Application of computer simulation methods to the study of Platinum Group minerals

Abstract

Computer simulation studies of a few representative Platinum-Group- Minerals (PGM), which are of industrial importance to the mining industry in South Africa were carried out. The electronic, structural and optical properties of PGM (PtS, PdPt3S4, PtAs2 and Pt4As4S4) were calculated within the framework of the density functional theory. We have used both the pseudopotential planewave and the Tight Binding Linear Muffin-Tin Orbital (TB-LMTO) methods to complement each other, since there is not much experimental data available for these systems. In the TB-LMTO method, the radii of overlapping Muffin Tin (MT) spheres were chosen to provide an efficient packing of space while ensuring that the overlap between the spheres remains small. The ground state structural properties were obtained by self-consistent energy minimization with respect to the atomic volume. The predicted equilibrium volume is within less than 15% of experiment. We have also found non-metallic semi-conducting behaviour for the three systems, PtS, PdPt3S4 and PtAs2 using both ab initio techniques. On the other hand, Pt4As4S4 was predicted to be metallic. We argue that the strong bonding between the Pt 5d and the S(As) 3p(4p) states plays a crucial role in the formation of the band gap in the semiconducting. The optical properties of PtS, PdPt3S4 and PtAs2 were calculated, and their reflectance spectra were found to be in good agreement with the experimental measurements. Full relaxation of both the volume and the internal parameters was carried out using the plane-wave pseudopotential method. It was found that the internal parameters as well as the bond lengths decrease with hydrostatic compression particularly for the cubic PtAs2, Pt4As4S4 and tetragonal PdPt3S4, and PtS has no internal parameters. The bulk moduli were calculated for these representatives of PGM and there are currently no experimental results available. We have derived a new set of potential models for PtS, PdPt3S4, PtAs2 and Pt4As4S4 structures. The models were used to study the pressure dependence of lattice constant for all the four systems and agree well with our electronic structure methods. However, PtS display anomaly under hydrostatic pressure, by expanding along c-lattice constant with increased pressure, for which there are no experimental evidence. We then modelled the structure and stabilities of PtAs2 and Pt4As4S4 of the dry and hydrated surfaces for low and high index surfaces, and predicted the {100} surface to be the most stable in both cases. It is further shown that molecular absorption of water has a stabilising effect on all the surfaces of the two structures. Stepped surfaces were created for {510} and {610} for both PtAs2 and Pt4As4S4 in order to model more realistic surfaces under dry condition with one dimensional defects, and then acute stepped were found to be the most stable compared to the obtuse steps. The three surfaces expressed in the equilibrium morphology of PtAs2, {100}, {210} and {102}, are in good agreement with experiment.