THERMAL-BEHAVIOR OF ANHYDROUS PHOSPHATES .8. SYNTHESIS AND CRYSTAL-STRUCTURE OF CR7(PO4)6 - 1ST MIXED-VALENCE PHOSPHATE WITH CR(II) AND CR(III)

Cr7(PO4)6 [Cr3(2+)Cr4(3+)(PO4)6] can be obtained reducing a mixture of Cr2O3 and CrPO4 by CrP (950-degrees-C, 48 h, 100 mg iodine as mineralizer). By means of chemical transport reactions (transport agent iodine; 1050 --> 950-degrees-C) the compound has been separated from its neighbour phases and crystallized (dark brown, transparent crystals; pinacoidal, edge length up to 0.5 mm). Cr7(PO4)6 [P 1BAR; Z = 1; a = 9.3184(19) angstrom, b = 8.0044(18) angstrom, c = 6.2532(14) angstrom, alpha = 100.982(9)degrees, beta = 107.954(11)degrees, gamma = 107.144(10)degrees] is isotypic to Fe7(PO4)6. The crystal structure refinement from single single crystal data led to a conventional residual R = 0.032 (3016 independent F(o), 170 parameters). For M = Cr2+ two different M-O-coordination polyhedra were observed; a compressed octahedron and a strongly distorted trigonal bipyramide. The influence of the Jahn-Teller effect on the coordination geometry of Cr2+ will be discussed. The Cr3+ ions (2 sites) are surrounded octahedrally by oxygen (1,884 angstrom less-than-or-equal-to d(Cr-O) less-than-or-equal-to 2,071 angstrom). Zn3Cr4(PO4)6 also isotypic to Fe7(PO4)6 has been synthesized and characterized by its Guinier powder pattern [a = 9.248(4) angstrom, b = 7.835(3) angstrom, c = 6.277(2) angstrom, alpha = 101.59(4)degrees, beta = 108.10(3)degrees, gamma = 105.34(4)degrees].