Thermodynamics of solid solution formation in NiOMgO and NiOZnO

High-temperature calorimetric measurements of the enthalpies of solution in molten 2PbO · B₂O₃ of (Ni_xMg_1-x)O and (Ni_xZn_1-x)O permit the calculation of the enthalpy of the zincite to rocksalt transformation in ZnO, and the enthalpies of mixing, relative to rocksalt standard states, in the two solid solution series. The enthalpy of the zincite to rocksalt transformation is 24,488 ± 3,592 J mole^-1 with a corresponding positive entropy change of 0.48 ± 3.3 J K^-1 mole^-1. The small positive entropy change for the transformation necessitates a very flat and perhaps negative dP dT slope for the phase boundary. Both solid solutions, when referred to rocksalt standard states, show negative enthalpies of mixing. For (Ni_xMg_1-x)O the negative enthalpies of mixing are fitted by a subregular model, where ΔH_mix = X_AX_B(BX_A + AX_B), with A = -21,971 ± 4,953 J mole^-1 and B = -5103 ± 1151 J mole^-1. The associated negative excess entropies of mixing, calculated from the heats of mixing and previously measured activity-composition relations, are similarly modeled with A = -10.7 J K^-1 mole^-1 and B = + 1.1 J K^-1 mole^-1. Negative enthalpies of mixing in (Ni_xZn_1-x)O conform to a regular solution model with W = -13520 ± 5581 J mole^-1. The negative enthalpies of mixing are interpreted in terms of a tendency toward ordering in the solid solutions, the proposed ordering scheme finding support in spectroscopic, structural, and magnetic data. These tendencies toward order are used to explain observed phase relations and thermodynamic properties in some other systems containing a transition metal cation and another ion of similar size, namely carbonates, hydrated sulfates and the systems CuOMO (M = Mg, Co, Ni).