Iron Pyrite from Iron Thin Films: Identification of Intermediate Phases and Associated Conductivity-type Transitions

In this work, the growth of FeS₂ by direct sulfuration of Fe thin films is examined with the purpose of elucidating the nature of the Fe to FeS₂ transformation and to state some of its characteristics. To this aim, the film Seebeck coefficient (<i>S</i>) was measured during the whole sulfuration treatment. The <i>S</i> value changes from positive (ca. +8 μV K^–1) to negative values (ca. −15 μV K^–1) and, then, to very positive (ca. +100 μV K^–1) at the end of the sulfuration process. To understand the film transformations and the resulting S evolution (mainly, the cause of its negative values) some Fe films were sulfurated during some selected times and, then, quickly cooled (to prevent changes in the chemical composition of the films) from the corresponding sulfuration temperature to room temperature (RT). Chemical composition and structural analyses of the quenched samples were accomplished at RT. Using the obtained data, it was concluded that the sulfuration transforms the original Fe film into hexagonal pyrrhotite (Fe_1–<i>x</i>S^H), which partially converts to its orthorhombic (Fe_1–<i>x</i>S^O) phase through a Néel transformation. Then, both the orthorhombic and hexagonal pyrrhotites react with sulfur to form pyrite (FeS₂). These chemical transformations are accompanied by changes of the film conductivity type: from p-type (Fe) to n-type (pyrrhotites) and finally to p-type (FeS₂). The intermediate pyrrhotite phases appear to be precursors of the final pyrite phase. Results are discussed in the light of former data on sulfurated thin films and Fe bulk samples