Crystal structures of double vanadates Ca9R(VO4)(7). IV. R = Er, Tm, Yb, and Lu

Crystal structures of Ca9R(VO4)(7) compounds with R = Er (I), Tm (II), Yb ( III), and Lu (IV) have been studied by the Rietveld method. All these compo unds are isostructural to Ca-3(VO4)(2) and crystallized in the trigonal sys tem (sp. gr. R3c, Z = 6) with the following unit-cell parameters: (I) a = 1 0.8554(5), c = 37.967(1) Angstrom; (II) a = 10.8550(5) and c 37.950(1) Angs trom; (III) a = 10.8564(5) and c = 37.924(1) Angstrom; (IV) a = 10.8566(5) and c = 37.880(2) Angstrom. In compounds I-IV, the rare-earth and calcium c ations statistically occupy three positions-M( 1), M(2), and M(5). The numb er of R3+ cations in the M(5) position increases from 1.3(1) for Er up to 3 .0(1) for Lu. The occupancy of the M(1) and M(2) positions with R3+ cations decreases with a decrease of the radius of R3+. The data on the distributi on of Raf cations for the whole series (R = RE and Y) are given and discuss ed in detail. (C) 2000 MAIK "Nauka/Interperiodica".