Phase relationships in the system Ni-W-O and thermodynamic properties of NiWO₄

Abstract

The phase diagram of the Ni-W-O system at 1200 K was established by metallographic and X-ray identification of the phases present after equilibration at controlled oxygen potentials. The oxygen partial pressures over the samples were fixed by metered streams of CO + CO₂ gas mixtures. There was only one ternary oxide, nickel tungstate (NiWO₄), in the Ni-W-O system at a total pressure of 1 atm, and this compound decomposed to a mixture of Ni + WO_2.72 on lowering the oxygen potential. The Gibbs' free energy of formation of NiWO₄ was determined from the measurement of the e.m.f, of the solid oxide galvanic cell, Pt, Ni + NiWO₄ + WO_2.72/CaO-ZrO₂/Ni + NiO, Pt and thermodynamic properties of tungsten and nickel oxides available in the literature. For the reaction, NiO(s) + WO₃(s) → NiWO₄(s) ΔG ° = - 10 500 - 0.708 T(±250) cal mo1^-1.