Phase relationships in the system Cr-W-O and thermodynamic properties of CrWO4 and Cr2WO6

The phase diagram of the Cr-W-O system at 1000° C was established by metallographic and X-ray identification of the phases present after equilibration in evacuated silica capsules. Two ternary oxide phases, CrWO₄ and Cr₂WO₆ were detected. The oxygen potential over the three-phase mixtures, W+Cr₂O₃ s+CrWO₄, WO_2.90+CrWO₄+Cr₂WO₆ and Cr₂O₃+CrWO₄+Cr₂WO₆, were measured by solid state cells incorporating Y₂O₃ stabilized ZrO₂ electrolyte and Ni+NiO reference electrode. The Gibbs' energies of formation of the two ternary phases can be represented by the following equations $$begin{gathered} W(s) + tfrac{1}{2} Cr_2 O_3 (s) + tfrac{5}{4} O_2 (g) to CrWO_4 (s) hfill Delta G^0 = - 172 047 + 48.725T ( pm 230) cal mol^{ - 1} hfill Cr_2 O_3 (s) + WO_3 (s) to Cr_2 WO_6 (s) hfill Delta G^0 = - 3 835 + 0.235{rm T} ( pm 500) cal mol^{ - 1} hfill end{gathered}$$