Subsolidus-Phase Equilibria in the System MgO-V2O5-MoO3

The phase relations in the system MgO-V2O5-MoO3 were studied and the subsolidus region of the phase diagram was constructed. The new compound Mg2.5VMoO8 was discovered on the join of MgMoO4-Mg3V2O8. Mg2.5VMoO8 crystallizes in the orthorhombic space group Pnma (No. 62) with cell dimensions a = 5.0515(1) Å, b = 10.3455(2) Å, c = 17.4683(4) Å, and Z = 6. The structure is a framework of MoO4 and VO4 tetrahedra with linking octahedral and trigonal prismatic MgO6 groups. The molybdenum and vanadium cations preserve their highest oxidation state, and the electrical neutrality of the crystalline lattice is maintained by partial occupancy of magnesium sites. Simple quasibinary equilibria were established between MgMoO4 and MoV2O8, V2-xMo2xO5, MgV2O6, and Mg2V2O7.