Heat capacity, enthalpy and entropy of calcium niobates

Abstract

Heat capacity and enthalpy increments of calcium niobates CaNb2O6 and Ca2Nb2O7 were measured by the relaxation time method (2–300 K), DSC (260–360 K) and drop calorimetry (669–1421 K). Temperature dependencies of the molar heat capacity in the form Cpm=200.4+0.03432T−3.450·106/T2 J K−1 mol−1 for CaNb2O6 and Cpm=257.2+0.03621T−4.435·106/T2 J K−1 mol−1 for Ca2Nb2O7 were derived by the least-squares method from the experimental data. The molar entropies at 298.15 K, Sm0(CaNb2O6, 298.15 K)=167.3±0.9 J K−1 mol−1 and Sm0(Ca2Nb2O7, 298.15 K)=212.4±1.2 J K−1 mol−1, were evaluated from the low temperature heat capacity measurements. Standard enthalpies of formation at 298.15 K were derived using published values of Gibbs energy of formation and presented heat capacity and entropy data: ΔfH0(CaNb2O6, 298.15 K)= −2664.52 kJ molt-1 and ΔfH0(Ca2Nb2O7, 298.15 K)= −3346.91 kJ mol−1.