Heat capacities and thermodynamic properties of antimony substituted lanthanum orthoniobates

The results of heat capacity measurements for the lanthanum orthoniobate substituted with 10, 20 and 30 mol% of antimony (LaNb0.9Sb0.1O4, LaNb0.8Sb0.2O4 and LaNb0.7Sb0.3O4) are presented and discussed. Temperature dependence of low temperature heat capacity was analyzed within the Debye and Einstein models. The Debye temperature decreased, whereas the Einstein temperature increased with antimony content. The decrease of the Debye temperature with increasing antimony content was correlated with decreasing scheelite-fergusonite transition temperature. The increase of the Einstein temperature of LaSbxNb1−xO4 with increasing antimony content may indicate increasing frequency of optical vibrations of Nb(Sb)-O4−2 polyhedra relative to La3+ cations. Using the heat capacity data, standard entropies of the phases were calculated and combined with previously measured enthalpies of formation to obtain Gibbs energies of formation. Standard thermodynamic properties were tabulated.