Structure and hydrogen bonding inCaSiD1+x: Issues about covalent bonding

Abstract

We report here our high-resolution neutron powder diffraction and neutron vibrational spectroscopy study of ${\mathrm{CaSiD}}_{1+x}$ along with first-principles calculations, which reveal the deuterium structural arrangements and bonding in this novel alloy hydride. Both the structural and spectroscopic results show that, for $xg0$, D atoms start occupying a ${\mathrm{Ca}}_{3}\mathrm{Si}$ interstitial site. The corresponding $\mathrm{Si}\text{\ensuremath{-}}\mathrm{D}$ bond length is determined to be $1.82\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, fully $0.24\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ larger than predicted by theory. Thus, our neutron measurements are at odds with the strongly covalent $\mathrm{Si}\text{\ensuremath{-}}\mathrm{H}$ bonding in ${\mathrm{CaSiH}}_{1+x}$ that such calculations suggest, a result which may have implications for a number of ongoing studies of metal-hydrogen systems destabilized by Si alloying.