Heat capacities ofLaDxandLaHx(1.9≤x≤3.0) from 1 to 300 K

The heat capacities of lanthanum hydrides in the ${\mathrm{LaD}}_{\mathrm{x}}$ and ${\mathrm{LaH}}_{\mathrm{x}}$ (1.9≤x≤3.0) series have been measured over the temperature range from 1.2 to 300 K. The electronic specific-heat coefficients showed nearly the same variation for both ${\mathrm{LaD}}_{\mathrm{x}}$ and ${\mathrm{LaH}}_{\mathrm{x}}$ over the entire composition range, having the values of γ=0.81±0.01 and 0.038±0.01 mJ/g-at. ${\mathrm{K}}^2$ at the stoichiometric compositions of ${\mathrm{LaD}}_2$ (${\mathrm{LaH}}_2$) and ${\mathrm{LaD}}_3$ (${\mathrm{LaH}}_3$), respectively. Some different variations were found in the composition dependence of the Debye temperatures of the ${\mathrm{LaD}}_{\mathrm{x}}$ and ${\mathrm{LaH}}_{\mathrm{x}}$ series, but, nevertheless, the same values of ${\mathrm{FTHETA}}_D$=348±2 and 381±2 K were found at the compositions of ${\mathrm{LaD}}_2$ (${\mathrm{LaH}}_2$) and ${\mathrm{LaD}}_3$ (${\mathrm{LaH}}_3$), respectively. The variations of γ and ${\mathrm{FTHETA}}_D$ do not show a monotonic behavior but depend on the formation of the three hydrogen-ordering phases, namely, LaD(H${)}_{2.25}$, LaD(H${)}_{2.5}$, and LaD(H${)}_{2.75}$. One sharp heat-capacity peak was found in ${\mathrm{LaD}}_{\mathrm{x}}$ between x=2.65 and 2.90, associated with octahedral hydrogen ordering centered at ${\mathrm{LaD}}_{2.75}$, whereas four sharp peaks were found in ${\mathrm{LaD}}_{2.95}$ at 215.5, 265.5, 286.8, and 293.8 K and were correlated with the four transitions observed in ${\mathrm{LaD}}_3$ by Ito et al. The metal-to-semiconductor transition, which is found in the alloys between 2.75≤x≤3.0, is discussed on the basis of the electronic specific-heat coefficient and recent theoretical work. We also report that ${\mathrm{LaH}}_{2.01}$ becomes a superconductor at 0.17 K and, if ${\mathrm{LaD}}_{2.06}$ goes superconducting, it does so below 0.06 K.