Formation of NaCl-Type Monodeuteride LaD by the Disproportionation Reaction of ${\mathrm{LaD}}_2$

Previous x-ray diffraction measurements revealed the pressure-induced decomposition of an fcc ${\mathrm{LaH}}_{2.3}$ into H-rich and H-poor fcc phases around 11 GPa. The present neutron diffraction measurements on ${\mathrm{LaD}}_2$ confirm the formation of NaCl-type LaD as a counterpart of the D-rich ${\mathrm{LaD}}_{2+\delta }$ by disproportionation. First-principles enthalpy and lattice dynamic calculations demonstrate that the NaCl-type LaH is stabilized at high pressures and can be recovered at ambient conditions. Finding the NaCl-type LaH will pave the way for investigations on the site-dependent nature of hydrogen-metal interactions.

Figure 1

Select NPD patterns measured for ${\mathrm{LaD}}_2$ at high pressures at room temperature. Several new peaks (denoted by arrows) appear when the pressure is increased to 11.5 GPa, indicating that fcc ${\mathrm{LaD}}_2$ decomposes into two phases. Vertical bars at the bottom show the locations of the diffraction peaks from the sintered diamond anvils.

Figure 2

(a) (top) SR-XRD, (middle) NPD profiles for the decomposed products of ${\mathrm{LaD}}_2$ at 13 GPa, and (bottom) calculated NPD profiles for ${\mathrm{LaD}}_{x<1}$. Intensities are on a logarithmic scale in arbitrary units. Solid lines at the bottom are the calculated NPD profiles for the NaCl-type structure of ${\mathrm{LaD}}_x$ with the D atoms partially ($x=0.6$, blue line) and fully ($x=1$, red line) occupying the $O$ sites of the fcc metal lattice. (b) NPD patterns at 13 and 15 GPa with black and blue solid lines, respectively.

Figure 3

Relative formation enthalpy $\Delta F$ of fcc ${\mathrm{LaH}}_x$ as a function of pressure. Insets show variations in the relative internal energy $\Delta E_0$ (lower left) and $\Delta (PV)$ (upper right) with pressure. Spacing of pressure data points is 1 GPa or denser, and the curve is a cubic-spline fit of the calculated data points.

Figure 4

Phonon dispersion curves and corresponding PDOS of (a) LaH at 15 GPa, (b) ${\mathrm{LaH}}_2$ at 4 GPa, and (c) ${\mathrm{LaH}}_3$ at 15 GPa. In all three panels, black solid lines show the stabilized phonon dispersion relations, and the corresponding shaded areas exhibit PDOS. Crystal structures are illustrated on the right side of each graph. Yellow, blue, and cyan spheres denote lanthanum, $T$-site, and $O$-site H atoms, respectively.