Phase transitions and polymorphism in LaH3.0and LaD3.0at low temperatures

The heat capacities of La${\mathrm{H}}_{3.0}$ and La${\mathrm{D}}_{3.0}$ have been measured in the temperature range from 1.2 to 300 K. Four sharp heat-capacity anomalies were found in La${\mathrm{D}}_{3.0}$ at 211, 230.5, 233.5, and 274 K, indicating the existence of five phases. For La${\mathrm{H}}_{3.0}$ two broad heat-capacity peaks were observed at 241 and 270 K, indicating the existence of three phases. The lowest transition in both La${\mathrm{H}}_{3.0}$ and La${\mathrm{D}}_{3.0}$ is thought to be a semiconductor-to-metal transition on warming, and the highest transition in La${\mathrm{D}}_{3.0}$ to be a transition in which the deuterium atoms change locations from an off-centered position to the center of the tetrahedral site on warming. The intermediate transitions in La${\mathrm{D}}_{3.0}$ and the uppertransition in La${\mathrm{H}}_{3.0}$ are probably transformations involving the rearrangement of the deuterium or hydrogen atoms, respectively, in different off-centered configurations. The thermodynamic functions for La${\mathrm{H}}_{3.0}$ and La${\mathrm{D}}_{3.0}$, respectively, are found to be as follows: $S_{298.15}^0=13.94\mathrm{} \mathrm{and} 16.58$ J/g-at. K, $\frac{(H_{298.15}^0-H_0^0)}{T}=7.54\mathrm{} \mathrm{and} 9.36$ J/g-at.K. The electronic specific-heat constant $\gamma$ and Debye temperature ${\Theta }_D$ for La${\mathrm{H}}_{3.0}$ and La${\mathrm{D}}_{3.0}$, respectively, are $\gamma =0\mathrm{}\pm 0.04 \mathrm{and} 0\pm 0.06$ mJ/g-at. K, ${\Theta }_D=241.5\mathrm{}\pm 0.8 \mathrm{and} 246.3\pm 0.2$ K. The isotope effect in the heat capacity can be approximately accounted for by a shift of $\frac{1}{\sqrt{2}}$ in the frequency of the hydrogen optic modes in both octahedral and tetrahedral sites of La${\mathrm{D}}_{3.0}$ relative to La${\mathrm{H}}_{3.0}$.