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Keywords:

  • Crystal growth;
  • Solid-state structures;
  • Chromium;
  • Electronic spectra;
  • Phosphates

Abstract

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.