Heat capacity, enthalpy and entropy of ternary bismuth tantalum oxides

Heat capacity and enthalpy increments of ternary bismuth tantalum oxides Bi4Ta2O11, Bi7Ta3O18 and Bi3TaO7 were measured by the relaxation time method (2–280 K), DSC (265–353 K) and drop calorimetry (622–1322 K). Temperature dependencies of the molar heat capacity in the form Cpm=445.8+0.005451T−7.489×106/T2 J K−1 mol−1, Cpm=699.0+0.05276T−9.956×106/T2 J K−1 mol−1 and Cpm=251.6+0.06705T−3.237×106/T2 J K−1 mol−1 for Bi3TaO7, Bi4Ta2O11 and for Bi7Ta3O18, respectively, were derived by the least-squares method from the experimental data. The molar entropies at 298.15 K, S°m(298.15 K)=449.6±2.3 J K−1 mol−1 for Bi4Ta2O11, S°m(298.15 K)=743.0±3.8 J K−1 mol−1 for Bi7Ta3O18 and S°m(298.15 K)=304.3±1.6 J K−1 mol−1 for Bi3TaO7, were evaluated from the low-temperature heat capacity measurements.