HIGH PRESSURE SYNTHESIS AND POLYMORPHISM OF COMPLEX OXIDES MN2BSBO6(B=FE,V,CR,GA,AL)

摘要

Conditions for formation of a synthetic analog of the natural mineral melanostibite with the composition Mn2FeSbO6 which crystallizes in the structural type of ilmenite and of some other compounds belonging to the series Mn2BSbO6, B = Cr, V, Ga, Al, have been found. It has been established that none of these compounds is formed from simple oxides when their mixtures are sintered in air at 900-1320 degrees C. The products of synthesis under these conditions were binary oxides MnSb2O6, Mn2Sb2O7, and spinel phases of the MnB2O4 composition. The compound Mn2FeSbO6 with the structure of ilmenite has been synthesized under a pressure of 3 GPa and temperature 900 degrees C. The lattice parameters of this phase (a = 5,238(1), c = 14,344(3) Angstrom, hexagonal unit cell) approximate those determined for the natural mineral. A phase transition from the ilmenite to tire perovskite modification was registered for Mn2FeSbO6 at 5 GPa and 900 degrees C. The perovskite phase in the pure form has been obtained at 6 GPa and 900 degrees C. The orthorhombic unit cell with a = 5.394(2), b = 5,246(1), c = 7,652(3) Angstrom and ordered distribution of Fe3+ and Sb5+ cations in octahedral sites of the perovskite structure was established for this phase. The other compounds from this series also form ilmenite and perovskite structures under high pressures. In the process of high pressure synthesis it was also found that all the investigated systems contain phases with the cubic pyrochlore structure, the composition of which was not identified. (C) 1996 Academic Press, Inc. [References: 12]