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 R_p = 0.0313-0.0421, R_wp = 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.