Beiträge zum thermischen Verhalten wasserfreier Phosphate VIII.* Darstellung und Kristallstruktur von Cr7(PO4)6. Das erste gemischtvalente Phosphat mit Cr(II) und Cr(III)

Abstract

Abstract Cr7(PO4)6 [Cr3 2+ Cr4 3+(PO4)6] can be obtained reducing a mixture of Cr2O3 and CrPO4 by CrP (950°C, 48h, 100 mg iodine as mineralizer). By means of chemical transport reactions (transport agent iodine; 1050 → 950°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 [unk]; Z = 1; a = 9.3184(19) Å, b = 8.0044(18) Å, c = 6.2532(14) Å, α = 100.982(9)°, β = 107.954(11)°, γ = 107.144(10)°] 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 0, 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 Å ≤ d Cr–O ≤ 2,071 Å). Zn3Cr4(PO4)6 also isotypic to Fe7(PO4)6 has been synthesized and characterized by its Guinier powder pattern [a = 9.248(4) Å, b = 7.835(3) Å, c = 6.277(2) Å, α = 101.59(4)°, β = 108.10(3)°, γ = 105.34(4)°].