Preparation and structural study from neutron diffraction data of R2MoO 6 ( R=Dy, Ho, Er, Tm, Yb, Y)

Abstract

The title compounds have been prepared as polycrystalline powders by thermal treatments of stoichiometric mixtures of R 2O 3 and MoO 3 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 α- R 2MoO 6, typified by Sm 2MoO 6. The structure contains four zigzag, one-dimensional MoO 5 polyhedral rows per unit cell, running through the RO 8 polyhedral framework along the [001] direction. MoO 5 form discrete units (i.e. do not share common oxygen), with Mo–O distances ranging from 1.77 to 2.24 Å, although the oxygen coordination can be extended to distances of about 3.1 Å, giving rise to strongly distorted MoO 8 scalenohedra. Thus, MoO 8 and RO 8 polyhedra are fully ordered in R 2MoO 6 compounds, which in fact can be considered as superstructures of fluorite (M 3O 6), containing 24 MO 2 fluorite units per unit cell, with unit-cell parameters related to that of cubic fluorite ( a f =5.5 Å). A bond valence study demonstrates that the present crystal structure is especially stable for small rare-earth cations, and becomes more unstable when the R 3+ size increases, thus explaining the observed preference of the large rare-earth molybdates for polymorphs β and γ with the same stoichiometry.