In spite of reports of significant deviations from ideal stoichiometry, no real proof existed that pyrite had an existence range which exceeded compositional variations of the order of 1at%. A theoretical estimate was made of the vacancy formation energy in this material. On the basis of the macroscopic cavity model of Van Vechten and Phillips, the vacancy formation energies of a S vacancy and an Fe vacancy were calculated to be equal to 1.66 and 2.18eV, respectively. The formation energy of a complete Schottky defect (Fe + 2S), 5.50eV, was found to be in good agreement with a value of 5.80eV which was deduced from Kröger’s rule that the vacancy formation energy was equal to half of the sublimation energy. These relatively high values supported the conclusion that pyrite exhibited only slight deviations from the nominal S/Fe ratio of 2.

On the Stoichiometry of the Semiconductor Pyrite (FeS2). K.Ellmer, C.Höpfner: Philosophical Magazine A, 1997, 75[4], 1129-51