Crystal structure of double vanadates Ca9R(VO4)7.: II.: R = Tb, Dy, Ho, and Y

Crystal structures of the compounds Ca9R(VO4)(7) (R = Tb (I), Dy (II), Ho (III) and Y (IV) have been studied by the method of the full-profile analysis. All the compounds are crystallized in the trigonal system (sp. gr. R3c, Z = 6) with the unit-cell parameters (I) a = 10.8592(1), c = 38.035(1), V = 3884.2(2) Angstrom(3); (II) a = 10.8564(1), c = 38.009(1) Angstrom, V = 3879.6(2) Angstrom(3) (III) a = 10.8565(1) and c = 37.995(1) Angstrom, V = 3878.3(2) Angstrom(3), and (IV) a = 10.8588(1), c = 37.995(1) Angstrom, V= 3879.9(2) Angstrom(3). In structures I-IV, rare earth and calcium cations occupy three positions-M(1), M(2), and M(5). Rare earth cations occupy the R3+ positions almost in the same way: 2.7-2.6(2) cations in the M(1) position; 2.7-2.3(2)cations in the M(2) position, and 0.6-1.0(1) cation in the M(5) position. At the same time, the occupancy of the M(5) position regularly increases with a decrease of the R3+ radius. (C) 2000 MAIK "Nauka/Interperiodica".