Structure and hydrogen bonding in ${\mathrm{CaSiD}}_{1+x}$: Issues about covalent bonding

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 $x>0$, D atoms start occupying a ${\mathrm{Ca}}_3\mathrm{Si}$ interstitial site. The corresponding $\mathrm{Si}\text{−}\mathrm{D}$ bond length is determined to be $1.82\mathrm{\AA }$, fully $0.24\mathrm{\AA }$ larger than predicted by theory. Thus, our neutron measurements are at odds with the strongly covalent $\mathrm{Si}\text{−}\mathrm{H}$ 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.

Figure 1

(Color online) Experimental (dots), calculated (line), and difference NPD profiles for (a) ${\mathrm{CaSiD}}_{1.2}$ and (b) $\mathrm{CaSiD}$. The low-angle portions of the diffraction patterns are also presented in insets. An additional small inset shows expanded-scale (200) peaks for (a) ${\mathrm{CaSiD}}_{1.2}$ and (b) $\mathrm{CaSiD}$ more clearly indicating the peak shift. Squares indicated the positions of some impurity peaks that are not related to the ${\mathrm{CaSiD}}_{1.2}$ and $\mathrm{CaSiD}$ phases. These impurity peaks were removed from the NPD patterns for clarity.

Figure 2

(Color online) Crystal structure of (a) ${\mathrm{CaSiD}}_{1.2}$ and (b) $\mathrm{CaSiD}$. The large black spheres, medium gray spheres, and small white spheres represent Ca, Si, and D atoms, respectively. ${\mathrm{Ca}}_4$-site tetrahedra are in dark gray; ${\mathrm{Ca}}_3\mathrm{Si}$-site tetrahedra in light gray.

Figure 3

(Color online) The Rietveld goodness-of-fit parameters ${\chi }^2$ and $R_{wp}$ vs the $\mathrm{Si}3$-D4 bond length fixed at various values along the $\mathrm{Si}3$-D4 axis as determined by the refined optimal position of D in the ${\mathrm{Ca}}_3\mathrm{Si}$ interstices (shown schematically).

Figure 4

(Color online) Neutron vibrational spectra of ${\mathrm{CaSiD}}_{1.2}$ and $\mathrm{CaSiD}$. Full width half maximum instrumental resolutions are depicted by the horizontal bars beneath the spectra. Calculated spectra for $\mathrm{CaSiD}$, ${\mathrm{CaSiD}}_{1.17}$, and ${\mathrm{CaSiD}}_{1.33}$ delineating both 1 phonon (dotted line) and 1+2 phonon (solid line) contributions are shown with the experimental data. The calculated splitting of D4 modes in ${\mathrm{CaSiD}}_{1.33}$ and the single mode for ${\mathrm{CaSiD}}_{1.17}$ (as discussed in the text) are indicated by arrows.