CRYSTAL-STRUCTURE REFINEMENTS OF ZIRCON-TYPE MVO4 (M=SC, Y, CE, PR, ND, TB, HO, ER, TM, YB, LU)

Improved structural parameters for the lanthanide orthovanadates (including also YVO4 and ScVO4 and new results for PrVO4, TmVO4, and the LuVO4 end members) have been determined by employing a full-profile Rietveld structure analysis of neutron powder-diffraction data. High-quality powders of the zircon-type MV O4 vanadates were prepared by a homogeneous coprecipitation in molten urea. The zircon structure has space group symmetry I 41/amd, Z = 4, with 3 atoms occupying the asymmetric unit. In all, 22 parameters were refined with final agreement values of Rp = 0.0313-0.0421, Rwp = 0.0361-0.0511, and G-of-F = 1.210-1.962. The average V-O bond length for all of the MVO4 samples is 1.709(2) Å, and the V-O distance exhibits a small systematic shortening with decreasing M atom size. The oxygen positional parameters, the cell dimensions, and the M-O distances vary systematically with the metal-ion atomic number. The anisotropic thermal motion of the oxygen atom has the largest amplitude normal to the shared polyhedral edge between the VO4 tetrahedron and MO8 bisdisphenoid. Empirical relations are given for the cell dimensions and oxygen atom position as a function of the size of the lanthanide ion which can be used to estimate structural parameters for other zircon-type lanthanide vanadate end-members and vanadate solid solutions. © 1994 Academic Press, Inc.