Low-temperature heat capacities of yttrium, lanthanum, and lutetium sesquisulfides

The heat capacities of $\delta -{\mathrm{YS}}_{1.491}$, $\alpha -{\mathrm{La}}_2{\mathrm{S}}_3$, $\beta -{\mathrm{La}}_2{\mathrm{S}}_3$, and $\epsilon -{\mathrm{Lu}}_2{\mathrm{S}}_3$ were measured from 1.2 to 20 K and that of $\delta -{\mathrm{YS}}_{1.497}$ from 0.3 to 20 K. The heat capacities of the two ${\mathrm{La}}_2$ ${\mathrm{S}}_3$ samples and the ${\mathrm{Lu}}_2$ ${\mathrm{S}}_3$ sample exhibited only a $T^3$ dependence, i.e., the electronic specific constant $\gamma$ was found to be zero within ±0.02 mJ/g-at.${\mathrm{K}}^2$. This indicates that these materials are insulators or semiconductors. The temperature dependence of the heat capacity of the two substoichiometric $\delta -{\mathrm{YS}}_x$ samples exhibited both $T$ and $T^3$ dependences. The small $\gamma$ values indicated that some conduction electrons were present, and that the stoichiometric $\delta -{\mathrm{Y}}_2{\mathrm{S}}_3$ would be a semiconductor or insulator.