| 1. |
- Aslandukov, Andrey, et al.
(författare)
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High-Pressure Yttrium Nitride, Y5N14, Featuring Three Distinct Types of Nitrogen Dimers
- 2021
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Ingår i: The Journal of Physical Chemistry C. - : AMER CHEMICAL SOC. - 1932-7447 .- 1932-7455. ; 125:32, s. 18077-18084
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Tidskriftsartikel (refereegranskat)abstract
- Yttrium nitride, Y5N14, was synthesized by direct reaction between yttrium and nitrogen at similar to 50 GPa and similar to 2000 K in a laser-heated diamond anvil cell. High-pressure single-crystal X-ray diffraction revealed that the crystal structure of Y5N14 (space group P4/mbm) contains three distinct types of nitrogen dimers. Crystal chemical analysis and ab initio calculations demonstrated that the dimers [N-2](x-) are crystallographically and chemically nonequivalent and possess distinct noninteger formal charges (x) that make Y5N14 unique among known compounds. Theoretical computations showed that Y5N14 has an anion-driven metallicity, with the filled part of its conduction band formed by nitrogen p-states. The compressibility of Y5N14, determined on decompression down to similar to 10 GPa, was found to be uncommonly high for dinitrides containing +3 cations (the bulk modulus K-0 = 137(6) GPa).
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| 2. |
- Aslandukova, Alena, et al.
(författare)
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High-pressure hP3 yttrium allotrope with CaHg2-type structure as a prototype of the hP3 rare-earth hydride series
- 2023
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Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 107:1
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Tidskriftsartikel (refereegranskat)abstract
- A high-pressure (HP) yttrium allotrope, hP3-Y (space group P6/mmm), was synthesized in a multi-anvil press at 20 GPa and 2000 K which is recoverable to ambient conditions. Its relative stability and electronic properties were investigated using density functional theory calculations. A hP3-Y derivative hydride, hP3-YHx, with a variable hydrogen content (x = 2.8, 3, 2.4), was synthesized in diamond anvil cells by the direct reaction of yttrium with paraffin oil, hydrogen gas, and ammonia borane upon laser heating to similar to 3000 K at 51, 45 and 38 GPa, respectively. Room-temperature decompression leads to gradual reduction and eventually the complete loss of hydrogen at ambient conditions. Isostructural hP3-NdHx and hP3-GdHx hydrides were synthesized from Nd and Gd metals and paraffin oil, suggesting that the hP3-Y structure type may be common for rare-earth elements. Our results expand the list of allotropes of trivalent lanthanides and their hydrides and suggest that they should be considered in the context of studies of HP behavior and properties of this broad class of materials.
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| 3. |
- Aslandukova, Alena, et al.
(författare)
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Novel High-Pressure Yttrium Carbide gamma-Y4C5 Containing [C-2] and Nonlinear [C-3] Units with Unusually Large Formal Charges
- 2021
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Ingår i: Physical Review Letters. - : AMER PHYSICAL SOC. - 0031-9007 .- 1079-7114. ; 127:13
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Tidskriftsartikel (refereegranskat)abstract
- Changes in the bonding of carbon under high pressure leads to unusual crystal chemistry and can dramatically alter the properties of transition metal carbides. In this work, the new orthorhombic polymorph of yttrium carbide, gamma-Y4C5, was synthesized from yttrium and paraffin oil in a laser-heated diamond anvil cell at similar to 50 GPa. The structure of gamma-Y4C5 was solved and refined using in situ synchrotron single-crystal x-ray diffraction. It includes two carbon groups: [C-2] dimers and nonlinear [C-3] trimers. Crystal chemical analysis and density functional theory calculations revealed unusually high noninteger charges ([C-2](5.2-) and [C-3](6.8-)) and unique bond orders (<1.5). Our results extend the list of possible carbon states at extreme conditions.
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| 4. |
- Meier, Thomas, et al.
(författare)
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Proton mobility in metallic copper hydride from high-pressure nuclear magnetic resonance
- 2020
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Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 102:16
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Tidskriftsartikel (refereegranskat)abstract
- The atomic and electronic structures of Cu2H and CuH have been investigated by high-pressure nuclear magnetic resonance spectroscopy up to 96 GPa, X-ray diffraction up to 160 GPa, and density functional theory-based calculations. Metallic Cu2H was synthesized at a pressure of 40 GPa, and semimetallic CuH at 90 GPa, found stable up to 160 GPa. For Cu2H, experiments and computations show an anomalous increase in the electronic density of state at the Fermi level for the hydrogen 1s states and the formation of a hydrogen network in the pressure range 43-58 GPa, together with high H-1 mobility of similar to 10(-7) cm(2)/s. A comparison of these observations with results on FeH suggests that they could be common features in metal hydrides.
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| 5. |
- Tidholm, Johan, 1991-
(författare)
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Lattice dynamics : From fundamental research to practical applications
- 2020
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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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| 6. |
- Trybel, Florian, et al.
(författare)
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Absence of proton tunneling during the hydrogen-bond symmetrization in delta-AlOOH
- 2021
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 104:10
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Tidskriftsartikel (refereegranskat)abstract
- delta-AlOOH is of significant crystallochemical interest due to a subtle structural transition near 10 GPa from a P2(1) nm to a Pnnm structure, the nature and origin of hydrogen disorder, the symmetrization of the O-H center dot center dot center dot O hydrogen bond and their interplay. We perform a series of density functional theory-based simulations in combination with high-pressure nuclear magnetic resonance (NMR) experiments on delta-AlOOH up to 40 GPa with the goal to better characterize the hydrogen potential and therefore the nature of hydrogen disorder. Simulations predict a phase transition in agreement with our NMR experiments at 10 - 11 GPa and hydrogen bond symmetrization at 14.7 GPa. Calculated hydrogen potentials do not show any double-well character and there is no evidence for proton tunneling in our NMR data.
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| 7. |
- Vlcek, Vojtech, et al.
(författare)
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Improved ground-state electronic structure and optical dielectric constants with a semilocal exchange functional
- 2015
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 91:3, s. 035107-
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Tidskriftsartikel (refereegranskat)abstract
- A recently published generalized gradient approximation functional within density functional theory (DFT) has shown, in a few paradigm tests, an improved KS orbital description over standard (semi) local approximations. The characteristic feature of this functional is an enhancement factor that diverges like s ln(s) for large reduced density gradients s which leads to unusual properties. We explore the improved orbital description of this functional more thoroughly by computing the electronic band structure, band gaps, and the optical dielectric constants in semiconductors, Mott insulators, and ionic crystals. Compared to standard semilocal functionals, we observe improvement in both the band gaps and the optical dielectric constants. In particular, the results are similar to those obtained with orbital functionals or by perturbation theory methods in that it opens band gaps in systems described as metallic by standard (semi) local density functionals, e. g., Ge, alpha-Sn, and CdO.
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