First-principles study on thermal vibration effects ofMgH2

Abstract

We report the results of the first-principles calculation on the atomic displacement parameters, the crystal structure factors, and the charge density distributions of $\mathrm{Mg}{\mathrm{H}}_{2}$ at finite temperature. Our calculations have been performed with the ultrasoft pseudopotential method and the linear response approach based on the density-functional perturbation theory (DFPT). The thermal vibration effects have been taken into consideration using phonon dispersions predicted by the DFPT and the harmonic approximation. To obtain the crystal structure factors, the pseudo charge density has been converted to the all electron one by utilizing the projector augmented wave formalism. The computed atomic displacement parameters of the hydrogen atom in $\mathrm{Mg}{\mathrm{H}}_{2}$ are in good agreement with the inelastic neutron data. The crystal structure factors at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ predicted from the atomic form factors and the Debye-Waller factors have been compared with those obtained from the synchrotron x-ray diffraction measurement. They are in good agreement for lower diffraction vector indices $hkl$, but there are slightly differences in higher $hkl$. We also discuss the temperature dependence of the charge density distribution. It is confirmed that thermal vibration is more effective to the H atom than the Mg atom. The ionic charge which is estimated from the number of electrons within the sphere around the H atom is well in agreement with the experimental result by the maximum entropy method.