Search for metal hydrides with short hydrogen–hydrogen separation: Ab initiocalculations

Abstract

The present investigation is a part of a series on metal hydrides with extraordinary short $\mathrm{H}\mathrm{H}$ separations. The electronic structure, chemical bonding, and ground state properties of $RT\mathrm{In}$ $(R=\mathrm{La},\phantom{\rule{0.3em}{0ex}}\mathrm{Ce},\phantom{\rule{0.3em}{0ex}}\mathrm{Pr},\phantom{\rule{0.3em}{0ex}}\mathrm{Nd};\phantom{\rule{0.3em}{0ex}}T=\mathrm{Ni},\phantom{\rule{0.3em}{0ex}}\mathrm{Pd},\phantom{\rule{0.3em}{0ex}}\mathrm{Pt})$ and their saturated hydrides ${R}_{3}{T}_{3}{\mathrm{In}}_{3}{\mathrm{H}}_{4}$ $(=3RT\mathrm{In}{\mathrm{H}}_{1.333})$ are systematically studied using the full-potential linear muffin-tin-orbital method. The effect of the metal matrix on the $\mathrm{H}\mathrm{H}$ separation in $RT\mathrm{In}{\mathrm{H}}_{1.333}$ is analyzed in terms of chemical bonding, and bond strength is quantitatively analyzed using the crystal-orbital-Hamilton population. Force and volume optimizations reveal that all these hydrides violate the ``$2\text{\ensuremath{-}}\mathrm{\AA{}}$ rule.'' The insertion of hydrogen in the metal matrix causes highly anisotropic lattice changes; a large expansion along $c$ and a small contraction in the $a$ direction. Among the 12 studied hydrides the hypothetical $\mathrm{La}\mathrm{Pt}\mathrm{In}{\mathrm{H}}_{1.333}$ phase exhibits the shortest $\mathrm{H}\mathrm{H}$ separation $(1.454\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})$. The optimized unit-cell parameters and atomic coordinates fit very well with the experimental findings for $R\mathrm{Ni}\mathrm{In}{\mathrm{H}}_{1.333}$, $R=\mathrm{La}$, Ce, and Nd. Examination of the effect of the metal matrix on the $\mathrm{H}\mathrm{H}$ separation in $RT\mathrm{In}{\mathrm{H}}_{1.333}$ suggests that on a proper choice of alloying element one may be able to reduce the $\mathrm{H}\mathrm{H}$ separation below $1.45\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. The $\mathrm{H}\mathrm{H}$ separation is reduced significantly by application of pressure.