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1.	Abrikosov, Igor, et al. (författare) Theoretical description of pressure-induced phase transitions: a case study of Ti-V alloys 2015 Ingår i: High Pressure Research. - : Taylor andamp; Francis: STM, Behavioural Science and Public Health Titles. - 0895-7959 .- 1477-2299. ; 35:1, s. 42-48 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.
2.	Walls, B., et al. (författare) Nanodomain structure of single crystalline nickel oxide 2021 Ingår i: Scientific Reports. - : Nature Portfolio. - 2045-2322. ; 11:1 Tidskriftsartikel (refereegranskat)abstract In this work we present a comprehensive study of the domain structure of a nickel oxide single crystal grown by floating zone melting and suggest a correlation between point defects and the observed domain structure. The properties and structure of domains dictate the dynamics of resistive switching, water splitting and gas sensing, to name but a few. Investigating the correlation between point defects and domain structure can provide a deeper understanding of their formation and structure, which potentially allows one to tailor domain structure and the dynamics of the aforementioned applications. A range of inhomogeneities are observed by diffraction and microscopy techniques. X-ray and low-energy electron diffraction reveal domains on the submicron- and nanometer-scales, respectively. In turn, these domains are visualised by atomic force and scanning tunneling microscopy (STM), respectively. A comprehensive transmission electron microscopy (TEM) study reveals inhomogeneities ranging from domains of varying size, misorientation of domains, variation of the lattice constant and bending of lattice planes. X-ray photoelectron spectroscopy and electron energy-loss spectroscopy indicate the crystal is Ni deficient. Density functional theory calculations-considering the spatial and electronic disturbance induced by the favourable nickel vacancy-reveal a nanoscale distortion comparable to STM and TEM observations. The different inhomogeneities are understood in terms of the structural relaxation induced by ordering of nickel vacancies, which is predicted to be favourable.
3.	Barannikova, S. A., et al. (författare) Influence of stresses on structure and properties of Ti and Zr-based alloys from first-principles simulations 2015 Ingår i: INTERNATIONAL SCIENTIFIC CONFERENCE OF YOUNG SCIENTISTS: ADVANCED MATERIALS IN CONSTRUCTION AND ENGINEERING. - : IOP Publishing. Konferensbidrag (refereegranskat)abstract Computer simulations in the framework of the Density Functional Theory have become an established tool for computer simulations of materials properties. In most cases, however, information is obtained at ambient conditions, preventing design of materials for applications at extreme conditions. In this work we employ ab initio calculations to investigate the influence of stresses on structure and stability of Ti-Mo and Zr-Nb alloys, an important class of construction materials. Calculations reproduce known phase stability trends in these systems, and we resolve the controversy regarding the stability of body-centered cubic solid solution in Mo-rich Ti-Mo alloys against the isostructural decomposition. Calculated results are explained in terms of the electronic structure effects, as well as in terms of physically transparent thermodynamic arguments that relate phase stability to deviations of concentration dependence of atomic volume from the linear behavior.
4.	Khvan, A. V., et al. (författare) Thermodynamic properties of tin: Part I Experimental investigation, ab-initio modelling of alpha-, beta-phase and a thermodynamic description for pure metal in solid and liquid state from 0 K 2019 Ingår i: Calphad. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0364-5916 .- 1873-2984. ; 65, s. 50-72 Tidskriftsartikel (refereegranskat)abstract Thermodynamic data for crystalline white and grey tin were assessed using an extended Einstein model from 0 K. Ab-initio simulations in the framework of density functional theory (DFT) with the quasiharmonic approximation (QHA) were carried out to define the heat capacities for both phases of tin from 0 K up to room temperatures. Good agreement was observed between theoretical and experimental heat capacities, which makes it possible to combine theoretical and experimental data to determine the standard entropies. Data for the liquid phase were described using a two state model. During the assessment, careful analysis of the experimental data was carried out. In order to fulfil the need for a precise evaluation of S-298(o) we needed to use an additional technique using multiple Einstein functions, which allows the experimental heat capacity and enthalpy data for the solid phase to be approximated accurately from 0 K up to the melting point and to estimate solid phase transition entropy and enthalpy which are difficult to measure due to a high activation barrier. Additional measurements of heat capacity were carried out where existing data were scarce.
5.	Sidnov, K. P., et al. (författare) Effect of alloying on elastic properties of ternary Ni-Al-Ti system: Experimental validation 2016 Ingår i: Journal of Alloys and Compounds. - : ELSEVIER SCIENCE SA. - 0925-8388 .- 1873-4669. ; 688, s. 534-541 Tidskriftsartikel (refereegranskat)abstract Using combustion synthesis approach we fabricated B2 NiAl intermetallic compound as well as quasibinary Ni(Al50Ti50) alloy, where half Al atoms are randomly substituted by transitional metal Ti. Youngs modulus for synthesized materials was measured and appeared to be 222 +/- 10 GPa for NiAl and 175 +/- 15 GPa for Ni(Al50Ti50) phases. Using first-principles simulations in the framework of the Density Functional Theory, we investigate the elastic properties of Ni(Al1-xTix) system, including single-crystal, as well as polycrystalline elastic moduli. Direct comparison of the experimental and theoretical values of the Youngs modulus demonstrates that the employed theoretical approach allows carefully predict elastic properties of NiAl-based intermetallics. In particular, we predict that alloying NiAl with Ti should increase the ductility of the intermetallic phase. (C) 2016 Elsevier B.V. All rights reserved.
6.	Skripnyak, Natalia, 1991-, et al. (författare) Mixing enthalpies of alloys with dynamical instability : bcc Ti-V system 2020 Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 188, s. 145-154 Tidskriftsartikel (refereegranskat)abstract Enthalpy of mixing is among the key materials parameters to determine phase stability and phase transformations in solid solutions. The possibility to predict it from first principles in the framework of the density functional theory is one of the corner stones of the modern materials modeling and the future data-driven materials design. Here we have considered body-centered cubic (bcc) Ti-V alloys, a system with high potential for aerospace, automotive biomedical and energy applications, which is known to exhibit the dynamical instability of the crystal lattice for Ti-rich alloys at low temperature. We have calculated the mixing enthalpies ΔH of bcc Ti-V alloys in the whole interval of concentration at high temperature using ab initio molecular dynamics (AIMD) simulations. A comparison with state-of-the-art static calculations at temperature 0 K shows drastic difference between the two methods: while AIMD predicts positive values of ΔH in the whole range of concentrations, the static zero-temperature simulations result in negative values of ΔH for Ti-rich alloys. We have measured the mixing enthalpy of bcc Ti-V alloys experimentally at 1073 K using an isoperibol high temperature Tian-Calvet calorimeter and found that the enthalpies are positive, in agreement with our finite temperature AIMD calculations. We attribute the failure of the standard static calculations of ΔH to lattice distortions associated with the dynamical instability of bcc Ti-V alloys at zero temperature and argue that the effect should be generally important in theoretical predictions of thermodynamic properties, especially for systems with dynamical instability.
7.	Yu Nikonov, A., et al. (författare) Calculation of Mechanical Properties of BCC Ti-Nb Alloys 2015 Ingår i: INTERNATIONAL CONFERENCE ON ADVANCED MATERIALS WITH HIERARCHICAL STRUCTURE FOR NEW TECHNOLOGIES AND RELIABLE STRUCTURES 2015. - : AMER INST PHYSICS. - 9780735413306 Konferensbidrag (refereegranskat)abstract We have calculated mechanical properties of bcc Ti-Nb alloys in the framework of the first-principles approach using the exact muffin-tin orbital method. The results obtained quantitatively correlate well with known experimental data and can be used in order to design new materials based on of Ti alloys intended for various applications, e.g. for bio-medical applications.
8.	Abrikosov, Igor, et al. (författare) Theoretical Modeling of Thermodynamic and Mechanical Properties of the Pure Components of Ti and Zr Based Alloys Using the Exact Muffin-Tin Orbitals Method 2014 Ingår i: Russian Physics Journal. - : Springer Verlag (Germany). - 1064-8887 .- 1573-9228. ; 56:9, s. 1030-1038 Tidskriftsartikel (refereegranskat)abstract The exact muffin-tin orbitals (EMTO) method belongs to the third and latest generation of first-principles methods of calculating the electronic structure of materials in the so-called approximation of muffin-tin (MT) orbitals within the framework of the density functional theory. A study has been performed of its applicability for modeling the thermodynamic and mechanical properties of the pure components of Ti and Zr based alloys. The total energies of Ti, Zr, Nb, V, Mo, and Al are calculated in three crystal structures - face-centered cubic (FCC), body-centered cubic (BCC), and hexagonal close-packed (HCP). For all of these elements and crystal structures, we have calculated the theoretical values of the lattice constants, elastic constants, and equations of state. The stable crystal structures have been determined. In all cases, calculations by the EMTO method predict the correct structure of the ground state. For stable structures we compared the obtained results with experiment and with calculations using full potential methods. We have demonstrated the reliability of the EMTO method and conclude that its further application for effective modeling of the properties of disordered alloys based on Ti and Zr is possible.
9.	Abrikosov, Igor A., et al. (författare) Finite Temperature, Magnetic, and Many-Body Effects in Ab Initio Simulations of Alloy Thermodynamics 2013 Ingår i: TMS2013 Supplemental Proceedings. - Hoboken, NJ, USA : John Wiley & Sons. - 9781118605813 - 9781118663547 ; , s. 617-626 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.
10.	Isaeva, E. I., et al. (författare) Ab initio phonon calculations for Ll(2) Ni3Al and B2NiAl 2004 Ingår i: Solid State Communications. - : Elsevier BV. - 0038-1098 .- 1879-2766. ; 129:12, s. 809-814 Tidskriftsartikel (refereegranskat)abstract The phonon spectra and phonon density of states of the Ni3Al and NiAl intermetallic compounds are calculated from first principles using the linear response method in conjunction with ultrasoft pseudopotentials. The calculated phonon dispersion curves are in good agreement with available experimental results from inelastic neutron scattering.
11.	Mukhamedov, B. O., et al. (författare) Thermodynamic and physical properties of Zr3Fe and ZrFe2 intermetallic compounds 2019 Ingår i: Intermetallics (Barking). - : ELSEVIER SCI LTD. - 0966-9795 .- 1879-0216. ; 109, s. 189-196 Tidskriftsartikel (refereegranskat)abstract Experimental differential scanning calorimetry measurements and ab-initio simulations were carried out to define the heat capacities of Zr3Fe and C15-ZrFe2 compounds from 0 K up to their maximum stability temperatures. Experimental measurements of heat capacity of each compound were performed for the first time in wide range of temperatures. Density functional theory and quasi-harmonic approximation (QHA) were employed to calculate the free energy of the studied systems as a function of volume and temperature. A good agreement was observed between theoretical and experimental heat capacities within validity range of the QHA. This makes it possible to combine theoretical and experimental data to determine the standard entropies of intermetallic compounds.
12.	Abrikosov, Igor, et al. (författare) Recent progress in simulations of the paramagnetic state of magnetic materials 2016 Ingår i: Current opinion in solid state & materials science. - : PERGAMON-ELSEVIER SCIENCE LTD. - 1359-0286 .- 1879-0348. ; 20:2, s. 85-106 Forskningsöversikt (refereegranskat)abstract We review recent developments in the field of first-principles simulations of magnetic materials above the magnetic order disorder transition temperature, focusing mainly on 3d-transition metals, their alloys and compounds. We review theoretical tools, which allow for a description of a system with local moments, which survive, but become disordered in the paramagnetic state, focusing on their advantages and limitations. We discuss applications of these theories for calculations of thermodynamic and mechanical properties of paramagnetic materials. The presented examples include, among others, simulations of phase stability of Fe, Fe-Cr and Fe-Mn alloys, formation energies of vacancies, substitutional and interstitial impurities, as well as their interactions in Fe, calculations of equations of state and elastic moduli for 3d-transition metal alloys and compounds, like CrN and steels. The examples underline the need for a proper treatment of magnetic disorder in these systems. (C) 2015 Elsevier Ltd. All rights reserved.
13.	Barannikova, S A, et al. (författare) Significant correlation between macroscopic and microscopic parameters for the description of localized plastic flow auto-waves in deforming alloys 2012 Ingår i: Solid State Communications. - : Elsevier. - 0038-1098 .- 1879-2766. ; 152:9, s. 784-787 Tidskriftsartikel (refereegranskat)abstract Understanding of mechanical properties of materials and a possibility to predicting them from ab initio calculations have fundamental importance for solid state theory. In this work we establish a significant correlation between the product of the macroscopic parameters of localized plastic flow auto-waves in deforming alloys, their length and propagation rate and the product of the microscopic (lattice) parameters of these materials, the spacing between close-packed planes of the lattice and the rate of transverse elastic waves. Thus, these products can be regard as invariants of plastic and elastic deformation processes, respectively. Moreover, the established regularity suggests that the elastic and the plastic processes simultaneously involved in the deformation are closely related. Our work also demonstrates that ab initio simulations can be used for the prediction of parameters of localized plastic flow auto-waves in deforming alloys.
14.	Baykov, Vitaly, et al. (författare) Ab Initio Studies of the Energy Characteristics and Magnetic Properties of Point Defects in GaAs 2005 Ingår i: Physics of the solid state. - : Pleiades Publishing Ltd. - 1063-7834 .- 1090-6460. ; 47:10, s. 1831-1836 Tidskriftsartikel (refereegranskat)abstract The formation energies of intrinsic point defects and solution energies of transition metal impurities in gallium arsenide are determined on the basis of ab initio calculations using the method of a locally self-consistent Green's function, which is a generalization of the coherent potential approximation. Based on the calculated energies, the conclusion is made that the As-Ga antisite defect is the most common intrinsic defect in GaAs. Calculations showed that transition metal impurities, except for Ni, preferentially occupy gallium sites substitutionally. The magnetic moments of impurity atoms are calculated as a function of the chemical environment. It is shown that, in compensated GaAs, Mn atoms tend to form clusters.
15.	Baykov, Vitaly, et al. (författare) Magnetic properties of 3d impurities in GaAs 2007 Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier. - 0304-8853 .- 1873-4766. ; 310:2, s. 2120-2122 Tidskriftsartikel (refereegranskat)abstract Electronic structure, thermodynamic, and magnetic properties of 3d-transition metal (TM) impurities in GaAs have been studied from first principles using Green's function approach. The studied TM impurities (V, Cr, Mn, and Fe) are found to form substitutional alloys on the Ga sublattice. The possibility of raising the Curie temperature TC in (GaMn) As by co-doping it with Cr impurities was examined on the basis of total energy difference between the disordered local moment (DLM) and the ferromagnetically ordered (FM) spin configurations. The calculated Curie temperature and magnetic moment have maxima for GaAs doped with Cr and Mn. The magnetic properties of Mn-doped GaAs are shown to be more sensitive to antisite As defects than those of Cr-doped GaAs. However, the Cr impurities are sensitive to the presence of acceptor defects, such as vacancies on the Ga sublattice. The investigation of the electronic structure of pseudo-ternary alloys (Ga(1-x-y)MnxCry) As has shown a mutual compensation of Mn and Cr impurities. Therefore, in order to reach the highest critical temperature, GaAs has to be separately doped with Cr or Mn impurities. The GaAs doped with Fe is found to be non-ferromagnetic.
16.	Bleskov, I. D., et al. (författare) Ab initio calculations of elastic properties of Ru1-xNixAl superalloys 2009 Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 94:16 Tidskriftsartikel (refereegranskat)abstract Ab initio total energy calculations based on the exact muffin-tin orbitals method, combined with the coherent potential approximation, have been used to study the thermodynamical and elastic properties of substitutional refractory Ru1-xNixAl alloys. We have found that the elastic constants C' and C11 exhibit pronounced peculiarities near the concentration of about 40 at. % Ni, which we ascribe to electronic topological transitions. Our suggestion is supported by the Fermi surface calculations in the whole concentration range. Results of our calculations show that one can design Ru-Ni-Al alloys substituting Ru by Ni (up to 40 at. %) with almost invariable elastic constants and reduced density.
17.	Bosak, A., et al. (författare) Possible artifacts in inferring seismic properties from X-ray data 2016 Ingår i: Physics of the Earth and Planetary Interiors. - : ELSEVIER SCIENCE BV. - 0031-9201 .- 1872-7395. ; 260, s. 14-19 Tidskriftsartikel (refereegranskat)abstract We consider the experimental and computational artifacts relevant for the extraction of aggregate elastic properties of polycrystalline materials with particular emphasis on the derivation of seismic velocities. We use the case of iron as an example, and show that the improper use of definitions and neglecting the crystalline anisotropy can result in unexpectedly large errors up to a few percent. (C) 2016 Elsevier B.V. All rights reserved.
18.	Dubrovinsky, Leonid, et al. (författare) Materials synthesis at terapascal static pressures 2022 Ingår i: Nature. - London, United Kingdom : Nature Publishing Group. - 0028-0836 .- 1476-4687. ; 605:7909, s. 274-278 Tidskriftsartikel (refereegranskat)abstract Theoretical modelling predicts very unusual structures and properties of materials at extreme pressure and temperature conditions(1,2). Hitherto, their synthesis and investigation above 200 gigapascals have been hindered both by the technical complexity of ultrahigh-pressure experiments and by the absence of relevant in situ methods of materials analysis. Here we report on a methodology developed to enable experiments at static compression in the terapascal regime with laser heating. We apply this method to realize pressures of about 600 and 900 gigapascals in a laser-heated double-stage diamond anvil cell(3), producing a rhenium-nitrogen alloy and achieving the synthesis of rhenium nitride Re7N3-which, as our theoretical analysis shows, is only stable under extreme compression. Full chemical and structural characterization of the materials, realized using synchrotron single-crystal X-ray diffraction on microcrystals in situ, demonstrates the capabilities of the methodology to extend high-pressure crystallography to the terapascal regime.
19.	Glazyrin, K., et al. (författare) Importance of Correlation Effects in hcp Iron Revealed by a Pressure-Induced Electronic Topological Transition 2013 Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 110:11, s. 117206- Tidskriftsartikel (refereegranskat)abstract We discover that hcp phases of Fe and Fe0.9Ni0.1 undergo an electronic topological transition at pressures of about 40 GPa. This topological change of the Fermi surface manifests itself through anomalous behavior of the Debye sound velocity, c/a lattice parameter ratio, and Mossbauer center shift observed in our experiments. First-principles simulations within the dynamic mean field approach demonstrate that the transition is induced by many-electron effects. It is absent in one-electron calculations and represents a clear signature of correlation effects in hcp Fe.
20.	Granas, O., et al. (författare) Theoretical study of the Mo-Ru sigma phase 2008 Ingår i: Calphad. - : Elsevier. - 0364-5916 .- 1873-2984. ; 32:1, s. 171-176 Tidskriftsartikel (refereegranskat)abstract The thermodynamic properties of the Mo-Ru binary sigma-phase are investigated using a combination of ab initio calculations and CALPHAD modeling. Total energy calculations have been performed for the complete set of 32 end-member compounds of a 5-sublattice compound energy model. The internal crystallographic parameters for each end-member compound have been determined by minimising the total energy. A simpler, 3-sublattice model of the Mo-Ru a-phase is formulated on the basis of calculated total energies. The site occupancy is acquired by minimising the free energy given by the compound energy model. A strong preference of Mo and Ru towards high-coordination sites and icosahedral sites in the Mo-Ru a-phase is found and analysed in terms of the electronic structure.
21.	Ingason, Arni Sigurdur, et al. (författare) Magnetic Self-Organized Atomic Laminate from First Principles and Thin Film Synthesis 2013 Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 110 Tidskriftsartikel (refereegranskat)abstract he first experimental realization of a magnetic Mn+1AXn (MAX) phase, (Cr0.75Mn0.25)2GeC, is presented, synthesized as a heteroepitaxial single crystal thin film, exhibiting excellent structural quality. This self-organized atomic laminate is based on the well-known Cr2GeC, with Mn, a new element in MAX phase research, substituting Cr. The compound was predicted using first-principles calculations, from which a variety of magnetic behavior is envisaged, depending on the Mn concentration and Cr/Mn atomic configuration within the sublattice. The analyzed thin films display a magnetic signal at room temperature.
22.	Isaev, E. I., et al. (författare) Anomalously enhanced superconductivity and ab initio lattice dynamics in transition metal carbides and nitrides 2005 Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 72, s. 064515- Tidskriftsartikel (refereegranskat)
23.	Ji, Fuxiang, 1991-, et al. (författare) Remarkable Thermochromism in the Double Perovskite Cs2NaFeCl6 2023 Ingår i: Advanced Optical Materials. - : Wiley-Blackwell. - 2162-7568 .- 2195-1071. Tidskriftsartikel (refereegranskat)abstract Lead-free halide double perovskites (HDPs) have emerged as a new generation of thermochromic materials. However, further materials development and mechanistic understanding are required. Here, a highly stable HDP Cs2NaFeCl6 single crystal is synthesized, and its remarkable and fully reversible thermochromism with a wide color variation from light-yellow to black over a temperature range of 10 to 423 K is investigated. First-principles, density functional theory (DFT)-based calculations indicate that the thermochromism in Cs2NaFeCl6 is an effect of electron–phonon coupling. The temperature sensitivity of the bandgap in Cs2NaFeCl6 is up to 2.52 meVK−1 based on the Varshni equation, which is significantly higher than that of lead halide perovskites and many conventional group-IV, III–V semiconductors. Meanwhile, this material shows excellent environmental, thermal, and thermochromic cycle stability. This work provides valuable insights into HDPs' thermochromism and sheds new light on developing efficient thermochromic materials.
24.	Ji, Fuxiang, 1991-, et al. (författare) Remarkable Thermochromism in the Double Perovskite Cs2NaFeCl6 2024 Ingår i: Advanced Optical Materials. - : John Wiley & Sons. - 2162-7568 .- 2195-1071. ; 12:8 Tidskriftsartikel (refereegranskat)abstract Lead-free halide double perovskites (HDPs) have emerged as a new generation of thermochromic materials. However, further materials development and mechanistic understanding are required. Here, a highly stable HDP Cs2NaFeCl6 single crystal is synthesized, and its remarkable and fully reversible thermochromism with a wide color variation from light-yellow to black over a temperature range of 10 to 423 K is investigated. First-principles, density functional theory (DFT)-based calculations indicate that the thermochromism in Cs2NaFeCl6 is an effect of electron-phonon coupling. The temperature sensitivity of the bandgap in Cs2NaFeCl6 is up to 2.52 meVK(-1) based on the Varshni equation, which is significantly higher than that of lead halide perovskites and many conventional group-IV, III-V semiconductors. Meanwhile, this material shows excellent environmental, thermal, and thermochromic cycle stability. This work provides valuable insights into HDPs' thermochromism and sheds new light on developing efficient thermochromic materials.
25.	Koemets, E., et al. (författare) Revealing the Complex Nature of Bonding in the Binary High-Pressure Compound FeO2 2021 Ingår i: Physical Review Letters. - : AMER PHYSICAL SOC. - 0031-9007 .- 1079-7114. ; 126:10 Tidskriftsartikel (refereegranskat)abstract Extreme pressures and temperatures are known to drastically affect the chemistry of iron oxides, resulting in numerous compounds forming homologous series nFeOmFe(2)O(3) and the appearance of FeO2. Here, based on the results of in situ single-crystal x-ray diffraction, Mossbauer spectroscopy, x-ray absorption spectroscopy, and density-functional theory + dynamical mean-field theory calculations, we demonstrate that iron in high-pressure cubic FeO2 and isostructural FeO2H0.5 is ferric (Fe3+), and oxygen has a formal valence less than 2. Reduction of oxygen valence from 2, common for oxides, down to 1.5 can be explained by a formation of a localized hole at oxygen sites.
26.	Laniel, Dominique, et al. (författare) Aromatic hexazine [N6]4− anion featured in the complex structure of the high-pressure potassium nitrogen compound K9N56 2023 Ingår i: Nature Chemistry. - : NATURE PORTFOLIO. - 1755-4330 .- 1755-4349. ; 15:5, s. 641-646 Tidskriftsartikel (refereegranskat)abstract The recent high-pressure synthesis of pentazolates and the subsequent stabilization of the aromatic [N-5](-) anion at atmospheric pressure have had an immense impact on nitrogen chemistry. Other aromatic nitrogen species have also been actively sought, including the hexaazabenzene N-6 ring. Although a variety of configurations and geometries have been proposed based on ab initio calculations, one that stands out as a likely candidate is the aromatic hexazine anion [N-6](4-). Here we present the synthesis of this species, realized in the high-pressure potassium nitrogen compound K9N56 formed at high pressures (46 and 61 GPa) and high temperature (estimated to be above 2,000 K) by direct reaction between nitrogen and KN3 in a laser-heated diamond anvil cell. The complex structure of K9N56-composed of 520 atoms per unit cell-was solved based on synchrotron single-crystal X-ray diffraction and corroborated by density functional theory calculations. The observed hexazine anion [N-6](4-) is planar and proposed to be aromatic.
27.	Mopoung, Kunpot, et al. (författare) Understanding Antiferromagnetic Coupling in Lead-Free Halide Double Perovskite Semiconductors 2024 Ingår i: The Journal of Physical Chemistry C. - : AMER CHEMICAL SOC. - 1932-7447 .- 1932-7455. ; 128:12, s. 5313-5320 Tidskriftsartikel (refereegranskat)abstract Solution-processable semiconductors with antiferromagnetic (AFM) order are attractive for future spintronics and information storage technology. Halide perovskites containing magnetic ions have emerged as multifunctional materials, demonstrating a cross-link between structural, optical, electrical, and magnetic properties. However, stable optoelectronic halide perovskites that are antiferromagnetic remain sparse, and the critical design rules to optimize magnetic coupling still must be developed. Here, we combine the complementary magnetometry and electron-spin-resonance experiments, together with first-principles calculations to study the antiferromagnetic coupling in stable Cs-2(Ag:Na)FeCl6 bulk semiconductor alloys grown by the hydrothermal method. We show the importance of nonmagnetic monovalence ions at the B-I site (Na/Ag) in facilitating the superexchange interaction via orbital hybridization, offering the tunability of the Curie-Weiss parameters between -27 and -210 K, with a potential to promote magnetic frustration via alloying the nonmagnetic B-I site (Ag:Na ratio). Combining our experimental evidence with first-principles calculations, we draw a cohesive picture of the material design for B-site-ordered antiferromagnetic halide double perovskites.
28.	Mosyagin, Igor, et al. (författare) Ab initio calculations of pressure-dependence of high-order elastic constants using finite deformations approach 2017 Ingår i: Computer Physics Communications. - : ELSEVIER SCIENCE BV. - 0010-4655 .- 1879-2944. ; 220, s. 20-30 Tidskriftsartikel (refereegranskat)abstract We present a description of a technique for ab initio calculations of the pressure dependence of second and third-order elastic constants. The technique is based on an evaluation of the corresponding Lagrangian stress tensor derivative of the total energy assuming finite size of the deformations. Important details and parameters of the calculations are highlighted. Considering body-centered cubic Mo as a model system, we demonstrate that the technique is highly customizable and can be used to investigate non-linear elastic properties under high-pressure conditions. (C) 2017 Elsevier B.V. All rights reserved.
29.	Mosyagin, Igor, et al. (författare) Reply to the comment by M. Mazdziarz on the article "Ab initio calculations of pressure-dependence of high-order elastic constants using finite deformations approach" [Computer Physics Communications 220 (2017) 20-30] 2019 Ingår i: Computer Physics Communications. - : ELSEVIER SCIENCE BV. - 0010-4655 .- 1879-2944. ; 235, s. 295-296 Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract Marcin Mazdziarz has published a comment on our recent paper by I. Mosyagin, A.V. Lugovskoy, O.M. Krasilnikov, Yu.Kh. Vekilov, S.I. Simak and L.A. Abrikosov titled "Ab initio calculations of pressure dependence of high-order elastic constants using finite deformations approach" [Computer Physics Communications 220 (2017)2030]. The author states that there are serious fundamental errors and flaws. In this reply we clarify all misunderstanding mentioned in the said comment. (C) 2018 Published by Elsevier B.V.
30.	Ponomareva, A. V., et al. (författare) Effect of pressure on phase stability in Fe-Cr alloys 2011 Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 84:9, s. 094422- Tidskriftsartikel (refereegranskat)abstract The effect of hydrostatic pressure on the phase stability of Fe-Cr alloys has been studied using ab initio methods. We show that while pressure decreases the tendency toward the phase separation in the paramagnetic state of bcc alloys, in the ferromagnetic state it reduces the alloy stability at low Cr concentration and vice versa, makes the solid solution more stable at higher concentrations. This behavior of the phase stability can be predicted from the deviation of the lattice parameter from Vegard's law in bcc Fe-Cr alloys. On the atomic level, the pressure effect can be explained by the suppression of the local magnetic moments on Cr atoms, which gives rise to a decrease of the Fe-Cr magnetic exchange interaction at the first coordination shell and, as a result, to the observed variation of the ordering tendency between the Fe and Cr atoms.
31.	Salamania, Janella, 1992-, et al. (författare) Elucidating dislocation core structures in titanium nitride through high-resolution imaging and atomistic simulations 2022 Ingår i: Materials & design. - : Elsevier. - 0264-1275 .- 1873-4197. ; 224 Tidskriftsartikel (refereegranskat)abstract Although titanium nitride (TiN) is among the most extensively studied and thoroughly characterizedthin-film ceramic materials, detailed knowledge of relevant dislocation core structures is lacking. Byhigh-resolution scanning transmission electron microscopy (STEM) of epitaxial single crystal (001)-oriented TiN films, we identify different dislocation types and their core structures. These include, besidesthe expected primary a/2{110}h110i dislocation, Shockley partial dislocations a/6{111}h112i and sessileLomer edge dislocations a/2{100}h011i. Density-functional theory and classical interatomic potentialsimulations complement STEM observations by recovering the atomic structure of the different disloca-tion types, estimating Peierls stresses, and providing insights on the chemical bonding nature at the core.The generated models of the dislocation cores suggest locally enhanced metal–metal bonding, weakenedTi-N bonds, and N vacancy-pinning that effectively reduces the mobilities of {110}h110i and {111}h112idislocations. Our findings underscore that the presence of different dislocation types and their effects onchemical bonding should be considered in the design and interpretations of nanoscale and macroscopicproperties of TiN.
32.	Skripnyak, Natalia, et al. (författare) Ab initio calculations of elastic properties of alloys with mechanical instability: Application to BCC Ti-V alloys 2018 Ingår i: Materials & design. - : Elsevier. - 0264-1275 .- 1873-4197. ; 140, s. 357-365 Tidskriftsartikel (refereegranskat)abstract Considering Ti-V alloys with the body-centered cubic crystal lattice, a system with mechanical instability for Tirich alloys, we calculate their elastic properties using Projector Augmented Wave method and the exact muffin tin orbital method in a complete interval of V concentrations. The substitutional disorder is modeled using the special quasi-random structures technique and the coherent potential approximation. The efficiency and accuracy of the simulation techniques is analyzed, and a strategy for efficient high-throughput calculations of elastic properties of disordered alloys is proposed. Dependences of the single crystal elastic moduli on V concentration and a set ofmechanical characteristics of polycrystalline alloys are presented and discussed. The effect of V content on themechanical stabilization of the bcc Ti-V alloys is investigated. In agreement with experiment, we find that titanium-rich alloys are mechanically unstable, however the alloys becomemechanically stablewith increasing content of V in the system. We observe a nonlinear dependence of the alloys Youngs moduli in a vicinity of the mechanical stabilization and suggest that this effect can be used to design alloys with low values of the elastic moduli. (C) 2017 Elsevier Ltd. All rights reserved.
33.	Vorotilo, S., et al. (författare) Ab-initio modeling and experimental investigation of properties of ultra-high temperature solid solutions TaxZr1-xC 2019 Ingår i: Journal of Alloys and Compounds. - : ELSEVIER SCIENCE SA. - 0925-8388 .- 1873-4669. ; 778, s. 480-486 Tidskriftsartikel (refereegranskat)abstract Due to their high melting temperature, high-temperature oxidation resistance and outstanding mechanical properties, TaxZr1-xC solid solutions are promising ultra-high temperature ceramics (UHTC). However, accelerated knowledge-based development of UHTCs solid solutions requires reliable data regarding the properties of the solution phases in the whole interval of concentrations. At present, there are contradictory reports regarding the existence of the miscibility gap in Ta-Zr-C system at temperatures below 900 degrees C. In this work, we carry out ab-initio calculations of the thermodynamic properties of TaxZr1-xC alloys and demonstrate that the solid solutions should not decompose into TaC and ZrC end member compounds. We synthesize single-phase samples of TaxZr1-xC with compositions x = 0.9, 0.8, 0.6, and 0.3 by self-propagating high-temperature synthesis (SHS) and anneal the samples for 40 h. We do not observe any sign of the decomposition of the solid solution during the annealing, corroborating the conclusions obtained by theoretical simulations. (C) 2018 Elsevier B.V. All rights reserved.
34.	Zarechnaya, E Yu, et al. (författare) Superhard Semiconducting Optically Transparent High Pressure Phase of Boron 2009 Ingår i: PHYSICAL REVIEW LETTERS. - 0031-9007. ; 102:18, s. 185501- Tidskriftsartikel (refereegranskat)abstract An orthorhombic (space group Pnnm) boron phase was synthesized at pressures above 9 GPa and high temperature, and it was demonstrated to be stable at least up to 30 GPa. The structure, determined by single-crystal x-ray diffraction, consists of B-12 icosahedra and B-2 dumbbells. The charge density distribution obtained from experimental data and ab initio calculations suggests covalent chemical bonding in this phase. Strong covalent interatomic interactions explain the low compressibility value (bulk modulus is K-300=227 GPa) and high hardness of high-pressure boron (Vickers hardness H-V=58 GPa), after diamond the second hardest elemental material.
35.	Abrikosov, Igor, et al. (författare) Importance of Thermally Induced Magnetic Excitations in First-principles Simulations of Elastic Properties of Transition Metal Alloys 2012 Ingår i: Solid State Phenomena. - 1012-0394 .- 1662-9779. ; 190, s. 291-294 Tidskriftsartikel (refereegranskat)abstract We demonstrate the importance of accounting for the complex magnetic ground state and finite temperature magnetic excitations in theoretical simulations of structural and elastic properties of transition metal alloys. Considering Fe72Cr16Ni12 face centered cubic (fcc) alloy, we compare results of first-principles calculations carried out for ferromagnetic and non-magnetic states, as well as for the state with disordered local moments. We show that the latter gives much more accurate description of the elastic properties for paramagnetic alloys. We carry out a determination of the magnetic ground state for fcc Fe-Mn alloys, considering collinear, as well as non-collinear states, and show the sensitively of structural and elastic properties in this system to the detailed alignment between magnetic moments. We therefore conclude that it is essential to develop accurate models of the magnetic state for the predictive description of properties of transition metal alloys.
36.	Abrikosov, Igor, et al. (författare) Magnetism in systems with reduced dimensionality and chemical disorder : the local environment effects 2006 Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier. - 0304-8853 .- 1873-4766. ; 300:1, s. 211-215 Tidskriftsartikel (refereegranskat)abstract We study influence of the local chemical environment, the so-called local environment effects, on the electronic structure and properties of magnetic systems with reduced dimensionality and chemical disorder, and show that they play a crucial role in a vicinity of magnetic instability. As a model, we consider Fe–Ni Invar. We present results obtained from ab initio calculations of the electronic structure, magnetic moments, and exchange interactions in random fcc Fe–Ni alloy, for a single monolayer alloy film on a Cu (0 0 1) substrate as well as in the bulk. We analyze the difference between the film and the bulk magnetization, which is found to be most pronounced for dilute alloys. We also analyze a sensitivity of the individual magnetic moments and effective exchange parameters to the local chemical environment of the atoms.
37.	Abrikosov, Igor, 1965-, et al. (författare) Phase Stabilities of Alloys From First-Principles 2008 Ingår i: XVII International Materials Research Congress,2008. - Mexico : Mexico. Konferensbidrag (refereegranskat)
38.	Alling, Björn, 1980-, et al. (författare) A unified cluster expansion method applied to the configurational thermodynamics of cubic TiAlN 2011 Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 83:10, s. 104203- Tidskriftsartikel (refereegranskat)abstract We use a study of the cubic Ti1−xAlxN system to illustrate a practical way of combining the major methodologies within alloy theory, the Connolly-Williams cluster expansion and the generalized perturbation method, in order to solve difficult alloy problems. The configurational, concentration dependent, Hamiltonian is separated into a fixed-lattice and a local lattice relaxation part. The effective cluster interactions of the first part is obtained primarily with a GPM-based approach while the later is obtained using cluster expansion. In our case the impact on the isostructural phase diagram of considering short range clustering beyond the mean field approximation, obtained from the mixing enthalpy and entropy of the random alloy, is rather small, especially in the composition region x ≤ 0.66, within reach of thin film growth techniques.
39.	Alling, Björn, 1980-, et al. (författare) Comparison of thermodynamic properties of cubic Cr 1-x Al x N and Ti 1-x Al x N from first-principles calculations 2007 Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 102:044314 Tidskriftsartikel (refereegranskat)abstract In order to investigate the stability of the cubic phase of Cr1−xAlxN at high AlN content, first principles calculations of magnetic properties, lattice parameters, electronic structure, and mixing enthalpies of the system were performed. The mixing enthalpy was calculated on a fine concentration mesh to make possible the accurate determination of its second concentration derivative. The results are compared to calculations performed for the related compound Ti1−xAlxN and with experiments. The mixing enthalpy is discussed in the context of isostructural spinodal decomposition. It is shown that the magnetism is the key to understand the difference between the Cr- and Ti-containing systems. Cr1−xAlxN turns out to be more stable against spinodal decomposition than Ti1−xAlxN, especially for AlN-rich samples which are of interest in cutting tools applications.
40.	Alling, Björn, 1980- (författare) Configurational and Magnetic Interactions in Multicomponent Systems 2010 Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract This thesis is a theoretical study of configurational and magnetic interactions in multicomponent solids. These interactions are the projections onto the configurational and magnetic degrees of freedom of the underlying electronic quantum mechanical system, and can be used to model, explain and predict the properties of materials. For example, the interactions govern temperature induced configurational and magnetic order-disorder transitions in Heusler alloys and ternary nitrides.In particular three perspectives are studied. The first is how the interactions can be derived from first-principles calculations at relevant physical conditions. The second is their consequences, like the critical temperatures for disordering, obtained with e.g. Monte Carlo simulations. The third is their origin in terms of the underlying electronic structure of the materials.Intrinsic defects in the half-Heusler system NiMnSb are studied and it is found that low-energy defects do not destroy the important half-metallic property at low concentrations. Deliberate doping of NiMnSb with 3d-metals is considered and it is found that replacing some Ni with extra Mn or Cr creates new strong magnetic interactions which could be beneficial for applications at elevated temperature. A self-consistent scheme to include the effects of thermal expansion and one-electron excitations in the calculation of the magnetic critical temperature is introduced and applied to a study of Ni1−xCuxMnSb.A supercell implementation of the disordered local moments approach is suggested and benchmarked for the treatment of paramagnetic CrN as a disordered magnetic phase. It is found that the orthorhombic-to-cubic phase transition in this nitride can be understood as a first-order magnetic order-disorder transition. The ferromagnetism in Ti1−xCrxN solid solutions, an unusual property in nitrides, is explained in terms of a charge transfer induced change in the Cr-Cr magnetic interactions.Cubic Ti1−xAlxN solid solutions displays a complex and concentration dependent phase separation tendency. A unified cluster expansion method is presented that can be used to simulate the configurational thermodynamics of this system. It is shown that short range clustering do influence the free energy of mixing but only slightly change the isostructural phase diagram as compared to mean-field estimates.
41.	Alling, Björn, et al. (författare) Electronic origin of the isostructural decomposition in cubic M1-xAlxN (M=Ti, Cr, Sc, Hf) : A first-principles study 2008 Ingår i: Surface and Coatings Technology. - : Elsevier BV. - 0257-8972. ; 203:5-7, s. 883-886 Tidskriftsartikel (refereegranskat)abstract We have used first-principles calculations to investigate the mixing enthalpies, lattice parameters and electronic density of states of the ternary nitride systems Ti1-xAlxN, Cr1-xAlxN, Sc1-xAlxN and Hf1-xAlxN in the cubic B1 structure where the transition metals and aluminium form a solid solution on the metal sublattice. We discuss the electronic origins of the possible isostructural decomposition in these materials relevant for hard coatings applications. We find that in the systems Ti1-xAlxN and Hf1-xAlxN the electronic structure effects strongly influences the phase stability as d-states are localised at the Fermi level in AlN-rich samples. This leads to a strongly asymmetric contribution to the mixing enthalpy, an effect not present in Cr1-xAlxN and Sc1-xAlxN. The lattice mismatch is large in Sc1-xAlxN and Hf1-xAlxN, giving a symmetric contribution to the mixing enthalpies in those systems.
42.	Alling, Björn, 1980-, et al. (författare) First-principles study of the effect of nitrogen vacancies on the decomposition pattern in cubic Ti1-xAlxN1-y 2008 Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 92, s. 071903-1-071903-3 Tidskriftsartikel (refereegranskat)abstract The effect of nitrogen substoichiometry on the isostructural phase stabilities of the cubic Ti1−xAlxN1−y system has been investigated using first-principles calculations. The preferred isostructural decomposition pattern in these metastable solid solutions was predicted from the total energy calculations on a dense concentration grid. Close to the stoichiometric Ti1−xAlxN1 limit, N vacancies increase the tendency for phase separation as N sticks to Al while the vacancies prefers Ti neighbors. For nitrogen depleated conditions, N sticks to Ti forming TiN (0<<1) while Al tends to form nitrogen-free fcc-Al or Al–Ti alloys.
43.	Alling, Björn, 1980-, et al. (författare) Mixing and decomposition thermodynamics of c-Ti1-xAlxN from first-principles calculations 2007 Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75:045123 Tidskriftsartikel (refereegranskat)abstract Wedescribe an efficient first-principles method that can be used tocalculate mixing enthalpies of transition metal nitrides with B1 structureand substitutional disorder at the metal sublattice. The technique isbased on the density functional theory. The independent sublattice modelis suggested for the treatment of disorder-induced local lattice relaxationeffects. It supplements the description of the substitutional disorder withinthe coherent potential approximation. We demonstrate the excellent accuracy ofthe method by comparison with calculations performed by means ofthe projector augumented wave method on supercells constructed as specialquasirandom structures. At the same time, the efficiency of thetechnique allows for total energy calculations on a very finemesh of concentrations which enables a reliable calculation of thesecond concentration derivative of the alloy total energy. This isa first step towards first-principles predictions of concentrations and temperatureintervals where the alloy decomposition proceeds via the spinodal mechanism.We thus calculate electronic structure, lattice parameter, and mixing enthalpiesof the quasibinary alloy c-Ti1−xAlxN. The lattice parameter follows Vegard'slaw at low fractions of AlN but deviates increasingly withincreasing Al content. We show that the asymmetry of themixing enthalpy and its second concentration derivative is associated withsubstantial variations of the electronic structure with alloy composition. Thephase diagram is constructed within the mean-field approximation.
44.	Alling, Björn, et al. (författare) Strong impact of lattice vibrations on electronic and magnetic properties of paramagnetic Fe revealed by disordered local moments molecular dynamics 2016 Ingår i: PHYSICAL REVIEW B. - : AMER PHYSICAL SOC. - 2469-9950. ; 93:22 Tidskriftsartikel (refereegranskat)abstract We study the impact of lattice vibrations on magnetic and electronic properties of paramagnetic bcc and fcc iron at finite temperature, employing the disordered local moments molecular dynamics (DLM-MD) method. Vibrations strongly affect the distribution of local magnetic moments at finite temperature, which in turn correlates with the local atomic volumes. Without the explicit consideration of atomic vibrations, the mean local magnetic moment and mean field derived magnetic entropy of paramagnetic bcc Fe are larger compared to paramagnetic fcc Fe, which would indicate that the magnetic contribution stabilizes the bcc phase at high temperatures. In the present study we show that this assumption is not valid when the coupling between vibrations and magnetism is taken into account. At the gamma-delta transition temperature (1662 K), the lattice distortions cause very similar magnetic moments of both bcc and fcc structures and hence magnetic entropy contributions. This finding can be traced back to the electronic densities of states, which also become increasingly similar between bcc and fcc Fe with increasing temperature. Given the sensitive interplay of the different physical excitation mechanisms, our results illustrate the need for an explicit consideration of vibrational disorder and its impact on electronic and magnetic properties to understand paramagnetic Fe. Furthermore, they suggest that at the gamma-delta transition temperature electronic and magnetic contributions to the Gibbs free energy are extremely similar in bcc and fcc Fe.
45.	Andersson, David A., et al. (författare) Modeling of Ce2, Ce2O3 ,and CeO2-x in the LDA+U formalism 2007 Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 75, s. 035109-1-035109-6 Tidskriftsartikel (refereegranskat)abstract The electronic structure and thermodynamic properties of CeO2 and Ce2O3 have been studied from first principles by the all-electron projector-augmented-wave (PAW) method, as implemented in the ab initio total-energy and molecular-dynamics program VASP (Vienna ab initio simulation package). The local density approximation (LDA)+U formalism has been used to account for the strong on-site Coulomb repulsion among the localized Ce 4f electrons. We discuss how the properties of CeO2 and Ce2O3 are affected by the choice of U as well as the choice of exchange-correlation potential, i.e., the local density approximation or the generalized gradient approximation. Further, reduction of CeO2, leading to formation of Ce2O3 and CeO2-x, and its dependence on U and exchange-correlation potential have been studied in detail. Our results show that by choosing an appropriate U it is possible to consistently describe structural, thermodynamic, and electronic properties of CeO2, Ce2O3, and CeO2-x, which enables modeling of redox processes involving ceria-based materials.
46.	Andersson, David A., et al. (författare) Optimization of ionic conductivity in doped ceria 2006 Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 103:10, s. 3518-3521 Tidskriftsartikel (refereegranskat)abstract Oxides with the cubic fluorite structure, e.g., ceria (CeO2), are known to be good solid electrolytes when they are doped with cations of lower valence than the host cations. The high ionic conductivity of doped ceria makes it an attractive electrolyte for solid oxide fuel cells, whose prospects as an environmentally friendly power source are very promising. In these electrolytes, the current is carried by oxygen ions that are transported by oxygen vacancies, present to compensate for the lower charge of the dopant cations. Ionic conductivity in ceria is closely related to oxygen-vacancy formation and migration properties. A clear physical picture of the connection between the choice of a dopant and the improvement of ionic conductivity in ceria is still lacking. Here we present a quantum-mechanical first-principles study of the influence of different trivalent impurities on these properties. Our results reveal a remarkable correspondence between vacancy properties at the atomic level and the macroscopic ionic conductivity. The key parameters comprise migration barriers for bulk diffusion and vacancy-dopant interactions, represented by association (binding) energies of vacancy-dopant clusters. The interactions can be divided into repulsive elastic and attractive electronic parts. In the optimal electrolyte, these parts should balance. This finding offers a simple and clear way to narrow the search for superior dopants and combinations of dopants. The ideal dopant should have an effective atomic number between 61 (Pm) and 62 (Sm), and we elaborate that combinations of Nd/Sm and Pr/Gd show enhanced ionic conductivity, as compared with that for each element separately.
47.	Andersson, David A., et al. (författare) Redox properties of CeO2-MO2 MO2 (M=Ti, Zr, Hf or Th) solid solutions from first principles calculations 2007 Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 90:3, s. 031909- Tidskriftsartikel (refereegranskat)abstract The authors have used density functional theory calculations to investigate how the redox thermodynamics and kinetics of CeO2 are influenced by forming solid solutions with TiO2, ZrO2, HfO2, and ThO2. Reduction is facilitated by dissolving TiO2 (largest improvement), HfO2, or ZrO2 (least improvement), while ThO2 makes reduction slightly more difficult. The migration barrier is much lower in the neighborhood of a Ti (largest decrease), Hf, or Zr (least decrease), while the binding energy of solute ions and vacancies increases in the same sequence. They rationalize the properties of ceria solid solutions in terms of defect cluster relaxations.
48.	Andersson, David A., et al. (författare) Theoretical study of CeO(2) doped with tetravalent ions 2007 Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 76, s. 1741191-17411910 Tidskriftsartikel (refereegranskat)abstract We have used density functional theory calculations within the LDA+U formulation to investigate how small amounts of dissolved SiO2, GeO2, SnO2, or PbO2 affect the redox thermodynamics of ceria (CeO2). Compared to pure ceria, reduction is facilitated and the reducibility increases in the sequence of CeO2-SnO2, CeO2-GeO2, and CeO2-SiO2, which correlates with the decrease of the ionic radii of the solutes. For low solute concentrations, there is an inverse relation between high reducibility and the solution energy of tetravalent solutes. CeO2-PbO2 is unique in the sense that the initial reduction occurs by Pb(IV)double right arrow Pb(II) instead of the usual Ce(IV)double right arrow Ce(III) reaction. Among the investigated ceria compounds, CeO2-PbO2 has the lowest reduction energy and rather low solution energy. We have studied how the solution and reduction energies depend on the concentration of Si, Ge, Sn, Pb, Ti, Zr, Hf, and Th solute ions. While the solution energy increases monotonously with concentration, the reduction energy first decreases, as compared to pure ceria (except for Th, which exhibits a small increase), and with further increase of solute concentration, it either remains almost constant (Zr, Hf, and Th) or slightly increases (Ti, Si, Ge, and Sn).
49.	Asker, Christian, et al. (författare) Elastic constants and anisotropy in FeNi alloys at high pressures from first-principles calculations 2009 Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 79:21 Tidskriftsartikel (refereegranskat)abstract The single-crystal and polycrystalline elastic constants and the elastic anisotropy in face-centered cubic and hexagonal close-packed FeNi alloys have been investigated at ultrahigh pressures by means of first-principles calculations using the exact muffin-tin orbitals method and the coherent-potential approximation. Comparisons with earlier calculations for pure Fe and experimental results are presented and discussed. We show that Ni alloying into Fe increases slightly the density and has very little effect on bulk moduli. Moreover, the relative decrease in c(44) elastic constant is much stronger in the hcp phase than in the fcc one. It is found that the elastic anisotropy is higher for face-centered cubic than for the hexagonal close-packed structure of FeNi, even though the face-centered cubic phase has a higher degree of symmetry. The anisotropy in face-centered cubic structure decreases with increasing nickel concentration while a very weak increase is observed for the hexagonal close-packed structure.
50.	Asker, Christian, 1979-, et al. (författare) Electronic and atomic structure of Mo from high-temperature molecular dynamics simulations Annan publikation (populärvet., debatt m.m.)abstract By means of ab initio molecular dynamics (AIMD) simulations we carry out a detailed stdly of the electronic and atomic structure of Mo upon the thermal stabilization of its dynamically unstable face-centered cubic (fcc) phase, Wc calculate how the atomic positions, radial distribution function, and the ei

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