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

Phase equilibria involving the pseudobrookite-type solid solution seri es 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 fugacitie s in the range 10(-14)-10(-19) atm were obtained using H2/CO2 and H2/H 2O mixtures. Members of the solid solution have orthorhombic symmetry for x < 0.72 and monoclinic symmetry for x > 0.72. From Rietveld refin ements, the origin of the symmetry lowering was found to be Ti3+-Ti3bonding in the M2 sites. M2-M2 is constant at 2.92 angstrom in the ort horhombic 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] rat io has a maximum value of 1.705 near the orthorhombic to monoclinic tr ansition. Constrained site occupation refinements gave (Mg in M1)/SIGM AMg = 0.6 at x = 0 and increasing with increasing x to 1.0, correspond ing to full ordering, at x = 0.9. An alternative interpretation of the results was considered: the apparent Mg ordering obtained in the refi nements was compensating for ordering of cation vacancies in M1. On th is basis, and assuming that the Mg ordering had the same dependency on x as for the corresponding Fe1-xTi2+xO5 solid solution, site occupanc y refinements led to calculated values of [O]/[M] that agreed with the experimental values obtained from analyses. (C) 1994 Academic Press, Inc.