Crystal structures of new double calcium and cobalt phosphates

New phosphates Ca9.5Co(PO4)(7) (I) and Ca9+2xCo1+x(PO4)(7) (x = 0.14 (II), x = 0.07 (III)) were synthesized by the solid-state method. Compounds II and III were obtained as mixtures with beta-Ca2P2O7. They are related to the mineral whitlockite and crystallize into trigonal system (space group R3c) with unit-cell parameters a = 10.3501(1), c = 37.179(1)Angstrom (I); a = 10.3374(1), c = 37.174(1) Angstrom (II); and a 10.3278(1), c = 37.101(1) Angstrom (III). The crystal structures of I and II were determined by the Rietveld analysis (R-WP = 3.11, R-I = 1.77, S = 1.85 (I); R-WP = 2.67, R-I = 2.70, S = 1.49 (II)). In Ca9.5Co(PO4)(7), cobalt occupies the octahedral site M(5); the M(4) site is half-filled by Ca2+. In Ca9.28Co1.14(PO4)(7), cobalt occupies the octahedral site M(5) and partially occupies the M(4) site. Some of the cobalt cations have the oxidation state +3. (C) 1998 Elsevier Science Ltd.