Energetics of cobalt phosphate frameworks: α, β, and red NaCoPO₄

Thermal behavior, relative stability, and enthalpy of formation of α (pink phase), β (blue phase), and red NaCoPO₄ are studied by differential scanning calorimetry, X-ray diffraction, and high-temperature oxide melt drop solution calorimetry. Red NaCoPO₄ with cobalt in trigonal bipyramidal coordination is metastable, irreversibly changing to α NaCoPO₄ at 827 K with an enthalpy of phase transition of -17.4±6.9 kJ mol^-1. α NaCoPO₄ with cobalt in octahedral coordination is the most stable phase at room temperature. It undergoes a reversible phase transition to the β phase (cobalt in tetrahedra) at 1006 K with an enthalpy of phase transition of 17.6±1.3 kJ mol^-1. Enthalpy of formation from oxides of α, β, and red NaCoPO₄ are -349.7±2.3, -332.1±2.5, and -332.3±7.2 kJ mol^-1; standard enthalpy of formation of α, β, and red NaCoPO₄ are -1547.5±2.7, -1529.9±2.8, and -1530.0±7.3 kJ mol^-1, respectively. The more exothermic enthalpy of formation from oxides of β NaCoPO₄ compared to a structurally related aluminosilicate, NaAlSiO₄ nepheline, results from the stronger acid-base interaction of oxides in β NaCoPO₄ (Na₂O, CoO, P₂O₅) than in NaAlSiO₄ nepheline (Na₂O, Al₂O₃, SiO₂).