Thermodynamic stability of selected ASb2O6 and A2Sb2O7 phases (A = Ca, Ba, Cd, Sr, Zn)

Abstract

– Antimonates with the stoichiometry ASb2O6 and A2Sb2O7 have a wide range of applications in many areas of materials sciences and geosciences. In this work, we have derived thermodynamic properties of ASb2O6 (A = Ba, Ca, Sr, Zn, Na2, Cd) and A2Sb2O7, (A = Ca, Cd) with a combination of high-temperature oxide-melt calorimetry, relaxation calorimetry, differential scanning calorimetry (DSC), and density-functional theory (DFT) calculations. The samples were synthesized by solid-state techniques, characterized by powder X-ray diffraction, and found to belong to different structural types (rosiaite, trirutile, ilmenite, pyrochlore, and weberite). The Gibbs free energies of formation at T = 298.15 K from elements are (all data in kJ·mol–1) –1691.7±4.8 (BaSb2O6), –1710.8±4.7 (CaSb2O6), –1219.3±4.1 (CdSb2O6), –1674.4±4.9 (SrSb2O6), –1302.0±3.4 (ZnSb2O6), –1596.9±5.4 (Na2Sb2O6), –2407.6±8.0 (Ca2Sb2O7), –1497.6±7.5 (Cd2Sb2O7). The DFT calculations allowed to extrapolate the experimental heat capacity up to T = 1000 K and to calculate phase diagrams. They show that the syntheses of these compounds should be feasible either by hydrothermal treatment or solid-state reaction (using carbonates) with Na2Sb2O6 as a precursor at relatively low temperatures. The only exception seems to be the phase SrSb2O6 that is predicted to be unstable in a solid-state reaction involving Na2Sb2O6 and SrCO3.