Chemical-bonding and high-pressure studies on hydrogen-storage materials

From gradient-corrected, all-electron, full-potential, density-functional calculations, including structural relaxation, it is shown that the metal-hydride series RTInH1.333 (R=La, Ce, Pr, or Nd; T=Ni, Pd, or Pt) violate the “2-Å” rule as well as the hole-size requirement. These hydrides possess unusually short H–H separations which in the most extreme case for LaPtInH1.333 is as short as 1.454 Å. These findings have been analyzed in terms of charge density, charge transfer, electron-localization function, crystal-orbital Hamilton population, and density of states analyses. From high-pressure studies it is predicted several successive pressure-induced structural transitions in MgH2 within the 20 GPa range. Calculations have also shown several pressure-induced structural transitions in alkali aluminum tetrahydrides with large volume reductions at the phase-transition points and small energy differences between the ambient-pressure and subsequent high-pressure phases.