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.