Structural and microstructural features of pyrite FeS_2−x thin films obtained by thermal sulfuration of iron

Abstract

Structural and microstructural properties of synthetic thin films of pyrite (FeS2−x), prepared by thermal sulfuration of iron layers, were investigated from Rietveld refinements of x-ray diffraction data, collected by step/scan mode. From this refinement lattice constant, a, and sulfur position parameter, u, nearest neighbor Fe–S and S–S bond distances and tetrahedral and octahedral bond angles have been determined. Moreover, sulfur deficit in the samples, surface and volume-weighted crystallite size and microstrains were also obtained. From these data, the influence of temperature and time of sulfuration and sulfur pressure on their structural and microstructural properties has been established. Stoichiometric pyrite thin films are obtained by sulfurating the iron films at low temperatures (Ts ∼ 600–700 K) during short times (ts ∼ 0.5–2 h). These experimental conditions yield films with the highest a, u, Fe–S bond distance, and microstrains, as well as S/Fe ratios about 2.00, i.e., null sulfur vacancies, the smallest S–S bond distances, and crystallite size. Finally, the possible influence of these structural and microstructural characteristics on some physical properties (optical absorption, electrical resistivity …) of the films is discussed.

Rietveld refinement study of pyrite crystals

“…It is noteworthy that the agreement with both, the most recent Rietveld refinement result for a synthetic bulk powdered pyrite [23] and the only existing study of a synthetic single crystal [50], is excellent. These values match, to less extent, the values observed in the past for synthetic thin films [15,16] (cf. Table 1).…”

“…There are numerous indications that pyrite is or can be nonstoichiometric (see e.g. [5,[13][14][15][16] and references therein). The deviation from stoichiometry is found to have a weak but well determined influence on the lattice parameter of bulk FeS 2 crystals [5].…”

“…Moreover, a number of papers are devoted to structure calculation from first principles [11,39,[42][43][44]. The experimental structural data concern natural crystals [13,23,45,46], synthetic bulk crystals [5,23,[47][48][49][50][51][52], and pure and doped polycrystalline [15,16,53] and monocrystalline [54] thin layers (see Table 1). Systematic, combined or comparative studies involve, e.g.…”

“…Systematic, combined or comparative studies involve, e.g. XPS and theoretical investigation of pyrite surface [36], X-ray diffraction (XRD) and XPS study of high-quality thin FeS 2 films [54], the variation of structural properties with Fe:S ratio [5,15,16] and a study of synthetic and natural crystals [23]. The (minor) discrepancies between the reported a and x values are due to both, the sample properties (nonstoichiometry, impurities in mineral samples) as well as to instrumental factors.…”

“…(Russia). The mentioned calculation method has already been found to be useful in studies of bulk FeS 2 crystals [23], arsenopyrite formation [55], in crystallographic/mineralogical studies including the pyrite phase (Rietveld refinement study of pyrite in coals [14]) and in mixture with marcasite [48] as well as in investigation of synthetic pyrite thin films [15,16].…”