The crystal structures of Ba2R2/3V2O8 (R = La, Nd) and Sr2La2/3V2O8; palmierite derivatives

摘要

The title phases Ba2R2/3V2O8 (R = La, Nd) and Sr2La2/3V2O8, synthesised by solid state reaction of oxides at 1350 degrees C, have structures derived from that of the palmierite-type of Ba3V2O8; the stoichiometry is often written as A(3)RV(3)O(12). Ba2La2/3V2O8 is hexagonal, spacegroup m, a = 5.75271(7), c = 21.0473(5) Angstrom, Z = 3; cation ordering was determined by joint Rietveld refinement using X-ray and neutron powder diffraction data, R-wp = 4.45%, R-p = 6.33%, chi(2) = 6.847. In the Ba3V2O8 structure, Ba occupies two sites: 3a and 6c. In Ba2La2/3V2O8, Ba wholly occupies the 3a site; the 6c site contains both Ba, La and vacancies. Bond valence analysis was inconclusive, but tends to support the presence of Ba on the 3a site. Sr2La2/3V2O8 and Ba2Nd2/3V2O8 are isostructural, as confirmed by Rietveld refinement using X-ray powder diffraction data. Ba2R2/3V2O8 phases could not be synthesised for lanthanides, R, smaller than Sm. (C) 2000 Kluwer Academic Publishers. [References: 27]