Synthesis and Physical Properties of the Oxofluoride Cu₂(SeO₃)F₂

Single crystals of the new compound Cu₂(SeO₃)F₂ were successfully synthesized via a hydrothermal method, and the crystal structure was determined from single-crystal X-ray diffraction data. The compound crystallizes in the orthorhombic space group Pnma with the unit cell parameters a = 7.066(4) Å, b = 9.590(4) Å, and c = 5.563(3) Å. Cu₂(SeO₃)F₂ is isostructural with the previously described compounds Co₂TeO₃F₂ and CoSeO₃F₂. The crystal structure comprises a framework of corner- and edge-sharing distorted [CuO₃F₃] octahedra, within which [SeO₃] trigonal pyramids are present in voids and are connected to [CuO₃F₃] octahedra by corner sharing. The presence of a single local environment in both the ¹⁹F and ⁷⁷Se solid-state MAS NMR spectra supports the hypothesis that O and F do not mix at the same crystallographic positions. Also the specific phonon modes observed with Raman scattering support the coordination around the cations. At high temperatures the magnetic susceptibility follows the Curie-Weiss law with Curie temperature of Θ = -173(2) K and an effective magnetic moment of μ_eff ∼ 2.2 μ_B. Antiferromagnetic ordering below ∼44 K is indicated by a peak in the magnetic susceptibility. A second though smaller peak at ∼16 K is tentatively ascribed to a magnetic reorientation transition. Both transitions are also confirmed by heat capacity measurements. Raman scattering experiments propose a structural phase instability in the temperature range 6-50 K based on phonon anomalies. Further changes in the Raman shift of modes at ∼46 K and ∼16 K arise from transitions of the magnetic lattice in accordance with the susceptibility and heat capacity measurements.