Crystal structures of double vanadates, Ca9R(VO4)(7). III. R = Nd, Sm, Gd,or Ce

The crystal structures of Ca9R(VO4)(7) (R = Nd (I), Sm (II), or Gd (III)) w ere studied by the Rietveld method. The compounds are isostructural to Ca-3 (VO4)(2) and are crystallized in the trigonal system (sp. gr. R3c Z = 6). T he unit-cell parameters are as follows: for I, a = 10.8720(5) Angstrom, c = 38.121(1) Angstrom; for II, a = 10.8652(5) Angstrom: c = 38.098(1) Angstro m; and for III, a = 10.8631(5) Angstrom, c = 38.072(1) In the structures of I and II, the M(1), M(2), and M(3) positions are statistically occupied by the rare-earth cations and calcium anions. In the structure of III, the Gd 3+ cations occupy the M(1) and M(2) positions. The distributions of the R3 cations over the positions are characteristic of each structure. The compo sition of the cerium-containing compound Ca9.81Ce0.42(VO4)(7) (a = 10.8552( 5) Angstrom, c = 38.037(1) Angstrom) was refined and its crystal structure was solved from the X-ray powder data. In this compound, cerium atoms are i n the oxidation states +3 and +4. (C) 2000 MAIK "Nauka/Interperiodica".