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1 Department of Mineralogy, South Australian Museum, North Terrace, Adelaide, South Australia 5000, Australia
2 School of Chemistry, Physics and Earth Sciences, The Flinders University of South Australia, GPO Box 2100 Adelaide, South Australia 5000, Australia
3 Dipartimento di Scienze della Terra, Università di Pavia, Via Ferrata 1, 27100 Pavia, Italy
4 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, U.K.
5 Research School of Chemistry, Australian National University, Canberra, ACT, 0200, Australia
Correspondence: * E-mail: pring.allan{at}saugov.sa.gov.au
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ABSTRACT |
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x 0.24, were examined. The IR spectrum of pure ZnS contains a single strong absorption band at 320 cm–1. With addition of FeS, the spectra become broader and shoulders appear. For compositions 
9 mol% FeS, a splitting of the main peak occurs, and the spectra show two absorption maxima at approximately 300 and 315 cm–1, respectively. The observation of such extra features does not correspond to the usual behavior observed in other ternary mixed crystals, where either one-, two-, or mixed-mode behavior is observed. The spectra can be deconvoluted into up to three peaks, main Peaks A and B at around 300 and 315 cm–1, respectively, and a shoulder at around 330 cm–1 (Peak C). The positions and area of the peaks do not change significantly with increasing Fe content. The peak at 315 cm–1 is the main absorption peak of the host ZnS structure, and the peak at 300 cm–1 is an impurity induced mode. An effective linewidth parameter 
corr was determined by autocorrelation analysis for each spectrum, but there are no obvious trends in the values of corr that can be interpreted in terms of an inhomogeneous distribution of Fe within the sphalerite structure.
Key Words: Fe-bearing sphalerites • infrared spectroscopy • autocorrelation analysis • solid solution
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INTRODUCTION |
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The extent as well as the mechanism underlying Fe substitution into sphalerite is therefore of great interest both from the point of view of economic geology and of materials technology (Lepetit et al. 2003; Bente and Doering 1993, 1995; Barton and Toulmin 1966; Barton and Bethke 1987; Di Benedetto et al. 2005a, 2005b). Phase equilibrium studies on the Fe-Zn-S system have thus been of considerable importance and early studies include Kullerud (1953), Barton and Kullerud (1958), and Barton and Toulmin (1966). These authors noted the importance of temperature and S fugacity on the extent of solubility. Lepetit et al. (2003) found that the upper solubility limit of Fe in ZnS at 700 °C ranged from 21 mol%, for a low sulfur fugacity up to as high as 52 mol% for a high-sulfur fugacity.
If sphalerite is formed in equilibrium with a suitable buffering mineral assemblage such as pyrrhotite and pyrite, then the Fe content can be used as a geothermometer (Scott 1973; Lusk et al. 1993). Several authors, however, have pointed out that great care must be exercised in using this geobarometer because of compositional and textural readjustments in sulfides (Toulmin et al. 1991; Lepetit et al. 2003).
The nature of the substitution of Fe and other cations for Zn in sphalerite has been probed using a wide variety of techniques. The relationship between the cubic unit-cell edge and Fe content was found to be linear by Skinner et al. (1959), but later workers reported non-linear behavior as a function of Fe content above ~10 mol%, which has been correlated with variable sulfur fugacities (Barton and Toulmin 1966; Lepetit et al. 2003). Higher sulfur fugacities in turn have been correlated with increased Fe3+ (and associated metal vacancy) concentrations at fixed Zn/Fe ratios. The Mössbauer spectra of Fe-containing sphalerites are known to change from a singlet to a doublet for Fe contents > ~6 mol% (Keys et al. 1968; Gerard et al. 1971; Di Benedetto et al. 2005b). This has been attributed to the onset of Fe-Fe bonding interactions and local Fe clustering above a critical Fe content, and has lead to speculation about possible "ordering phenomena depending on the Fe content" in Fe-bearing sphalerites (Lepetit et al. 2003).
Furdyna (1988) and Twardowksi (1990) discussed possible inhomogeneous distributions of paramagnetic cations replacing Zn in sphalerite, Balabin and Sack (2000) used cluster variation method (CVM) calculations to suggest thermodynamic stabilization of pairs and small clusters of Fe, in agreement with Twardowski et al. (1988). Withers et al. (2005) found no evidence for clustering using diffuse scattering, instead they observed structured diffuse scattering associated with the excitation of low-energy phonon modes. Di Benedetto et al. (2005b) found that magnetic susceptibility measurements at low temperature indicated the presence of clustered Fe even in the most dilute compositions.
The absorption bands in IR spectra are highly sensitive to structural variations, and substitution or partial ordering in a solid-solution system usually introduces local strains and structural relaxations. These distortions could be reflected in spectral line shifts and broadenings in the IR spectra. The method can be applied even to complex IR spectra containing broad peak overlapping, as seen in many perovskite-related phases (Meyer et al. 2002; Tenailleau et al. 2005). The method uses an autocorrelation function, defined as
 | (1) |
where 
(
) is the spectrum itself and ( + ') is the same spectrum offset in frequency by ', to extract and analyze spectral changes in a quantitative manner from complex infrared spectra (Salje et al. 2000). In theory, any peak position and/or width variation can be associated with individual lattice modes, polyhedral distortions, and/or local heterogeneities in the structure. The spectroscopic data obtained can then be compared with structural properties, such as lattice parameter evolution as a function of composition and macroscopic strain arising from composition-induced structural phase transitions. The purpose of the present study was to investigate the structural evolution of the substitution of Fe into ZnS on a microscopic length scale using hard-mode infrared (IR) powder-absorption spectroscopy. We use the autocorrelation analysis method to look for evidence for Fe clustering.
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SAMPLES AND EXPERIMENTAL METHODS |
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TopAbstractIntroductionSamples and experimental methodsResultsDiscussionAcknowledgmentsReferences cited |
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and 2). All samples lie close to the ZnS-FeS join. Great care was taken in selecting the natural samples. Over 100 specimens were initially examined by powder X-ray diffraction to establish their purity and approximate compositions from the cell parameters. Samples containing significant quantities (>1 mol%) of other substituting ions were excluded as were samples showing compositional zoning or microscopic inclusions of other minerals such as chalcopyrite. Compositional data for the natural specimens were obtained using a Cameca CAMEBAX SX51 electron microprobe at Adelaide Microscopy, University of Adelaide. The analyses were undertaken using an accelerating potential of 20 kV and a specimen current of ~20 nA. The spot size was set at 1 µm, but the effective resolution of the beam due to beam spread in the sample was of the order of 3 m. The following standards were used for wavelength dispersive analysis: natural pentlandite (Ni, Fe), natural sphalerite (Zn, S), natural cobaltite (Co), natural rhodonite (Mn), natural chalcopyrite (Cu), and Cd metal (Cd).
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X-ray powder diffraction patterns of the natural samples were obtained using a Huber HTC 9634 X-ray diffractometer with CoK1 radiation (
= 1.78897 Å). The cell parameters were determined by Rietveld refinement (Rietveld 1969; Hunter 1998). The cell parameters of the synthetic samples were determined by X-ray powder diffraction using a Guinier-Hägg XRD camera with CuK1 radiation. Silicon (NBS no.640) was added as an internal standard for accurate determination of the unit-cell dimensions, which were refined using the least squares method.
A total of 20 pellets for IR powder absorption spectroscopy were prepared with care following a method developed previously (Zhang et al. 1996; Blanch et al. 2007) to obtain the best spectra for autocorrelation analysis. Initially, around 50 mg of each of the samples were ground separately for 5 min. After testing several potential sample grinding times and dilutions with CsI as a matrix, about 2 mg of the ground samples and ~600 mg of CsI (sample:matrix ratio of 1:300) were then mixed and pressed into pellets weighing ~300 mg. The mixtures were left for 7 min in a vacuum press before 13 mm diameter pellets were created under 10 tons of pressure.
A reference IR spectrum of CsI was run prior to collecting a primary IR spectrum (512 scans per spectrum) for each sample. Each spectrum was then recorded as absorbance under vacuum at room temperature at an instrumental resolution of 2 cm–1 using a Bruker IFS 113v FT-IR spectrometer with a DTGS detector in the region 100–700 cm–1.
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RESULTS |
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. Although minor amounts of Cd and Mn were detected in most samples, the compositions all lie very close to the ZnS-FeS join. The Fe content of the samples ranges from effectively zero (0.1 mol%) up to a maximum of 22 mol%. Cell dimension data for the natural and synthetic samples are summarized in Table 2 and plotted against composition in Figure 1. Also superimposed on Figure 1 is the trend from Barton and Toulmin (1966). In general, the measured cell dimensions are in good agreement with this trend. Some of the natural samples plot slightly above the trend. This probably reflects the effects of low 
S on the sample during crystallization as increasing S leads to the oxidation of Fe2+ to Fe3+ and the simultaneous formation of vacancies on metal sites.
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x 0.24, in the far-IR region are shown in Figures 2 and 3, respectively. The observed IR absorption spectrum from the synthetic ZnS sample, depicted in Figure 3, shows a single strong absorbance band centered at ca. 320 cm–1. With the introduction of Fe, the spectrum acquires new features. The center of the main peak seems to slightly shift to lower frequency, but intensity is retained at the position of the peak in pure ZnS. The spectra also exhibit a shoulder at around 330 cm–1. For more Fe-rich compositions ( 9 mol% FeS), a splitting of the main peak occurs, and the spectra show two absorption maxima at approximately 300 and 315 cm–1, labeled A and B, respectively. The position of Peak B seems to stay close to the position of pure ZnS across the whole composition range. With further increase in the FeS content, the relative intensities of the two peaks also change and Peak B becomes more intense. The described spectral variations seem to develop in a fairly continuous manner. It is worth noting also that for compositions 9 mol% FeS there is a marked change in the macroscopic appearance of the sphalerites, i.e., the luster changes from adamantine to semi-metallic. On careful examination of the spectra at higher wavenumbers, it is possible to identify the presence of two weak peaks at 610 and 635 cm–1, respectively. These peaks might correspond to second-order effects that have been already observed in the spectrum of pure ZnS by Nilsen (1969) and assigned to TO overtone and TO-LO combination bands.
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Peak fitting
The broad absorption band at around 300 cm–1 was deconvoluted into three separate individual peaks for each spectra (see Fig. 4) and fitted using Voigt profiles with the MultiPeakFitting routine from IGOR Pro software (Wavemetrics, Inc., Oregon). Figure 5 shows the variation in the measured frequencies of the three peaks plotted against the mole fraction of Fe. It is worth noting that while there is some scatter in the position of Peak A, the position of Peak B at different XFe is consistent between natural and synthetic suites of samples. As evident from Figure 5 the measured frequencies of the A and B peaks slightly shift with composition as the FeS content increases to 24 mol% although variations are quite small (less than 4 cm–1). In their Raman study on the (Fe,Zn)S system, Jiménez-Sandoval et al. (2003) observed no peak shift and therefore spoke of a "frozen mode," but they examined only two compositions. The ratios of peak amplitudes between Peaks A and B, and between Peaks C and B are summarized in Table 2 and plotted in Figure 6. With increasing FeS content, the relative intensities of Peak A and of the Peak C seem to decrease, as already observed simply from visual inspection of the spectra.
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corr was determined by autocorrelation analysis (Salje et al. 2000; Meyer et al. 2002). It is proportional to the average width of the peak(s) in the chosen segment of the primary spectrum. After subtracting a baseline for the spectral region analyzed an autocorrelation spectrum was calculated from each primary spectrum. A Gaussian curve, G, was used to fit to the central peak of the autocorrelation spectrum:
 | (2) |
where k2 is proportional to the linewidth. corr was then obtained by extrapolation of k2 vs. to the origin (' = 0 in Eq. 1). The values of corr calculated in the range 130–450 cm–1 for the natural and synthetic samples are listed in Table 2 and plotted against FeS content together with the average of the full width at half maximum values from fitting three overlapping peaks in the same range in Figure 7. As evident from a simple visual inspection of the spectra, addition of a very small amount of FeS to the sphalerite induces a large increase in average linewidth. The values of corr for the natural samples and for the synthetic samples show a wide scatter, but those of the natural samples are generally 10% larger than those of the synthetic suite.
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corr can be associated with strain in the lattice, however, there are obvious differences between the data for the series of natural and synthetic Fe-sphalerites and particularly the scatter in the data. These differences are considerably larger than have been reported in other autocorrelation infrared studies on oxysalts (Boffa Ballaran et al. 1999, 2001; Tenailleau et al. 2005; Meyer et al. 2002; Tarantino et al. 2002, 2003, 2005). The autocorrelation calculations were carefully checked, and we are confident that the scatter is not an artifact of the method. However, it is worth noting that the form of corr variation and that of the average linewidths show similar trends. |
DISCUSSION |
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, shows a single strong absorbance band centered at ca. 320 cm–1. Such a shift of the powder absorption peak toward higher energies (i.e., toward the LO position) with respect to the reported TO frequency of 271 cm–1 is expected considering the large TO-LO splitting in ZnS (Piriou 1974) and the medium effect of the matrix. With addition of Fe, the sphalerite spectra become more complex and extra features appear. Historically, two limits of phonon mode behavior have been distinguished in spectra from samples forming solid solutions. In the one-mode case, samples of intermediate compositions behave like a pure crystal, and the zone-center optical phonon frequencies vary continuously and approximately linearly with concentration between the frequencies of the two end-members. In two-mode behavior, each TO-LO phonon mode degenerates to an impurity mode of the other end-member. The intensity of each mode scales with the concentration of its parent compound.
The vibrational spectra of solid solutions of II–VI and III–V semiconductors have been extensively analyzed in the literature and show various phenomena. By applying the criterion of Chang and Mitra (1968) to predict the occurrence of two-mode behavior, based on the difference of reduced masses between the two end-members, it is found that the system FexZn1–xS (µZnS = 21.51 amu; µFeS = 20.37 amu) should exhibit one-mode behavior. However, the observed spectra show impurity-induced peaks, even at very low Fe content, and neither two-mode or mixed mode behavior seems to occur. It is worth noting that the rationalization of the vibrational modes of mixed ternary crystals was based on systems in which the two end-members have the same structure and similar force constants. This is not the case along the ZnS-FeS join, as there is no zinc-blended form of FeS. The presence of impurity-induced modes in the vibrational spectra from doped cubic ZnS has been observed by Król et al. (1978), Zigone et al. (1981), and Jiménez-Sandoval et al. (2003) by means of Raman scattering. These authors found that new modes with frequencies in the range between those of the TO and LO modes appeared in the spectra with the addition of small quantities (ca. 1%) of impurities such as Cr, Mn, Fe, Co, and Ni. For Fe-doped samples, Zigone et al. (1981) and Jiménez-Sandoval et al. (2003) observed three lines at ca. 300, 312, and 332 cm–1, respectively. The positions of Peaks A and C are in good agreement with the positions of the two impurity peaks of symmetry F that Zigone et al. (1981) detected and assigned to resonant modes. The position of Peak B, as already observed above, is close to the position of the main peak of the pure ZnS sample and its variation with increasing FeS is consistent between natural and synthetic suites of samples. This reinforces the proposition that Peak B is the main absorption peak of the host ZnS structure while Peak A, though seemingly more intense at low iron content, might be due to an impurity induced mode. Peak A is also considerably broader than Peak B, as expected for an impurity induced resonant mode.
Zigone et al. (1981) observed that the positions of the impurity-induced modes in sphalerite varied with changing dopant, and therefore it might be reasonable to expect the presence of further bands at different wavenumbers. The broad peak at ca. 200 cm–1 in the spectra from the natural suite of samples, might be due to the presence of impurities.
Is the pattern of line broadening revealed by autocorrelation perhaps indicative of some clustering of Fe atoms in the synthetic samples? The peak positions for both synthetic and natural samples vary smoothly with FeS content and do not show the same scattered pattern of corr, which could be reasonably expected in the presence of clustering. The position of Peak B in particular shows an increase in wavenumber with increasing FeS content up to 10 12 mol% and then remains almost constant across the analyzed compositions. Information about the macroscopic volume of mixing behavior should be reflected in the phonon wavenumber systematics. Both the unit-cell dimension and wavenumber shifts of the IR bands appear to vary in a continuous and, essentially, linear manner upon Fe-Zn exchange. No major structural breaks have been observed either at the X-ray or phonon length scales.
Impurity-induced modes have been attributed by Król et al. (1978) and Zigone et al. (1981) to resonance bands due to a change in the force constant of the sulfur impurity bond. A resonance band mode may be considered as the usual vibration of the defect (Hayes and Loudon 1978). It is very intense in the vicinity of the defect but then fades and appears as broad peaks. The extra modes in our spectra (Figs. 2 and 3) appear at very diluted compositions and are fairly broad. Furthermore their intensities, as evident from Figure 6, do not increase with increasing FeS content, thus suggesting that their presence cannot be related to a short-range ordering process. Nonetheless broad impurity-induced modes could well lead to a wide variation in the effective linewidth corr. This is fairly evident in the spectra from the natural samples, the presence of the impurity induced bands at 200 cm–1 might account for the larger corr values than those of the synthetic suite. Spectra from samples with low FeS (up to 4% mol FeS) content show on the low energy side an asymmetry, which decreases with increasing FeS and this might have contributed to slightly increase the linewidth. The shoulder of Peak B (Peak C) is slightly more evident in samples with low FeS content and then again in the two highly doped specimens.
From visual inspection of the spectra as well as from peak fitting it seems that with increasing FeS, the linewidth of Peak A slightly decreases and then remains quite constant. It is therefore necessary to be cautious in interpreting changes in linewidth, but it has been possible to extract a limited amount of quantitative information from the spectra. Differences in the variation of corr might be related to how the structure accommodates Fe-Zn substitution at the length scale sampled by IR spectra. As discussed elsewhere (Salje 1992; Carpenter and Boffa Ballaran 2001), the characteristic length scales of phonons are not known precisely, but should vary with 1/ where is the wavenumber. Therefore the small line broadening of IR absorption bands might suggest that substitution of Fe for Zn actually causes very little structural distortion of the long-range sphalerite structure. Crystal-chemical considerations also indicate that there should be little strain associated with the substitution of Fe2+ for Zn2+ in the sphalerite structure. Fe2+ is only 5% larger than Zn2+ [ionic radii 0.63 and 0.6 Å for Fe2+ and Zn2+ in tetrahedral coordination (Shannon 1976)], and the S2– anion is considerably larger and more polarizable ("softer") than the O2– anion so the strain associated with substitution of cations of different size should be less significant than they would be in an oxysalt. This is also reflected in the bond lengths. Iwanowski et al. (1998) determined, from EXAFS data, that the individual Fe-S and Zn-S bond lengths remain constant across the solid solution and differ by only ~0.5%: Fe-S = ca. 2.357 Å, Zn-S = ca. 2.345 Å. These values are also in accord with those calculated from the unit-cell parameters in the limit of the Vegard’s law model.
This is all at least consistent with the view that an important structural response of the sphalerite structure to exchange of Zn for Fe involves the maintenance of individual tetrahedra more or less as they are in the end-member phase and with the actual presence of an Fe-induced change in the bond force constant which causes the extra peaks. It was noted by Withers et al. (2005) that the frequency of the phonon modes they observed by diffuse scattering, due to vibrations of tetrahedral linkages, do not change with Fe content.
A random distribution of cations, giving a bimodal distribution of bond lengths in zinc-blende-type solid solutions, typical of several pseudobinary semiconductor alloys, have been rationalized in terms of distortions of the anions sublattice and proved to be in good agreement with EXAFS data for (Cd,Mn)Te, (In,Ga)As (Balzarotti et al. 1985), and (Cd,Zn)Te (Koteski et al. 2004). Thus, while we remain uncertain on the origin of the line broadening we have observed, our preferred hypothesis is that the variation of corr with composition is not associated with Fe clustering but is due to the effects of dilute impurity modes.
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ACKNOWLEDGMENTS |
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Footnotes |
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MANUSCRIPT RECEIVED February 15, 2007; MANUSCRIPT ACCEPTED November 26, 2007
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2 = 0.004.
