Phase Stability and Thermoelectric Properties of the Mineral FeS2: An Ab Initio Study

First-principles calculations were carried out to study the phase stability and thermoelectric properties of the naturally occurring marcasite phase of FeS2 at ambient conditions as well as under pressure. Two distinct density functional approaches were used to investigate these properties. The plane wave pseudopotential approach was used to study the phase stability and structural, elastic, and vibrational properties. The full potential linear augment plane wave method has been used to study the electronic structure and thermoelectric properties. From the total energy calculations, it is clearly seen that marcasite FeS2 is stable at ambient conditions, and it undergoes a first-order phase transition to pyrite FeS2 at around 3.7 GPa with a volume collapse of about 3%. The calculated ground-state properties, such as lattice parameters, bond lengths, and bulk modulus of marcasite FeS2, agree quite well with experimental values. In addition, phonon dispersion curves unambiguously indicated that the marcasite phase is stable under ambient conditions. Furthermore, we did not observe any phonon softening across the marcasite-to-pyrite transition. The possible reason for the transition was analyzed in the present study, which has not been attempted previously. In addition, we calculated the electronic structure and thermoelectric properties of both marcasite and pyrite FeS2. We found a high thermopower for both phases, especially with p-type doping, which enabled us to predict that FeS2 might have promising applications as a good thermoelectric material.