Contributions on Thermal Behaviour and Crystal Chemistry of Anhydrous Phosphates. XXXIV [1] Oxygen Equilibrium Pressures in Ternary Systems M / P / O (M = Co,Ni) and Heats of Formation of Anhydrous Cobalt(II) and Nickel(II) Phosphates

Oxygen equilibrium pressures have been measured in the temperature range 800 °C to 1000 °C by coulometric/potentiometric techniques for several equilibrium regions in the ternary systems M / P / O (M = Co, Ni). In both systems oxygen coexistence pressures of three‐phase equilibrium solids phosphide/phosphate are about 3 to 5 orders of magnitude smaller than p(O₂) above the corresponding M_s / MO_s system. Heats of formation Δ_fH°₂₉₈ and standard entropies S°₂₉₈ for the phosphates have been obtained from 2^nd and 3^rd law evaluation of the temperature dependence of the oxygen coexistence pressures. Thermodynamic data from literature for the phosphides of cobalt and nickel and estimated heat capacities for the anhydrous phosphates Co₃(PO₄)₂, Co₂P₂O₇, Ni₃(PO₄)₂, Ni₂P₂O₇ and Ni₂P₄O₁₂ were used for these calculations. Thus obtained enthalpies and entropies are compared to results from thermodynamic modelling of observed solid phase equilibria in the ternary systems M / P / O (M = Co, Ni).