thermal conductivity of materials based on ab initio calculations.
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| 25. |
- Kerje, Susanne, et al.
(författare)
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Is low molecular hyaluronan an early indicator of disease in avian systemic sclerosis?
- 2016
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Ingår i: Connective Tissue Research. - : Taylor & Francis. - 0300-8207 .- 1607-8438. ; 57:5, s. 337-346
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Tidskriftsartikel (refereegranskat)abstract
- AIM OF THE STUDY: To further elucidate the pathogenesis of systemic sclerosis (SSc) an experimental avian model was used. University of California at Davies line 200-chicken (UCD-200) spontaneously develops a SSc like disease that has most features of human SSc with vascular effects, inflammation, autoimmunity and fibrosis. The first signs of disease in UCD-200 are swelling and ischemic lesions of the comb, a tissue containing high amounts of the glycosaminoglycan hyaluronan. The aim was to evaluate inflammatory and fibrotic processes of the disease with regard to the molecular weight of hyaluronan.MATERIAL AND METHODS: Comb biopsies from UCD-200 and healthy White Leghorn (WL) chickens as controls at different ages were studied with histochemical localization of hyaluronan, hyaluronidase 1, CD3, IgY and collagen I and III. Hyaluronan molecular weight distribution was estimated with gas phase electrophoretic analysis.RESULTS: At 2 days of age hyaluronan was visualized in UCD-200 at the dermal part of the comb with no simultaneous staining of Hyal-1. In adult UCD-200 the comb skin was almost totally devoid of hyaluronan compared to WL of the same age. An increase of low molecular weight (LMW) hyaluronan was detected in comb tissue from UCD-200 at 1 day, 1, 2, 4 weeks in contrast to adult animals.CONCLUSIONS: An early inflammatory process involving LMW hyaluronan was confirmed as a possible profibrotic process. This indicates that hyaluronan might be an important participant in early inflammatory events of SSc in UCD-200 chicken and that disappearance of hyaluronan in skin predisposes to fibrosis.
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| 26. |
- Klarbring, Johan, et al.
(författare)
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Anharmonicity and Ultralow Thermal Conductivity in Lead-Free Halide Double Perovskites
- 2020
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Ingår i: Physical Review Letters. - : AMER PHYSICAL SOC. - 0031-9007 .- 1079-7114. ; 125:4
-
Tidskriftsartikel (refereegranskat)abstract
- The lead-free halide double perovskite class of materials offers a promising venue for resolving issues related to toxicity of Pb and long-term stability of the lead-containing halide perovskites. We present a first-principles study of the lattice vibrations in Cs2AgBiBr6, the prototypical compound in this class and show that the lattice dynamics of Cs2AgBiBr6 is highly anharmonic, largely in regards to tilting of AgBr6 and BiBr6 octahedra. Using an energy- and temperature-dependent phonon spectral function, we then show how the experimentally observed cubic-to-tetragonal phase transformation is caused by the collapse of a soft phonon branch. We finally reveal that the softness and anharmonicity of Cs2AgBiBr6 yield an ultralow thermal conductivity, unexpected of high-symmetry cubic structures.
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| 27. |
- Knoop, Florian, et al.
(författare)
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TDEP:Temperature Dependent Effective Potentials
- 2024
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Ingår i: Journal of Open Source Software. - : Open journals. - 2475-9066. ; 9:94
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Tidskriftsartikel (refereegranskat)abstract
- The Temperature Dependent Effective Potential (TDEP) method is a versatile and efficient approach to include temperature in a binitio materials simulations based on phonon theory. TDEP can be used to describe thermodynamic properties in classical and quantum ensembles, and several response properties ranging from thermal transport to Neutron and Raman spectroscopy. A stable and fast reference implementation is given in the software package of the same name described here. The underlying theoretical framework and foundation is briefly sketched with an emphasis on discerning the conceptual difference between bare and effective phonon theory, in both self-consistent and non-self-consistent formulations. References to numerous applications and more in-depth discussions of the theory are given.
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| 28. |
- Li, C.W., et al.
(författare)
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Phonon Self-Energy and Origin of Anomalous Neutron Scattering Spectra in SnTe and PbTe Thermoelectrics
- 2014
-
Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 112:17, s. 175501-
-
Tidskriftsartikel (refereegranskat)abstract
- The anharmonic lattice dynamics of rock-salt thermoelectric compounds SnTe and PbTe are investigated with inelastic neutron scattering (INS) and first-principles calculations. The experiments show that, surprisingly, although SnTe is closer to the ferroelectric instability, phonon spectra in PbTe exhibit a more anharmonic character. This behavior is reproduced in first-principles calculations of the temperature-dependent phonon self-energy. Our simulations reveal how the nesting of phonon dispersions induces prominent features in the self-energy, which account for the measured INS spectra and their temperature dependence. We establish that the phase space for three-phonon scattering processes, combined with the proximity to the lattice instability, is the mechanism determining the complex spectrum of the transverse-optic ferroelectric mode.
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| 29. |
- Marten, Tobias, 1979-, et al.
(författare)
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Double-segregation effect in AgxPd1-x/Ru(0001) thin film nanostructures
- 2008
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : APS. - 1098-0121 .- 1550-235X. ; 77:12
-
Tidskriftsartikel (refereegranskat)abstract
- We study the structural properties of ultrathin AgxPd1-x films on top of a Ru(0001) substrate. Effective interatomic interactions, obtained from first-principles calculations, have been used in Monte Carlo simulations to derive the distribution of the alloy components in a four-monolayer (4-ML) Ag-Pd film. Though Ag-Pd alloys show complete solubility in the bulk, the thin film geometry leads to a pronounced segregation between Ag and Pd atoms with a strong preference of Ag atoms toward the surface and Pd atoms toward the interface. The theoretical prediction of this double-segregation effect is strongly supported by photoelectron spectroscopy experiments carried out for 4-ML thin films. We also show, in an additional experiment, that even in the case where initially 1 ML Ag is buried under 6 ML Pd, the whole Ag ML segregates to the surface.
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| 30. |
- Miranda, Alonso L., et al.
(författare)
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Ab initio calculation of the thermal conductivity of indium antimonide
- 2014
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Ingår i: Semiconductor Science and Technology. - : IOP Publishing: Hybrid Open Access. - 0268-1242 .- 1361-6641. ; 29:12, s. 124002-
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Tidskriftsartikel (refereegranskat)abstract
- A theoretical study based on the density functional theory and the temperature-dependent effective potential method is performed to analyze the changes in the phonon band structure as a function of temperature for indium antimonide. In particular, we show changes in the thermal expansion coefficient and the thermal resistivity that agree rather well with experimental measurements. From the theoretical side, we show a weak dependence with respect to the chosen thermostat used to obtain the inter-atomic force constants, which strengthens our conclusions.
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| 31. |
- Mosyagin, Igor, et al.
(författare)
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Highly Efficient Free Energy Calculations of the Fe Equation of State Using Temperature-Dependent Effective Potential Method
- 2016
-
Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 120:43, s. 8761-8768
-
Tidskriftsartikel (refereegranskat)abstract
- Free energy calculations at finite temperature based on ab initio molecular dynamics (AIMD) simulations have become possible, but they are still highly computationally demanding. Besides, achieving simultaneously high accuracy of the calculated results and efficiency of the computational algorithm is still a challenge. In this work we describe an efficient algorithm to determine accurate free energies of solids in simulations using the recently proposed temperature-dependent effective potential method (TDEP). We provide a detailed analysis of numerical approximations employed in the TDEP algorithm. We show that for a model system considered in this work, hcp Fe, the obtained thermal equation of state at 2000 K is in excellent agreement with the results of standard calculations within the quasiharmonic approximation.
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| 32. |
- Nilsson, Johan O., et al.
(författare)
-
Ionic conductivity in Gd-doped CeO2 : Ab initio color-diffusion nonequilibrium molecular dynamics study
- 2016
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 93:2, s. 024102-
-
Tidskriftsartikel (refereegranskat)abstract
- A first-principles nonequilibrium molecular dynamics (NEMD) study employing the color-diffusion algorithm has been conducted to obtain the bulk ionic conductivity and the diffusion constant of gadolinium-doped cerium oxide (GDC) in the 850-1150 K temperature range. Being a slow process, ionic diffusion in solids usually requires simulation times that are prohibitively long for ab initio equilibrium molecular dynamics. The use of the color-diffusion algorithm allowed us to substantially speed up the oxygen-ion diffusion. The key parameters of the method, such as field direction and strength as well as color-charge distribution, have been investigated and their optimized values for the considered system have been determined. The calculated ionic conductivity and diffusion constants are in good agreement with available experimental data.
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| 33. |
- Olovsson, Weine, 1975-, et al.
(författare)
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Vibrational Effects in X-ray Absorption Spectra of Two-Dimensional Layered Materials
- 2019
-
Ingår i: The Journal of Physical Chemistry C. - : ACS Publications. - 1932-7447 .- 1932-7455. ; 123:15, s. 9688-9692
-
Tidskriftsartikel (refereegranskat)abstract
- With the examples of the C K-edge in graphite and the B K-edge in hexagonal boron nitride, we demonstrate the impact of vibrational coupling and lattice distortions on the X-ray absorption near-edge structure (XANES) in two-dimensional layered materials. Theoretical XANES spectra are obtained by solving the Bethe–Salpeter equation of many-body perturbation theory, including excitonic effects through the correlated motion of the core hole and excited electron. We show that accounting for zero-point motion is important for the interpretation and understanding of the measured X-ray absorption fine structure in both materials, in particular for describing the σ*-peak structure.
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| 34. |
- Palumbo, M., et al.
(författare)
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Thermodynamic modelling of crystalline unary phases
- 2014
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Ingår i: Physica status solidi. B, Basic research. - : Wiley. - 0370-1972 .- 1521-3951. ; 251:1, s. 14-32
-
Tidskriftsartikel (refereegranskat)abstract
- Progress in materials science through thermodynamic modelling may rest crucially on access to a database, such as that developed by Scientific Group Thermodata Europe (SGTE) around 1990. It gives the Gibbs energy G(T) of the elements in the form of series as a function of temperature, i.e. essentially a curve fitting to experimental data. In the light of progress in theoretical understanding and first-principles calculation methods, the possibility for an improved database description of the thermodynamics of the elements has become evident. It is the purpose of this paper to provide a framework for such work. Lattice vibrations, which usually give the major contribution to G(T), are treated in some detail with a discussion of neutron scattering studies of anharmonicity in aluminium, first-principles calculations including ab initio molecular dynamics (AIMD), and the strength and weakness of analytic model representations of data. Similarly, electronic contributions to G(T) are treated on the basis of the density of states N(E) for metals, with emphasis on effects at high T. Further, we consider G(T) below 300K, which is not covered by SGTE. Other parts in the paper discuss metastable and dynamically unstable lattices, G(T) in the region of superheated solids and the requirement on a database in the calculation of phase diagrams.
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| 35. |
- Romero, A. H., et al.
(författare)
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Thermal conductivity in PbTe from first principles
- 2015
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 91:21, s. 214310-
-
Tidskriftsartikel (refereegranskat)abstract
- We investigate the harmonic and anharmonic contributions to the phonon spectrum of lead telluride and perform a complete characterization of how thermal properties of PbTe evolve as temperature increases. We analyze the thermal resistivitys variationwith temperature and clarify misconceptions about existing experimental literature. The resistivity initially increases sublinearly because of phase space effects and ultra strong anharmonic renormalizations of specific bands. This effect is the strongest factor in the favorable thermoelectric properties of PbTe, and it explains its limitations at higher T. This quantitative prediction opens the prospect of phonon phase space engineering to tailor the lifetimes of crucial heat carrying phonons by considering different structure or nanostructure geometries. We analyze the available scattering volume between TO and LA phonons as a function of temperature and correlate its changes to features in the thermal conductivity.
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| 36. |
- Sangiovanni, Davide, et al.
(författare)
-
Efficient and accurate determination of lattice-vacancy diffusion coefficients via non equilibrium ab initio molecular dynamics
- 2016
-
Ingår i: PHYSICAL REVIEW B. - : AMER PHYSICAL SOC. - 2469-9950. ; 93:9, s. 094305-
-
Tidskriftsartikel (refereegranskat)abstract
- We revisit the color-diffusion algorithm [Aeberhard et al., Phys. Rev. Lett. 108, 095901 (2012)] in non equilibrium ab initio molecular dynamics (NE-AIMD) and propose a simple efficient approach for the estimation of monovacancy jump rates in crystalline solids at temperatures well below melting. Color-diffusion applied to monovacancy migration entails that one lattice atom (colored atom) is accelerated toward the neighboring defect site by an external constant force F. Considering bcc molybdenum between 1000 and 2800 K as a model system, NE-AIMD results show that the colored-atom jump rate k(NE) increases exponentially with the force intensity F, up to F values far beyond the linear-fitting regime employed previously. Using a simple model, we derive an analytical expression which reproduces the observed k(NE)(F) dependence on F. Equilibrium rates extrapolated by NE-AIMD results are in excellent agreement with those of unconstrained dynamics. The gain in computational efficiency achieved with our approach increases rapidly with decreasing temperatures and reaches a factor of 4 orders of magnitude at the lowest temperature considered in the present study.
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| 37. |
- Saunders, C. N., et al.
(författare)
-
Thermal expansion and phonon anharmonicity of cuprite studied by inelastic neutron scattering and ab initio calculations
- 2022
-
Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 105:17
-
Tidskriftsartikel (refereegranskat)abstract
- Inelastic neutron scattering measurements were performed with a time-of-flight chopper spectrometer to observe phonons in all parts of the Brillouin zone of a single crystal of cuprite Cu2O. We reduced the experimental data to phonon dispersions in the high-symmetry directions, and changes between 10 and 300 K are reported. In this paper, we show ab initio quasiharmonic (QH) and anharmonic (AH) calculations of phonon dispersions. We performed all AH calculations with a temperature-dependent effective potential method. Both QH and AH calculations account for the small negative thermal expansion of cuprite at low temperatures. However, the measured temperature-dependent phonon behavior was predicted more accurately with the AH calculations than the QH ones. Nevertheless, at 300 K, the cubic AH used in this paper did not entirely account for the experimental phonon dispersions in cuprite.
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| 38. |
- Shulumba, Nina, et al.
(författare)
-
Anharmonicity changes the solid solubility of an alloy at high temperatures
- 2015
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We have developed a method to accurately and efficiently determine the vibrational free energy as a function of temperature and volume for substitutional alloys from first principles. Taking Ti1−xAlxN alloy as a model system, we calculate the isostructural phase diagram by finding the global minimum of the free energy, corresponding to the true equilibrium state of the system. We demonstrate that the anharmonic contribution and temperature dependence of the mixing enthalpy have a decisive impact on the calculated phase diagram of a Ti1−xAlxN alloy, lowering the maximum temperature for the miscibility gap from 6560 K to 2860 K. Our local chemical composition measurements on thermally aged Ti0.5Al0.5N alloys agree with the calculated phase diagram.
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| 39. |
- Shulumba, Nina, et al.
(författare)
-
Impact of anharmonic effects on the phase stability, thermal transport, and electronic properties of AlN
- 2016
-
Ingår i: Physical Review B. - 2469-9950 .- 2469-9969. ; 94:10
-
Tidskriftsartikel (refereegranskat)abstract
- Wurtzite aluminium nitride is a technologically important wide band gap semiconductor with an unusually high thermal conductivity, used in optical applications and as a heatsink substrate. Many of its properties depend on an accurate description of its lattice dynamics, which have thus far only been captured in the quasiharmonic approximation. In this work, we demonstrate that anharmonicity has a considerable impact on its phase stability and transport properties, since anharmonicity is much stronger in the rocksalt phase. We compute a pressure-temperature phase diagram of AlN, demonstrating that the rocksalt phase is stabilised by increasing temperature, with respect to the wurtzite phase. We demonstrate that including anharmonicity, we can recover the thermal conductivity of the wurtzite phase (320 Wm−1K−1 under ambient conditions), and compute the hitherto unknown thermal conductivity of the rocksalt phase (96 Wm−1K−1). We also show that the electronic band gap decreases with temperature. These findings provide further evidence that anharmonic effects cannot be ignored in high temperature applications.
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| 40. |
- Shulumba, Nina, et al.
(författare)
-
Influence of vibrational free energy on the phase stability of alloys from first principles
- 2015
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We have developed a method to accurately and efficiently determine the vibrational free energy as a function of temperature and pressure for substitutional alloys from first principles. Taking the example of the technologically important hard coating alloy Ti1-xAlxN as an example, we investigate the effect on the vibrational free energy of substituting Ti for other group IV elements. By constructing the phase diagrams for these three alloys, we show why Zr1-xAlxN and Hf1-xAlxN are so difficult to experimentally synthesise in a metastable solid solution: both have solubility regions that span only a small low-AlN concentration range at temperatures above 1500 K. Moreover, Hf1-xAlxN is dynamically unstable at low temperatures and across most of the concentration range. We also show the chemical and thermal expansion effects dominate mass disorder in the Gibbs free energy of mixing.
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| 41. |
- Shulumba, Nina, et al.
(författare)
-
Intrinsic localized mode and low thermal conductivity of PbSe
- 2017
-
Ingår i: PHYSICAL REVIEW B. - : AMER PHYSICAL SOC. - 2469-9950. ; 95:1
-
Tidskriftsartikel (refereegranskat)abstract
- Lead chalcogenides such as PbS, PbSe, and PbTe are of interest for their exceptional thermoelectric properties and strongly anharmonic lattice dynamics. Although PbTe has received the most attention, PbSe has a lower thermal conductivity and a nonlinear temperature dependence of thermal resistivity despite being stiffer, trends that prior first-principles calculations have not fully reproduced. Here, we use ab initio calculations that explicitly account for strong anharmonicity and a computationally efficient stochastic phase-space sampling scheme to identify the origin of this low thermal conductivity as an anomalously large anharmonic interaction, exceeding in strength that in PbTe, between the transverse optic and longitudinal acoustic branches. The strong anharmonicity is reflected in the striking observation of an intrinsic localized mode that forms in the acoustic frequencies. Our work shows the deep insights into thermal phonons that can be obtained from ab initio calculations that do not rely on perturbations from the ground-state phonon dispersion.
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| 42. |
- Shulumba, Nina, et al.
(författare)
-
Lattice Vibrations Change the Solid Solubility of an Alloy at High Temperatures
- 2016
-
Ingår i: Physical Review Letters. - : AMER PHYSICAL SOC. - 0031-9007 .- 1079-7114. ; 117:20
-
Tidskriftsartikel (refereegranskat)abstract
- We develop a method to accurately and efficiently determine the vibrational free energy as a function of temperature and volume for substitutional alloys from first principles. Taking Ti1-xAlxN alloy as a model system, we calculate the isostructural phase diagram by finding the global minimum of the free energy corresponding to the true equilibrium state of the system. We demonstrate that the vibrational contribution including anharmonicity and temperature dependence of the mixing enthalpy have a decisive impact on the calculated phase diagram of a Ti1-xAlxN alloy, lowering the maximum temperature for the miscibility gap from 6560 to 2860 K. Our local chemical composition measurements on thermally aged Ti0.5Al0.5N alloys agree with the calculated phase diagram.
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| 43. |
- Shulumba, Nina, et al.
(författare)
-
Temperature-dependent elastic properties of Ti1−xAlxN alloys
- 2015
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 107:23
-
Tidskriftsartikel (refereegranskat)abstract
- Ti1−xAlxN is a technologically important alloy that undergoes a process of high temperature age-hardening that is strongly influenced by its elastic properties. We have performed first principles calculations of the elastic constants and anisotropy using the newly developed symmetry imposed force constant temperature dependent effective potential method, that include lattice vibrations and therefore the effects of temperature, including thermal expansion and intrinsic anharmonicity. These are compared with in situ high temperature x-ray diffraction measurements of the lattice parameter. We show that anharmonic effects are crucial to the recovery of finite temperature elasticity. The effects of thermal expansion and intrinsic anharmonicity on the elastic constants are of the same order, and cannot be considered separately. Furthermore, the effect of thermal expansion on elastic constants is such that the volume change induced by zero point motion has a significant effect. For TiAlN, the elastic constants soften non-uniformly with temperature: C11 decreases substantially when the temperature increases for all compositions, resulting in an increased anisotropy. These findings suggest that an increased Al content and annealing at higher temperatures will result in a harder alloy.
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| 44. |
- Shulumba, Nina, et al.
(författare)
-
Vibrational free energy and phase stability of paramagnetic and antiferromagnetic CrN from ab initio molecular dynamics
- 2014
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 89:17, s. 174108-
-
Tidskriftsartikel (refereegranskat)abstract
- We present a theoretical first-principles method to calculate the free energy of a magnetic system in its high-temperature paramagnetic phase, including vibrational, electronic, and magnetic contributions. The method for calculating free energies is based on ab initio molecular dynamics and combines a treatment of disordered magnetism using disordered local moments molecular dynamics with the temperature-dependent effective potential method to obtain the vibrational contribution to the free energy. We illustrate the applicability of the method by obtaining the anharmonic free energy for the paramagnetic cubic and the antiferromagnetic orthorhombic phases of chromium nitride. The influence of lattice dynamics on the transition between the two phases is demonstrated by constructing the temperature-pressure phase diagram.
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| 45. |
- Steneteg, Peter, et al.
(författare)
-
Temperature dependence of TiN elastic constants from ab initio molecular dynamics simulations
- 2013
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 87:9
-
Tidskriftsartikel (refereegranskat)abstract
- Elastic properties of cubic TiN are studied theoretically in a wide temperature interval. First-principles simulations are based on ab initio molecular dynamics (AIMD). Computational efficiency of the method is greatly enhanced by a careful preparation of the initial state of the simulation cell that minimizes or completely removes a need for equilibration and therefore allows for parallel AIMD calculations. Elastic constants C11, C12, and C44 are calculated. A strong dependence on the temperature is predicted, with C11 decreasing by more than 29% at 1800 K as compared to its value obtained at T=0 K. Furthermore, we analyze the effect of temperature on the elastic properties of polycrystalline TiN in terms of the bulk and shear moduli, the Young's modulus and Poisson ratio. We construct sound velocity anisotropy maps, investigate the temperature dependence of elastic anisotropy of TiN, and observe that the material becomes substantially more isotropic at high temperatures. Our results unambiguously demonstrate the importance of taking into account finite temperature effects in theoretical calculations of elastic properties of materials intended for high-temperature applications.
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| 46. |
- Strömberg, Thomas, et al.
(författare)
-
IGF-1 receptor tyrosine kinase inhibition by the cyclolignan PPP induces G2/M-phase accumulation and apoptosis in multiple myeloma cells.
- 2006
-
Ingår i: Blood. - : American Society of Hematology. - 0006-4971 .- 1528-0020. ; 107:2, s. 669-78
-
Tidskriftsartikel (refereegranskat)abstract
- Emerging evidence suggests the insulin-like growth factor-1 receptor (IGF-1R) to be an important mediator of tumor-cell survival and resistance to cytotoxic therapy in multiple myeloma (MM). Recently, members of the cyclolignan family have been shown to selectively inhibit the receptor tyrosine kinase (RTK) activity of the IGF-1R beta-chain. The effects of the cyclolignan picropodophyllin (PPP) were studied in vitro using a panel of 13 MM cell lines and freshly purified tumor cells from 10 patients with MM. PPP clearly inhibited growth in all MM cell lines and primary MM samples cultured in the presence or absence of bone marrow stromal cells. PPP induced a profound accumulation of cells in the G(2)/M-phase and an increased apoptosis. Importantly, IGF-1, IGF-2, insulin, or IL-6 did not reduce the inhibitory effects of PPP. As demonstrated by in vitro kinase assays, PPP down-regulated the IGF-1 RTK activity without inhibiting the insulin RTK activity. This conferred decreased phosphorylation of Erk1/2 and reduced cyclin dependent kinase (CDK1) activity. In addition, the expression of mcl-1 and survivin was reduced. Taken together, we suggest that interfering with the IGF-1 RTK by using the cyclolignan PPP offers a novel and selective therapeutic strategy for MM.
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| 47. |
- Szymanski, N. J., et al.
(författare)
-
Dynamical stabilization in delafossite nitrides for solar energy conversion
- 2018
-
Ingår i: Journal of Materials Chemistry A. - : ROYAL SOC CHEMISTRY. - 2050-7488 .- 2050-7496. ; 6:42, s. 20852-20860
-
Tidskriftsartikel (refereegranskat)abstract
- Delafossite structured ternary nitrides, ABN(2), have been of recent experimental investigation for applications such as tandem solar and photoelectrochemical cells. We present a thorough first principles computational investigation of their stability, electronic structure, and optical properties. Nine compounds, where A = Cu, Ag, Au and B = V, Nb, Ta, were studied. For three of these compounds, CuTaN2, CuNbN2, and AgTaN2, our computations agree well with experimental results. Optimized lattice parameters, formation energies, and mechanical properties have been computed using the generalized gradient approximation (GGA). Phonon density of states computed at zero-temperature shows that all compounds are dynamically unstable at low temperatures. Including finite-temperature anharmonic effects stabilizes all compounds at 300 K, with the exception of AgVN2. Analysis of Crystal Orbital Hamiltonian Populations (COHP) provides insight into the bonding and antibonding characters of A-N and B-N pairs. Instability at low temperatures can be attributed to strong A-N antibonding character near the Fermi energy. B-N bonding is found to be crucial in maintaining stability of the structure. AgVN2 is the only compound to display significant B-N antibonding below the Fermi energy, as well as the strongest degree of A-N antibonding, both of which provide explanation for the sustained instability of this compound up to 900 K. Hybrid functional calculations of electronic and optical properties show that real static dielectric constants in the semiconductors are related to corresponding band gaps through the Moss relation. CuTaN2, CuNbN2, AgTaN2, AgNbN2, AgVN2, AuTaN2, and AuNbN2 exhibit indirect electronic band gaps while CuVN2 and AuVN2 are metallic. Imaginary parts of the dielectric function are characterized by d-d interband transitions in the semiconductors and d-d intraband transitions in the metals. Four compounds, CuTaN2, CuNbN2, AgTaN2, and AgNbN2, are predicted to exhibit large light absorption in the range of 1.0 to 1.7 eV, therefore making these materials good candidates for solar-energy conversion applications. Two compounds, AuTaN2 and AuNbN2, have band gaps and absorption onsets near the ideal range for obtaining high solar-cell conversion efficiency, suggesting that these compounds could become potential candidates as absorber materials in tandem solar cells or for band-gap engineering by alloying.
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| 48. |
- Tidholm, Johan, 1991-
(författare)
-
Lattice dynamics : From fundamental research to practical applications
- 2020
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The reason to perform calculations in material science usually falls into one of two categories: to predict or explain the origin of material properties. This thesis covers first-principle calculations for solids at extreme conditions, from both of the two mentioned categories. I primarily have studied the effects of high-pressure and high-temperature on lattice dynamics, mechanical and electronic properties. To treat the effects of temperature, ab initio molecular dynamics (AIMD) simulations and self-consistent phonon calculations, based on density functional theory, have been utilised. These approaches account for the temperature effects by considering thermally excited supercells as samples of a statistical ensemble. To extract properties from this representation, I have used methods which maps the supercell data to a unit cell representation or fits it to a simple model Hamiltonian.The small displacement method was used to analyse the dynamical stability for nitrides and polymorphs of silica, synthesised at high-pressure in a diamond anvil cell. The nitride compounds consist of a high amount of nitrogen either as chains, forming a porous framework together with transition metal atoms or as dinitrogen molecules, occupying the channels of the framework. The nitrogen chains consist of single- or double-bonded nitrogen atoms, making these compounds highly energetic. Polymorphs of silica can be used to model deep Earth liquids. These new polymorphs, named coesite-IV and coesite-V, consist of four-, five-, and six-oriented silicon. Some of the octahedra of the six-oriented silicon atoms, of these new phases, are sharing faces, which according to Pauling's third rule would make them highly unstable. My phonon calculations indicate these phases to be dynamically stable. Furthermore, my calculations predict higher compressibility for these new phases compared to the competing ones. By modelling silicate melts with coesite-IV and coesite-V, a more complex and compressible structure is expected, affecting the predicted seismic behaviour.I studied Kohn anomalies for body-centered cubic niobium by simulating this material with self-consistent phonon calculations. The electronic structure was studied by using a band unfolding technique, for which I obtained an effective unit cell representation of the electronic structure at elevated temperatures. Temperature primarily smeared the electronic states but did not induce significant shifts of the bands. In parallel, the anharmonicity of this system was studied using the temperature dependent effective potential method. Even close to the melting temperature, this element is remarkably harmonic. The experimentally observed disappearance of the Kohn anomalies with increased temperature is predominantly dependent, according to my calculations, on the temperature-induced smearing of the electronic states.Using stress-strain relations, accurate high-temperature elastic properties were predicted for Ti0.5Al0.5N. The simulations were performed with AIMD. The stresses were fitted using the least-squares method to a linear expression from which the elastic constants were derived. The results were compared with previously performed calculations that employed additional approximations. The results of the symmetry imposed force constant temperature dependent effective potential (SIFC-TDEP) method agrees well with our results. I also compared my results with TiN calculations that employed a similar methodology. My and the SIFC-TDEP results are reporting lower values for the polycrystalline moduli than the calculations for TiN. The data I generated were also used for a machine learned interatomic potential method, where moment tensor potentials were trained and evaluated, using this data.
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| 49. |
- Tidholm, Johan, et al.
(författare)
-
Temperature dependence of the Kohn anomaly in bcc Nb from first-principles self-consistent phonon calculations
- 2020
-
Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 101:11
-
Tidskriftsartikel (refereegranskat)abstract
- Using ab initio calculations, we have analyzed the influence of anharmonic effects on the electronic structure and the phonon-dispersion relations of body-centered-cubic (bcc) niobium (Nb) and investigated the temperature dependence of the Kohn anomaly in this metal. A comparison of the results obtained in the framework of the temperature-dependent effective potential method with those derived within the quasiharmonic approximation demonstrates the importance of the explicit treatment of the finite-temperature effects upon the theoretical description of bcc Nb lattice dynamics. In agreement with experimental results, the inclusion of anharmonic vibrations in our calculations leads to the disappearance of the Kohn anomaly for the acoustic mode in a vicinity of the Gamma point with increasing temperature. Moreover, the calculated phonon self-energy indicates that the origin of the temperature dependence is related to the change of the electronic structure. We have calculated the temperature dependence of the electronic spectral function and analyzed the Fermi surface of Nb. A significant temperature-induced smearing of the electronic states has been identified as the origin of the disappearance of the Kohn anomaly in Nb at elevated temperature.
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| 50. |
- von Zur-Mühlen, Bengt, Docent, 1966-, et al.
(författare)
-
Treating diabetes with islet transplantation : Lessons learnt from the Nordic network for clinical islet transplantation
- 2019. - 1
-
Ingår i: Transplantation, Bioengineering, and Regeneration of the Endocrine Pancreas. - : Academic Press. - 9780128148334 ; , s. 599-611
-
Bokkapitel (refereegranskat)abstract
- In this chapter we present the Nordic experiences withclinical islet transplantation after starting networks. TheNordic Network for Clinical Islet Transplantation (NNCIT)was established in 2000. In addition, we also present a localmultidisciplinary effort organized as part of a Centerof Excellence (CoE) in Type 1 Diabetes. Even though careof type 1 diabetic patients has improved over time thereare still patients who have substantial problems withhypoglycemic incidents, unawareness, and glycemiclability and need treatment other than injected exogenousinsulin.1 In the Nordic countries it is now consideredas innovative rather than an experimental therapy.Islet transplantation still has limitations, only making ita treatment suitable for a small part of type 1 diabeticpatients. The main limitations are the need of immunosuppressanttherapy (as in all organ transplants betweenindividuals), early loss of substantial amounts of transplantedislets, and a limited source of islets (deceasedorgan donors). To have a favorable clinical result there isoften a need for retransplantation and or complementarytherapies. Islet transplantation as such has great potentialin the near future for modification, encapsulation ofthe islets, or stem cell therapies.2,3 Experiences made sofar will be useful when this new era evolves. Even if islettransplantation is a minimally invasive and surgicallysimple technique there are technically complex partssuch as islet isolation that require highly specialized staffand a need for multidisciplinary expertise, which is whyit for the foreseeable future will continue to be centralizedto a few units to ensure high quality and continueddevelopment. The aim of our collaborations, local,regional, and international, is to offer the best treatmentfor those patients for whom conventional treatment isinadequate and take advantage of the expertise distributedin several different disciplines.
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