Heat Capacities and Thermodynamic Functions of YH2 and YD2 from 5° to 350°K and the Hydrogen Vibration Frequencies

The heat capacities of pure YH2 and YD2 were measured by an adiabatic method from 5° to 350°K. The compounds were found by x-ray analyses to consist of a single phase, with the yttrium atoms in face-centered cubic positions. Below 16°K the heat capacities of the two compounds are equal within experimental error, between 16° and 90°K the heat capacity of the hydride is about 0.02 cal deg—1 mole—1 greater than that of the deuteride, and at higher temperatures that of the deuteride becomes appreciably greater. The difference in the heat capacities above 100°K is ascribed to a single optical vibration of the hydrogen atoms, whose frequency is shifted upon isotopic substitution. The difference in the heat capacities of YH2 and YD2 in the temperature range 100° to 350°K is fitted to ±0.02 cal deg—1 mole—1 with Einstein heat capacity functions and a frequency of 1030±30 cm—1 for YH2 and 1030/√2 for YD2. These results are compared with those obtained by inelastic neutron scattering experiments. The values obtained for the heat capacity, entropy, enthalpy and Gibbs free-energy function for YH2 at 298.15°K are 8.243±0.016 cal deg—1 mole—1, 9.175±0.018 cal deg—1 mole—1, 1403±3 cal mole—1 and —4.470±0.009 cal deg—1 mole—1, respectively. The corresponding values for YD2 are 10.773±0.022, 10.294±0.021, 1659±3 and —4.729±0.010. The enthalpy of formation of YH2 minus the enthalpy of formation of YD2 is calculated to be 0.78±0.08 kcal mole—1 at 0°K and 0.55±0.08 kcal mole—1 at 298.15°K.