β‐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.
The chromium(II) orthophosphate α-Cr 3 (PO 4 ) 2 is a weak ferrimagnet with the Curie temperature T C = 29 K confirmed by a λ-type peak in specific heat. Dominant antiferromagnetic interactions in this system are characterized by the Weiss temperature = 96 K, indicating an intermediate frustration ratio | |/T C ∼ 3. In its magnetically ordered states α-Cr 3 (PO 4 ) 2 exhibits a remarkable sequence of temperature-induced magnetization reversals sensitive to the protocol of measurements, i.e., either field-cooled or zero-field-cooled regimes. The reduction of the effective magnetic moment 4.5 μ B /Cr 2+ , as compared to the spin-only moment 4.9 μ B /Cr 2+ , cannot be ascribed to the occurrence of the low-spin state in any crystallographic site of the Jahn-Teller active 3d 4 Cr 2+ ions. X-ray absorption spectra at the K edge indicate divalent chromium and unravel the high-spin state of these ions at the L 2,3 edges. Weak ferrimagnetism and multiple magnetization reversal phenomena seen in this compound could be ascribed to incomplete cancellation and distortion of partial spontaneous magnetization functions of Cr 2+ in its six crystallographically inequivalent positions.
Syntheses and Characterization of β-Chromium(II)-orthophosphate. -Light blue single crystals of β-Cr3(PO4)2 are prepared by reduction of CrPO4 with elemental Cr at about 1300°C (sealed double-wall silica tubes, CrI2 as mineralizer). The new modification crystallizes in the monoclinic space group P21/n with Z = 2. The structure consists of elongated [Cr II O6] octahedra. The compound is further characterized by UV/VIS/NIR spectroscopy and magnetic measurements. -(GLAUM*, R.; HAMMER, E.; HERMES, W.; POETTGEN, R.; Z. Anorg. Allg. Chem. 637 (2011) 7-8, 1052-1061, http://dx.
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