Abstract
An extended Hückel high spin band approach is used to investigate the effects of oxygen octahedral distortions in the Fe 3O 2BO 3 ludwigite. Owing to distortion, a 0.2 eV stabilizing gap (above the spin down Fermi level) is found to appear in a 1D sub-unit, formed by the strongly interacting Fe 3+–Fe 2+–Fe 3+ triad. Through a detailed analysis of the crystal wave functions, the gap is found to be a result of 3 d( σ)–3 d( π) orbital mixing, which generates a narrow band for the extra (spin down) Fe 2+ electron. Charge localization is obtained in the 1D 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 1D gap.
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