Abstract
The oxidation state and local atomic environment of admixtures of In, Cu, and Ag in synthetic sphalerite crystals were determined by X-ray absorption spectroscopy (XAS). The sphalerite crystals doped with In, Cu, Ag, In–Cu, and In–Ag were synthesized utilizing gas transport, salt flux, and dry synthesis techniques. Oxidation states of dopants were determined using X-ray absorption near edge structure (XANES) technique. The local atomic structure was studied by X-ray absorption fine structure spectroscopy (EXAFS). The spectra were recorded at Zn, In, Ag, and Cu K-edges. In all studied samples, In was in the 3+ oxidation state and replaced Zn in the structure of sphalerite, which occurs with the expansion of the nearest coordination shells due to the large In ionic radius. In the presence of In, the oxidation state of Cu and Ag is 1+, and both metals can form an isomorphous solid solution where they substitute for Zn according to the coupled substitution scheme 2Zn2+ ↔ Me+ + In3+. Moreover, Ag K-edges EXAFS spectra fitting, combined with the results obtained for In- and Au-bearing sphalerite shows that the Me-S distances in the first coordination shell in the solid solution state are correlated with the ionic radii and increase in the order of Cu < Ag < Au. The distortion of the atomic structure increases in the same order. The distant (second and third) coordination shells of Cu and Ag in sphalerite are split into two subshells, and the splitting is more pronounced for Ag. Analysis of the EXAFS spectra, coupled with the results of DFT (Density Function Theory) simulations, showed that the In–In and Me+–In3+ clustering is absent when the metals are present in the sphalerite solid solution. Therefore, all studied admixtures (In, Cu, Ag), as well as Au, are randomly distributed in the matrix of sphalerite, where the concentration of the elements in the “invisible” form can reach a few tens wt.%.
Highlights
Sphalerite ZnS is a unique mineral which can reach high concentrations of elements that are in high demand in the hi-tech industry
23 ± 1 gas transport method, synthesis at 850/750 ◦ C; b salt flux method (KCl/NaCl eutectic mixture), synthesis at 790/730 ◦ C; c dry synthesis method synthesis at 550 ◦ C; d bdl—below the limit of detection; dash—not measured; for Samples 3757 and 4186, the amount of synthesis products was insufficient to perform chemical analyses by means of electron probe micro-analysis (EPMA) and LA-ICP-MS; only X-ray diffraction (XRD) data are available for these samples
Ag synthesis products was insufficient to perform chemical analyses by means of EPMA and LA-ICP-MS; only XRD data are available for these samples
Summary
Sphalerite ZnS is a unique mineral which can reach high concentrations of elements that are in high demand in the hi-tech industry. These admixtures include “critical” metals In, Cd, Ga, and Ge. Indium, being a rare element in the Earth’s crust, is not concentrated enough to form its ore minerals, but it can be recovered as a byproduct during the refinement of Zn ores, where it is contained in sphalerite. X-ray spectroscopy studies of In’s charge state and its local atomic structure were presented in our previous study [1]. The concentration of In is directly correlated to the concentration of Cu [2,3].
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