Hydrogen Bonding in CaSiH(D)$_{1+x}$: Is there Covalent Character?

We report here our neutron powder diffraction and neutron vibrational spectroscopy study of CaSiH(D)$_{1+x}$ along with first-principles calculations, which reveal the hydrogen structural arrangements and bonding in this novel alloy hydride. Both structural and spectroscopic results show that, for $x>$0, H(D) atoms start occupying a Ca$_{3}$Si interstitial site. The corresponding Si-H(D) bond length is determined to be 1.82~{\AA}, fully 0.24~{\AA} larger than predicted by theory. Here we discuss in detail our neutron spectroscopic measurements, which are also generally at odds with strongly covalent Si-H bonding in CaSiH$_{1+x}$ that such calculations suggest. These results may have implications for a number of ongoing studies of metal-hydrogen systems destabilized by Si alloying.