Heat capacity, enthalpy and entropy of strontium niobate Sr2Nb2O7 and calcium niobate Ca2Nb2O7

The heat capacity and the enthalpy increments of strontium niobate Sr2Nb2O7 and calcium niobate Ca2Nb2O7 were measured by the relaxation time method (2–300 K), DSC (260–360 K) and drop calorimetry (720–1370 K). Temperature dependencies of the molar heat capacity in the form Cpm = 248.0 + 0.04350T − 3.948 × 106/T2 J K−1 mol−1 for Sr2Nb2O7 and Cpm = 257.2 + 0.03621T − 4.434 × 106/T2 J K−1 mol−1 for Ca2Nb2O7 were derived by the least-square method from the experimental data. The molar entropies at 298.15 K, Sm°Sm°(298.15 K) = 238.5 ± 1.3 J K−1 mol−1 for Sr2Nb2O7 and Sm°Sm°(298.15 K) = 212.4 ± 1.2 J K−1 mol−1 for Ca2Nb2O7, were evaluated from the low-temperature heat capacity measurements.