Synthese und Charakterisierung von β‐Chrom(II)‐orthophosphat

Abstract

Abstractβ‐Cr3(PO4)2 was obtained by reducing CrPO4 with elemental chromium. The new modification is stable between 1250 °C and its melting point at 1350 °C. Light‐blue single crystal were grown by isothermal heating (T = 1300 °C, sealed double‐wall silica tubes, CrI2 as mineralizer). The crystal structure is closely related to the Mg3(PO4)2 (Farringtonite) structure type (P21/n, Z = 2, a = 4.9641(5), b = 9.492(1), c = 6.4849(5) Å, β = 91.412(8)°; 2868 independent reflections, 62 variables, seven atoms in the asymmetric unit, R1 = 0.028, wR2 = 0.061). The structure consists of elongated octahedra [CrIIO6] (2.054(1) ≤ d(Cr–O) ≤ 2.743(4) Å). Electronic absorption spectra measured in the NIR/vis range for α‐ and β‐Cr3(PO4)2, α‐Cr2P2O7, and SrCrP2O7 allow an estimation of 10Dq = 9000 cm–1 for the [CrIIO6] chromophore. The observed transition energies for the low‐symmetry chromophores match remarkably well with calculations within the framework of the angular overlap model. Comparison of the magnetic behavior of α‐ and β‐Cr3(PO4)2 (μexp = 4.90 μB) reveals significant differences.