Heat capacity, enthalpy and entropy of bismuth niobate and bismuth tantalate

The heat capacity and the heat content of bismuth niobate BiNbO4 and bismuth tantalate BiTaO4 were measured by the relaxation method and Calvet-type heat flux calorimetry. The temperature dependencies of the heat capacities in the form Cpm=128.628+0.03340 T−1991055/T2+136273131/T3 (J K–1 mol–1) and 133.594+0.02539 T−2734386/T2+235597393/T3 (J K–1 mol–1) were derived for BiNbO4 and BiTaO4, respectively, by the least-squares method from the experimental data. Furthermore, the standard molar entropies at 298.15 K Sm(BiNbO4)=147.86 J K–1 mol–1 and Sm(BiTaO4)=149.11 J K–1 mol–1 were assessed from the low temperature heat capacity measurements. To complete a set of thermodynamic data of these mixed oxides an attempt was made to estimate the values of the heat of formation from the constituent binary oxides.