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

Abstract

The average and local crystal structure of the Co2Co1-xFex(BO3)O2 (0.0 < x < 1.0) were studied using X-ray diffraction and Mӧssbauer spectroscopy combined with density functional theory calculations to obtain a detailed description of the cation distribution. The calculations explain the observation of several distinct ranges of quadrupole splitting and predict a preferential partitioning of the Fe into distinct crystallographic positions. The presence of the confined oxygens is the main reason for the difference in the tetragonal distortions of the oxygen octahedra. The large ligand contribution and pronounced charge anisotropy due to the covalent admixing of the spin-down 3d orbitals with 2p orbitals of the axial oxygens are responsible for the large quadrupole splitting for Fe1 and Fe3 ions. The solid solutions undergo the magnetic phase transitions at T1 = 48, 66, and 82 K for x = 0.3, 0.5, and 0.7, respectively, as characterized by the magnetization rise along the b-axis and λ-peak in the heat capacity. Additional magnetic anomalies have been observed at T2 = 21 K (x = 0.3) and 37 K (x = 0.5) for a magnetic field applied in the a-direction. The magnetic properties appear to be sensitive to the cation distribution.