Dft clearly discriminates between the mg2 nih 4 lti

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DFT clearly discriminates between the Mg 2 NiH 4 LTI and LTII structures: LTII is the preferred structure at both electronic and vibrational levels of analysis Excellent approximate structure can be derived from pursuit of soft modes in Mg-substituted CaMgNiH 4 -type structure PW91 GGA provides most accurate results Density Functional Theory: Structure Determination and Discrimination for Mg 2 NiH 4 *
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Thermophysical Properties: C p (T) (J/K mol) and ij (T) (1/K) Required properties for a wide range of applications, e.g. fuel cell and battery systems, constitutive models of structural alloys, thermometers, building, ship, aircraft construction, microelectronics fabrication, … Einstein (1907)* got the ball rolling on a QM description of heat capacity: all atoms vibrate as independent (non-interacting) quantum harmonic oscillators with frequency E Debye (1912)** theory of heat capacity: heat (atomic vibrations) due to phonons (or collective vibrations or sound waves) in a box (breaks down at high T due to anharmonicity): Modern theory based upon lattice dynamics gives the isochoric heat capacity, C v (T), but experimentalists always measure C p (T) Use quasi-harmonic approach with VASP/DFT/phonon to compute C p (T) and ij (T) Neglect electron-phonon many body enhancement factor (1.4 to 2.5) We have critically evaluated LDA, PBE, PBEsol functionals for selected metals, insulators, and semiconductors: most continuum mech. theories ignore T-dependence
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Thermophysical Properties: C p (T) and ij (T) The quasi-harmonic approximation: anharmonic effects are included via the volume dependence of phonon frequencies ! Requires lots of VASP calculations! elec elec phonon ZPE mag p V V o elec vib V V V elec F V B B vib V V P F(V,T) U V F V,T E V,T E V F C T C T V,T TB V,T V T where C T C T C T and f ,T C T k T n d k T and C T is taken from phonons V(T) T V (T) T         2 2 2 0 1 L V P For cubic systems V(T) T T V (T) T Note that for hexagonal, tetragonal, and orthorhombic systems, there are three components of the thermal expansion coefficient tensor \ that must be compu 0 1 1 3 3 ted!
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Thermophysical Properties: C p (T) and ij (T) Tungsten, Metallic, Cubic, T m ~ 3860 K Results from PBEsol C p (T) Thermal expansion [1] [2] present present A. Debernardi et al., Phys. Rev. B 63 (2001) 064305. A.P. Miiller, A. Cezairliyan, Int. J. Thermophysics 11 (1990) 619-628. G.K. White, M.L. Mingers, Int. J. Thermophysics 15 (1994) 1333-1343 [1] [3]
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Thermophysical Properties: C p (T) and ij (T) LiC 6 : Li-Ion Battery Anode Material, Metallic, Hexagonal, T m ~ 700 K Results from PBEsol (a,c, are lattice parameters, V is volume) Contributions to the C p (T) (no experimental data available) C p C v C v (elec) C p -C v data pt.
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  • VASP, density functional theory

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