THE ELECTRONIC-STRUCTURES OF THE PYRITE-TYPE DISULFIDES (MS2, WHERE M= MN, FE, CO, NI, CU, ZN) AND THE BULK PROPERTIES OF PYRITE FROM LOCAL-DENSITY APPROXIMATION (LDA) BAND-STRUCTURE CALCULATIONS

A local density approximation (LDA) band structure method, the Linear Muffin-Tin Orbital Atomic Sphere Approximation (LMTO-ASA) method has b een used to calculate the electronic structures of the pyrite-type dis ulphides (MS2, where M = Mn, Fe, Co, Ni, Cu, Zn). The total density of states has been calculated for 10 eV above and below the Fermi Level, along with the separate contributions from metal and sulphur and show s that the metal d band occurs above the sulphur p bands in MnS2, FeS2 , CoS2 and NiS2, whereas in CuS2, the d band passes through the sulphu r p band and in ZnS2, it lies below the sulphur p band. Substantial hy bridization of the metal d states with the sulphur states occurs. FeS2 is calculated to be a semiconductor with a direct band gap of 0.64 eV in good agreement with experiment. The calculated local densities of states have been used in turn to calculate X-ray photoelectron spectra and Bremsstrahlung Isochromat spectra for this series of compounds, a nd these also show reasonable agreement with experimental data. A part icular strength of the LMTO-ASA method is the ability to calculate and predict certain bulk properties of solids of interest in mineral phys ics. This has enabled the first reasonably accurate calculations of th e total energy of the valence electrons of the system for pyrite (FeS2 ), given as - 345.885 rydbergs per unit cell, and the equilibrium unit cell volume which is within 3.3% of that determined experimentally. A theoretical pressure vs. volume curve for pyrite was also calculated along with values for the bulk modulus. However, our calculations pred ict a bulk modulus of 6.75 Mbar which is too high by a factor of 4.6 d ue to the simplifying assumption of a uniform scaling of interatomic d istances on compression.