Non-stoichiometry in anion-excess ReO 3 phases; the structure of Zr 0.8Yb 0.2F 3.2O 0.3(MX 3.5) by powder neutron diffraction
The structure of cubic Zr 0.8Yb 0.2F 3.2O 0.3(MX 3.5) (space group Pm3m, a0 = 4.011 Å), one of several known anion-excess phases based on ZrF 4, with diffraction symmetry appropriate to the ReO 3 structure, has been determined by powder neutron diffraction. The anion-excess non-stoichiometry over that of the parent ReO 3-type structure is found to be accommodated by interstitial anions as indicated previously by density measurements. Two anion sites are found, one, F1, slightly displaced (by 0.31 Å) from an ideal lattice site, and the other, F2, more considerably so (by 1.11 Å). The populations of the two sites (2.50 and 1.00 respectively) indicate that the principal structural feature explaining the nonstoichiometry is the existence of F2-F2 pairs across a vacant F1 site, with a maximum separation of 2.23 Å, shorter than normally observed in ionic fluorides. Short fluorine-fluorine distances have previously been observed in CaF 2 heavily doped with YF 3, and it sems that short fluorine-fluorine separations are allowed in anion-excess nonstoichiometric fluorides where no mechanism for their relaxation is available. Such non-stoichiometry may not be uncommon where the host structure does not have a close-packed anion lattice and contains an ion such as zirconium, yttrium or a rare earth ion which can accept a co-ordination by fluorine higher than that demanded by the basic structure type. The several possible near-neighbor anion-anion distances in Zr 0.8Yb 0.2F 3.2O 0.3 are tabulated, and the most likely arrangements of neighboring atoms assessed. The previously observed limit of non-stoichiometry in these phases (about MX 3.8) is rationalized on this basis. Implications of the results are mentioned both for chemical bonding, including possible features in common with crystallographic shear phases in oxides, and for the preparation of anion-excess perovskites or bronzes.