The State of Trace Elements (In, Cu, Ag) in Sphalerite Studied by X-Ray Absorption Spectroscopy of Synthetic Minerals

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.%.

Probing the Local Atomic Structure of In and Cu in Sphalerite by XAS Spectroscopy Enhanced by Reverse Monte Carlo Algorithm

“…The methods of synthesis of the doped sphalerites and reference materials, analytical methods, and the methods of XAS measurements and preliminary spectra treatment (normalization, background removing) are described in detail in our previous publication (Section 2 in Trofimov et al [2], this issue). The sample IDs are identical throughout both the contributions.…”

“…In our previous studies [1,2], it was demonstrated by the analysis of the X-ray absorption spectra of In-Cu-bearing sphalerites that the formation of the sphalerite solid solution takes place according to the charge compensation schemes: 3Zn 2+ ↔2In 3+ + and 2Zn 2+ ↔In 3+ + Cu + . In the present study, the formation energy of In-and In-Cu-bearing sphalerite solid solutions was calculated by means of Density Functional Theory (DFT).…”

“…In the present work, we investigated three samples of synthetic sphalerite. The compositions of the samples are given in Table 1 in Trofimov et al [2]. One sample was doped with one element-In (Sample 3757-0.7 wt.% In), and the other two samples were doped with two elements-In + Cu (Sample 4108-0.1 wt.…”

“…The detected intensity was normalized to the incident photon flux. Normalization and extraction of the χ(k) signal were performed using the ATHENA program (IFEFFIT software package [3], see details in Trofimov et al [2]).…”

“…In our previous works [1,2], we studied the local atomic environment and electronic state of dopants (In, Cu, Ag, Au, Cd) in natural and synthetic ZnS crystals by means of X-ray absorption spectroscopy (XAS). Analysis of the X-ray absorption near edge structure (XANES)/Extended X-ray absorption fine structure (EXAFS) spectra revealed that in substitutes for Zn in the structure of sphalerite.…”