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

Robert Glaum

Abstract

Cr7(PO4)6 [Cr32+ Cr43+(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 F0, 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 ≤ dCr–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)°].

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Zeitschrift für Kristallographie – Crystalline Materials offers a place for researchers to present results of their crystallographic studies. The journal includes theoretical as well as experimental research. It publishes Original Papers, Letters and Review Articles in manifold areas of crystallography.

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