Thermodynamic properties of calcium antimonates from experiments and first principles

Calcium antimonates with fluorescence characteristic are potential candidates for lighting and display devices. Their thermodynamic properties were investigated using both experiments and first-principles calculations in this study. High-purity CaSb₂O₆ and Ca₂Sb₂O₇ were synthesized via solid-state sintering in air. Their structure Rietveld refinement, microstructure, and elemental valence states were conducted. Heat capacities at high temperature were precisely measured using a MHTC 96 Line calorimeter. The fitting equation are as follows: C_p (CaSb₂O₆) = 218.807 $+$ 3.370$\times$ 10⁻³T $-$ 4.770$\times$ 10⁶/T², C_p(Ca₂Sb₂O₇) = 265.610 $+$ 1.024$\times$ 10⁻²T $-$ 4.985$\times$ 10⁶/T², respectively. The enthalpies of formation (−1892.060 kJ mol⁻¹ and -2576.297 kJ mol⁻¹), entropies (163.045 J mol⁻¹ K⁻¹ and 218.973 J mol⁻¹ K⁻¹) at 298 K, and low temperature (0–298 K) heat capacities were obtained through first-principles calculations based on density functional theory (DFT) combined with phonon calculations. Furthermore, the thermal expansion coefficient of Ca₂Sb₂O₇ is significantly higher than that of CaSb₂O₆ during the heating process. Their Gibbs energy were also estimated, at 298 K, Ca₂Sb₂O₇ exhibits a negative value of −701.178 kJ mol⁻¹ relative to CaSb₂O₆.