PHASE-EQUILIBRIA AND STRUCTURAL STUDIES ON THE SOLID-SOLUTION MGTI2O5-TI3O5

Phase equilibria involving the pseudobrookite-type solid solution series Mg1-xTi2+xO5, 0.1 < x < 1, were determined as a function of oxygen fugacity at 1473 K. The quenching method was used and oxygen fugacities in the range 10(-14)-10(-19) atm were obtained using H2/CO2 and H2/H2O mixtures. Members of the solid solution have orthorhombic symmetry for x < 0.72 and monoclinic symmetry for x > 0.72. From Rietveld refinements, the origin of the symmetry lowering was found to be Ti3+-Ti3+ bonding in the M2 sites. M2-M2 is constant at 2.92 angstrom in the orthorhombic phases and decreases monotonically in the monoclinic phases to a value of 2.74 angstrom at x = 1. The solid solution members were found to be nonstoichiometric with the oxygen to metal atomic ratio, [O]/[M], higher than the stoichiometric value of 1.667. The [O]/[M] ratio has a maximum value of 1.705 near the orthorhombic to monoclinic transition. Constrained site occupation refinements gave (Mg in M1)/SIGMAMg = 0.6 at x = 0 and increasing with increasing x to 1.0, corresponding to full ordering, at x = 0.9. An alternative interpretation of the results was considered: the apparent Mg ordering obtained in the refinements was compensating for ordering of cation vacancies in M1. On this basis, and assuming that the Mg ordering had the same dependency on x as for the corresponding Fe1-xTi2+xO5 solid solution, site occupancy refinements led to calculated values of [O]/[M] that agreed with the experimental values obtained from analyses. (C) 1994 Academic Press, Inc.