Electron-lattice interaction and structural stability of the oxy-borate Co3O2BO3

A theoretical study is carried out in the homometallic mixed valent ludwigite Co3O2BO3 using a modified tight binding methodology. The study focuses on the electronic properties of bulk, 1D and molecular units to describe differences between Co3O2BO3 and another homometallic ludwigite, Fe3O2BO3. The latter is known to present a structural instability which has not been found in Co3O2BO3. Our results show that bulk band structures present no significant differences. Differences are found in the calculation of 1D stripes formed by 3+ 2+ 3+ triads, owed to different 3d occupancy of Fe and Co cations. Conditions for 1D semiconducting transport observed in Fe3O2BO3 are not present in Co3O2BO3. More important differences were found to be related to local octahedral geometry. Larger distortions in Co3O2BO3 lead to higher 2+ 3+ hopping barriers in the triads and consequent localization of charge. In Fe3O2BO3, site equivalence provides easy paths for inter-cation hopping. The present results indicate that local geometry of the cation sites is the main cause of differences in these two compounds; dimerization in the triads, which characterizes the structural change in Fe3O2BO3, could be structurally hindered in Co3O2BO3. An analogy is made with two mixed-valent warwickites, Fe2OBO3 and Mn2OBO3, which show the same structural stability/site equivalence relationship.