Ab initio study of phonon dispersion and thermodynamic properties of pure and doped pyrites

Pyrites (FeS₂) are solid minerals that are found abundantly in Nigeria and are easy to prepare in laboratories. In this work, FeS₂ is studied extensively in its pure state as well as when iron is substitutionally doped with zinc and calcium at concentrations of 0, 0.25, 0.5, 0.75 and 1. Using density functional theory, the eectronic, dynamic and thermodynamic properties were calculated. The results revealed that the lattice parameters and bulk modulus increases with increasing concentration and the obtained values are in agreement with available experimental and theoretical values. Though pyrite, when doped with zinc, obeys Vegard’s law, doping with calcium revealed pronounced deviation from this law. The calculated band structures showed that FeS₂ has an indirect band gap whose size decreases after introducing zinc while doping with calcium increases the band gap. The phonon dispersion of the end members FeS₂ and ZnS₂ indicate that the systems are dynamically stable while CaS₂ is dynamically unstate. Also, the thermodynamic properties of the pure and doped pyrites were calculated and the ranges of temperature at which the lattice and electronic degrees of freedom contribute to the specific heat capacity are presented.