Transition metal incorporation into mackinawite (tetragonal FeS)

Abstract

Naturally occurring mackinawite (tetragonal FeS) with incorporated transition metals is an important precursor to the formation of metal sulfides in ore deposits, but experimental results have not been sufficient to establish clear trends in the structure and stability of the transition-metal-enriched mineral. Using density functional theory with dispersion corrections, we report the first systematic examination of the relationship between composition and structure for FeS incorporating bivalent transition metals. Our method was validated by successful calculations of the structures of FeS, FeSe, FeSe 1-x S x , Fe 1-x Me x Se (Me = Co, Ni, Cu), and FeNi x Te. Two classes of transition-metal-incorporated FeS structures then were investigated: Fe 1-x Me x S (metal-substituted FeS) and FeMe x S (FeS intercalated by a metal at either a tetrahedral or square-pyramidal interstitial site), where Me = Co, Ni, and Cu, and x ≤ 0.25. We find that incorporated transition metals can both increase and decrease lattice parameters, depending on the metal and how it is incorporated into the FeS structure. As the mole fraction of substituting metal increases, the FeS unit-cell volume decreases for Co, is nearly constant for Ni, but increases for Cu. Metal substitution changes the c lattice parameter, which is sensitive to interactions between the mackinawite sheets, as much as it does the a and b lattice parameters. Upon intercalation, the unit-cell volume and c parameter increase but the a and b parameters decrease. Experimental structural data are consistent with our results for metal-substituted FeS. We determined the thermodynamic stability of metal-incorporated FeS by computing the free energy involved in the metal incorporation reactions as a function of chemical potential of sulfur. The thermodynamic results lead to the general conclusions that metal incorporation into mackinawite most likely occurs via substitution, which may be important to influence phase transformation pathways of mackinawite.