Determination of an adequate bornite model for computational simulations – Cu5FeS4 or Cu8Fe4S8

Abstract In the positional disordered unit cell of the natural bornite (Cu 5 FeS 4 ) there are 10 copper and 2 iron atoms in the asymmetric unit. The positions M(4) and M(5) occupied by the iron atoms and the other ten by copper atoms represent the arrangement that is 30 kcal mol −1 more stable in energy than other different arrangement in the orthorhombic unit cell. The perfect ideal cubic structure of bornite (Cu 8 Fe 4 S 8 ) proposed by Ding et al. was also investigated and compared to the most stable Cu 5 FeS 4 structure. Topological analysis of the electron density indicates that the M-S bonds have an ionic character and that there are no metal-metal bonds. The sulfur basins occupy the largest volume in both cells. However, the local properties of the copper basins together with the sulfur basins govern the bulk compressibility of the two structures. The cleavage energy of Cu 5 FeS 4 has been estimated for different surfaces indicating that 121 (3) and 101 surfaces are preferentially formed. These two surfaces have the sulfur atoms assume the outside part of them after relaxation process and PDOS analysis shown that the sulfur atoms being nucleophilic centers. The metal atoms move downward in order to increase the sulfur−metal−sulfur angle. This relaxation process is similar to one of the reconstruction surface mechanism of chalcopyrite, reported recently.