Thermodynamic data from redox reactions at high temperatures. V. Thermodynamic properties of NiO-MnO solid solutions from emf measurements

Divariant oxide plus metal assemblages potentially make useful redox sensors for use in hydrothermal and other high pressure experiments. Here we report the calibration of the (Ni, Mn)O/Ni redox sensor in which the Ni/NiO (NNO) oxygen buffer is displaced to lower oxygen chemical potentials (μO₂), by the solid solution of MnO in the oxide phase. This assemblage was chosen because: (1) it covers a useful range of μO₂; (2) the system can be calibrated very accurately. Values of μO₂ defined by the (Ni, Mn)O/Ni assemblage were determined electrochemically, from 900 to 1300 K, using calcia-stabilized zirconia solid electrolytes. The oxide compositions (8 in total, ranging from 0.1≤X_NiO≤0.8) were analysed afterwards by electron microprobe, and were checked for internal consistency by measuring the lattice parameters (a₀), using powder XRD. The accuracies of the measurements, both assessed theoretically and established empirically, are (1σ): ±80J/mol in μO₂, ±0.0002 Å in a₀ and ±0.002 to 0.005 in X_NiO. Activity-composition relations were fitted to the Redlich-Kister formalism. There is a slight asymmetry (corresponding to a subregular model) across the solution with A₀^G=9577(±45) J/mol, and A₁^G=-477(±80) J/mol. The experimental data were also used to derive the parameters V^ex, H^ex and S^ex. There is no obvious relationship between excess volumes and enthalpies of mixing, nor between excess volumes and excess entropies. The experimental data from this study have been used to formulate the (Ni, Mn)O/Ni redox sensor expression: μO₂ = μ₂(NNO) + 2 RTln X_NiO + 2(1 - X_NiO)²[11483 - 1.697 T] - 477(4 X_NiO - 1)(900 < T(K) < 1300) where μO₂(NNO)=-478967+248.514 T-9.7961 T In T, from O'Neill and Pownceby(1993a).