SYNTHESIS, CRYSTAL-GROWTH AND CRYSTAL-STRUCTURE OF A SCHEELITE TYPE COMPOUND IN LANTHANUM-NIOBIUM-VANADIUM-OXYGEN SYSTEM

Synthesis and crystal growth of La(Nb0.70V0.30)O-4 with the scheelite type (CaWO4) were performed by direct interaction of La2O3, Nb2O5, V2O5 oxides at 1640 degrees C followed by a slow cooling down to the room temperature. La(Nb0.70V0.30)04 crystallizes in the tetragonal system, space group I4(1)/a, with a = 5.3283(4)Angstrom A, c = 11.717(2)Angstrom A, V = 332.66(5)Angstrom A(3), Z = 4. The crystal structure has been refined to R = 0.031 using 324 independant hkl reflections and 15 variable parameters. This structure, isomorphous with LaNbO4 high temperature form, exhibits statistical distribution of Nb5+ and V5+ in the tetrahedron of oxygens part of a stella quadrangula polyhedron. The La atom occupies a site coordinated to 8 oxygen atoms forming a bisdisphenoid. The substitution of vanadium to niobium in LaNbO4 stabilizes this high temperature form with the unusual tetrahedral coordination for niobium 5+.