Thermochemistry of holmium bismuthides

Thermodynamic properties of Ho–Bi intermediate phases (HoBi and Ho5Bi3) have been studied by means of calorimetric and tensimetric techniques. The heats of formation at T=298 K were determined by high temperature direct synthesis calorimetry and the molar heat capacity of HoBi was measured by differential scanning calorimetry (DSC) in the temperature range 330–920 K. The equilibrium vapour pressures over the two-phase region HoBi+Ho5Bi3 were measured by Knudsen Effusion–Mass Spectrometry in the temperature range 1357–1631 K. These data were analyzed by the second-law method to obtain the enthalpy changes for the atomization processes and, thereafter, the heats of formation for the two compounds. From the calorimetric measurements the heats of formation at 298 K were determined to be −93±3 and −112±4 kJ/mol atoms, respectively for the Ho5Bi3 and HoBi compounds, consistent with ΔfH298°Ho5Bi3=−91.8±4.0 and ΔfH298°HoBi=−110.3±3.6 obtained from the tensimetric measurements.