Thermodynamics of Sr2NiMoO6 and Sr2CoMoO6 and their stability under reducing conditions

Abstract

The values of standard enthalpy of formation at 298.15 K and 1 atm for the double perovskites Sr₂NiMoO₆ and Sr₂CoMoO₆, measured by means of drop solution calorimetry, were found to be −2418.1 ± 12.4 and −2422.9 ± 9.6 kJ mol⁻¹, respectively. Heat capacity of Sr₂NiMoO₆ and Sr₂CoMoO₆ was measured between 2 and 370 K using relaxation and adiabatic calorimetry, and the enthalpy increments – between 373 and 1273 K using drop calorimetry. Low-temperature magnetic and higher-temperature structural phase transformations in Sr₂NiMoO₆ and Sr₂CoMoO₆ were discussed from the thermodynamic point of view. Specific heat (C_p), standard enthalpy (Δ^T₀H⁰) and standard entropy (S⁰) functions were derived from the experimental data for both double perovskites. The values of C_p, Δ^T₀H⁰ and S⁰ at 298.15 K were determined to be 202.31 ± 0.61 J mol⁻¹ K⁻¹, 36.12 ± 0.18 kJ mol⁻¹ and 231.3 ± 1.6 J mol⁻¹ K⁻¹ for Sr₂NiMoO₆, and 212.66 ± 0.64 J mol⁻¹ K⁻¹, 38.25 ± 0.19 kJ mol⁻¹ and 244.4 ± 1.7 J mol⁻¹ K⁻¹ for Sr₂CoMoO₆, respectively. Additionally, using the thermodynamic data available, phase diagrams with respect to T and p_O2 showing stability limits and decomposition products were calculated for Sr₂NiMoO₆ and Sr₂CoMoO₆. Though the cobaltite's stability range is wider than nickelate's both in terms of T and p_O2, both complex oxides were found to be stable only at reasonably high temperatures and in oxidizing conditions, and metastable at low temperatures.