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

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+WO2.72 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+WO2.72/CaO-ZrO2/Ni+NiO, Pt and thermodynamic properties of tungsten and nickel oxides available in the literature. For the reaction, NiO(s)+WO3(s)rarrNiWO4(s) DeltaG°=–10500–0.708 T (±250) cal mol–1.