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

Abstract

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≤XNiO≤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 XNiO. Activity-composition relations were fitted to the Redlich-Kister formalism. There is a slight asymmetry (corresponding to a subregular model) across the solution with A0G=9577(±45) J/mol, and A1G=-477(±80) J/mol. The experimental data were also used to derive the parameters Vex, Hex and Sex. 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 XNiO + 2(1 - XNiO)2[11483 - 1.697 T] - 477(4 XNiO - 1)(900 < T(K) < 1300) where μO2(NNO)=-478967+248.514 T-9.7961 T In T, from O'Neill and Pownceby(1993a). Show more