Insight into the cation distribution in Co2Co1−xFex(BO3)O2 (0.0 < x < 1.0): X-ray diffraction, Mössbauer spectroscopy, and DFT investigations

Abstract

Single crystals of the solid solutions Co₂Co_1−xFe_x(BO₃)O₂ (0.0 < x < 1.0) were grown using flux. The average and local crystal structures were studied using X-ray diffraction and Mössbauer spectroscopy combined with density functional theory calculations. The calculated quadrupole splittings are QS ≈ 1.81–2.60 mm s⁻¹ for the Fe³⁺ ions at the Fe1 and Fe3 positions, QS ≈ 0.92—1.17 mm s⁻¹ at the Fe4 position, QS ≈ 0.50–0.70 mm s⁻¹ at the Fe2 position, and are in quantitative agreement with the experimental values. A detailed analysis revealed that the confined oxygen atoms contribute to the tetragonal distortions of the coordinated octahedra. The large ligand contribution and charge anisotropy due to the covalent admixing of the spin-down 3d orbitals with 2p orbitals of the unconfined oxygen atoms are responsible for the large value of the electric field gradient at Fe1 and Fe3. The cation distribution over the entire series of the solid solution was established, which reflects the order of preference for Fe³⁺ ions: Fe4 ≫ Fe2 > Fe1 ≈ Fe3. A preliminary magnetic study using dc magnetization and heat capacity measurements revealed that the solid solutions undergo magnetic phase transitions at T₁ = 48, 66, and 82 K for x = 0.3, 0.5, and 0.7, respectively. Additional magnetic anomalies are observed upon cooling. The magnetic properties appear to be sensitive to the cation distribution.