Calculations of defect clustering in Fe_1-xO

Lattice energy calculations based on the Born model are used to examine the possible structures of defect clusters in non-stoichiometric FeO. The basic cluster is identified as a complex of four cation vacancies and one tetrahedral Fe³⁺ ion (or 4:1 cluster) which is formed as a result of a large Coulomb term favouring the occupation of the tetrahedral site when all nearest-neighbour cations are vacant. The aggregation of these 4:1 clusters occurs by vacancy-sharing, and the calculations suggest that the most stable small aggregates are formed by edge- rather than corner-sharing (6:2 or 8:3 clusters). If more extended clusters are formed they are likely to involve corner-sharing, but the binding energy for a cluster closely related to the Fe₃O₄ structure is larger than that calculated for the Koch-Cohen cluster.