Preparation and structural study from neutron diffraction data of R2MoO6 (R= Dy, Ho, Er, Tm, Yb, Y)

The title compounds have been prepared as polycrystalline powders by thermal treatments of stoichiometric mixtures of R2O3 and MoO3 in air. The room-temperature crystal structure for all the series has been refined from high-resolution neutron powder diffraction data. All the phases are isostructural (space group C2/c, Z = 8) with the polymorph alpha-R2MoO6, typified by Sm2MoO6. The structure contains four zigzag, one-dimensional MoO5 polyhedral rows per unit cell, running through the RO8 polyhedral framework along the [001] direction. MoO5 form discrete units (i.e. do not share common oxygen), with Mo-O distances ranging from 1.77 to 2.24 Angstrom, although the oxygen coordination can be extended to distances of about 3.1 Angstrom, giving rise to strongly distorted MoO8 scalenohedra. Thus, MoO8 and RO8 polyhedra are fully ordered in R2MoO6 compounds, which in fact can be considered as superstructures of fluorite (M3O6), containing 24 MO2 fluorite units per unit cell, with unit-cell parameters related to that of cubic fluorite (a(f) = 5.5 Angstrom). A bond valence study demonstrates that the present crystal structure is especially stable for small rare-earth cations, and becomes more unstable when the R3+ size increases, thus explaining the observed preference of the large rare-earth molybdates for polymorphs beta and gamma with the same stoichiometry. (C) 2004 Elsevier Inc. All rights reserved.