Inflexible stoichiometry in bulk pyrite FeS₂ as viewed by in situ and high-resolution X-ray diffraction

Non-stoichiometry is considered to be one of the main problems limiting iron pyrite, FeS₂, as a photovoltaic absorber material. Although some historical diffraction experiments have implied a large solubility range of FeS_2-δ with δ up to 0.25, the current consensus based on calculated formation energies of intrinsic defects has lent support to line-compound behavior. Here it is shown that pyrite stoichiometry is relatively inflexible in both reductive conditions and in autogenous sulfur partial pressure, which produces samples with precise stoichiometry of FeS₂ even at different Fe/S ratios. By properly standardizing in situ gas-flow X-ray diffraction measurements, no significant changes in the lattice parameter of FeS₂ can be resolved, which portrays iron pyrite as prone to forming sulfur-deficient compounds, but not intrinsic defects in the manner of NiS_2-δ.