Semiconductor-metal transition of pyrite FeS2 under high pressure by full-potential linearized-augmented plane wave calculations

The effects of hydrostatic pressure on the electronic band structure of the semiconductor mineral iron pyrite FeS2 have been investigated theoretically by an ab initio full-potential linearized-augmented plane wave (FPLAPW) method within a local approximation (LDA/GGA) to the density functional theory. The calculations predict that at a pressure of 94.1 GPa the indirect band gap of pyrite FeS2 vanishes and the material becomes a metal. This is due to the presence of the S-S and Fe-S bonds, which provide novel energy band distortions in the process of attaining the metallic state. Analysis indicates that, under increasing high pressure, the conduction bands (3p(z) of sulfur and 3d(x2-y2)+3d(xy) of iron) intrude downwards into the valence bands, which are predominantly 3d in nature. At normal pressure, the lattice constant, the bulk modulus, sulfur position parameter u, S-S bond length, and the indirect band gap of pyrite FeS2 are calculated using a fully relaxed unit cell and found to be equal to 541.8 pm, 159.7 GPa, u = 0.383, 219.5 pm and 0.45 eV, respectively. Apart from the gap, which is too small (the usual 'LDA error'), these results agree well with recent experiments. The effective masses of an electron at selected points in the conduction band are reported.