Electronic structure and stability of the pentlandites Co9S8 and (Fe,Ni)9S8 -: art. no. 155105

First-principle electronic structure investigations of transition-metal sulfides Co9S8 and related alloys with the unique structure of pentlandite are carried out using density-functional theory within the local-density approximation. The total-energy calculations for Co9S8 and (Fe,Ni)(9)S-8 alloys have been computed and we predict equilibrium lattice parameters that are on average 1% smaller than in the experiment. The heats of formation have been calculated, the theoretical prediction for Co9S8 being in excellent agreement with that available in the Thermocalc database. The predicted heat of formation for the Fe5Ni4S8 alloy is very close to Co9S8, reflecting the fact that the Fermi level is found to fall in a pseudogap for an average number of valence electrons per atom e/a=7.58. Furthermore, we determined the individual bond energies for Co9S8 and Co8S8 to stress the contribution of the octahedral metal cobalt to the stability of the Co9S8 phase.