Octahedral distortions in the homometallic Fe ludwigite

摘要

An extended Huckel high spin band approach is used to investigate the effects of oxygen octahedral distortions in the Fe3O2BO3 ludwigite. Owing to distortion, a 0.2 eV stabilizing gap (above the spin down Fermi level) is found to appear in a ID sub-unit, formed by the strongly interacting Fe3+-Fe2+-Fe3+ triad. Through a detailed analysis of the crystal wave functions, the gap is found to be a result of 3d(sigma)-3d(pi) orbital mixing, which generates a narrow band for the extra (spin down) Fe2+ electron. Charge localization is obtained in the I D sub-unit but not in the whole crystal (3D) calculation. It is suggested that the high barrier for electron hopping, experimentally found in the literature to occur around 220 K, be related to the I D gap. (C) 2004 Elsevier Inc. All rights reserved.