Energetics of formation of alkali and ammonium cobalt and zinc phosphate frameworks

Abstract

Alkali and ammonium cobalt and zinc phosphates show extensive polymorphism. Thermal behavior, relative stabilities, and enthalpies of formation of KCoPO 4, RbCoPO 4, NH 4CoPO 4, and NH 4ZnPO 4 polymorphs are studied by differential scanning calorimetry, high-temperature oxide melt solution calorimetry, and acid solution calorimetry. α-KCoPO 4 and γ-KCoPO 4 are very similar in enthalpy. γ-KCoPO 4 slowly transforms to α-KCoPO 4 near 673 K. The high-temperature phase, β-KCoPO 4, is 5–7 kJ mol −1 higher in enthalpy than α-KCoPO 4 and γ-KCoPO 4. HEX phases of NH 4CoPO 4 and NH 4ZnPO 4 are about 3 kJ mol −1 lower in enthalpy than the corresponding ABW phases. There is a strong relationship between enthalpy of formation from oxides and acid–base interaction for cobalt and zinc phosphates and also for aluminosilicates with related frameworks. Cobalt and zinc phosphates exhibit similar trends in enthalpies of formation from oxides as aluminosilicates, but their enthalpies of formation from oxides are more exothermic because of their stronger acid–base interactions. Enthalpies of formation from ammonia and oxides of NH 4CoPO 4 and NH 4ZnPO 4 are similar, reflecting the similar basicity of CoO and ZnO.