MOSSBAUER STUDY OF Fe-BEARING SYNTHETIC TENNANTITE

Abstract

The valence of iron in synthetic Fe-bearing tennantite was investigated by the Mössbauer method. Tennantite compositions were weighed out in the reference system Cu 12 As 4 S 13-Cu 14 As 4 S 13-Cu 10 Fe 2 As 4 S 13 at the levels with 0.0, 0.5, 1.0, 1.5 and 1.7 Fe apfu. The tennantite compositions obtained were characterized by electron-microprobe analysis and powder-diffraction data. Compositions with a low-level substitution of iron for copper contain Fe 3+. Evidence of divalent iron appears at the level of 1.0 Fe apfu, and for Cu-rich charges, even at 0.5 Fe apfu. It becomes dominant in Cu-rich tennantite at ~1.0 Fe apfu, whereas in Cupoor tennantite, only at ~1.4 Fe apfu. The balance of the iron is in both cases represented by mixed-valence iron, with the values of isomer-shift and quadrupole splitting intermediate between those for Fe 2+ and Fe 3+. Effects of electron delocalization and net charge-transfer in superexchange interactions on the effective valence of iron were modeled using the relation between isomer shift and valence proposed by Goodenough & Fatseas for sulfides.

Role of iron in synthetic tetrahedrites revisited

“…Each of those can be described as a superposition of two quadrupole doublets, Fe1 and Fe2, characterized by different values of isomer shifts δ and quadrupole splitting Δ, thus corresponding to two types of iron atoms. The minor component Fe1, which provides about 15% of the spectral area, exhibits the average values of the isomer shift, < δ 1 >  0.17 mm/s, expected for the high-spin Fe 3+ cation in environment of sulfur atoms, though its quadrupole splitting < Δ 1 >  0.69 mm/s (Table 4) is quite high for the symmetric sulfur coordination [11,21]. At the same time, the major Fe2 component of the spectra (I 2  85%) has a noticeably larger isomer shift than typically observed for Fe 3+ sulfides (0.15-0.25 mm/s), whereas its quadrupole splitting is typical for the high-spin Fe 3+ in symmetric tetrahedral environment [11,22].…”

“…The minor component Fe1, which provides about 15% of the spectral area, exhibits the average values of the isomer shift, < δ 1 >  0.17 mm/s, expected for the high-spin Fe 3+ cation in environment of sulfur atoms, though its quadrupole splitting < Δ 1 >  0.69 mm/s (Table 4) is quite high for the symmetric sulfur coordination [11,21]. At the same time, the major Fe2 component of the spectra (I 2  85%) has a noticeably larger isomer shift than typically observed for Fe 3+ sulfides (0.15-0.25 mm/s), whereas its quadrupole splitting is typical for the high-spin Fe 3+ in symmetric tetrahedral environment [11,22]. Although the hyperfine parameters for both components of the spectra could be crudely ascribed to Fe 3+ , we note that these parameters are not typical for the high-spin Fe 3+ in symmetrical tetrahedral environment of four sulfur atoms.…”

“…Both Fe2 and Fe3 doublets have very broad components. Such a broadening can be related to the relaxation effects due to electron hopping [11]. The relaxation process is most obvious in the case of Cu 10.5 Fe 1.5 Sb 4 S 13 (Figure 5), where the Fe2 doublet and one of the Fe3 lines are not well resolved, and a continuous absorption appears between the right-hand Fe2…”

“…Whereas all other substituting 3d metals, including manganese, form the M 2+ cations, the oxidation state of iron depends on the level of substitution [10]. It is generally believed [8,10,11] that in the Cu 12-x Fe x Sb 4 S 13 series, Fe 3+ forms until x reaches 1, while Fe 3+ and Fe 2+ coexist up to the maximum doping level of x = 2, at which Fe 2+ is present exclusively. According to this model, one assumes that the parent compound Cu 12 Sb 4 S 13 , formulated as (Cu + ) 10 (Cu 2+ ) 2 (Sb 3+ ) 4 (S 2-) 13 , transforms into (Cu + ) 11 (Cu 2+ ) 1 (Sb 3+ ) 4 (S 2-) 13 upon substitution of one iron atom for copper with a concomitant change in the oxidation state of one copper atom from +2 to +1.…”

“…The analysis of the literature data on the crystal structure and various properties of pristine and Fe-substituted tetrahedrites [8][9][10][11][12][13][14][15][16] makes us believe that the above description is unrealistic or, at least, oversimplified. In particular, the cubic unit cell parameter increases linearly with iron content, though Fe 3+ is smaller than copper for which it substitutes [8,9], and the hyperfine parameters of Mössbauer spectra do not match well with the values expected for high-spin iron tetrahedrally coordinated by sulfur atoms [10,11]. Taking these considerations into account, we have undertaken a comprehensive study of Fe-substituted tetrahedrites by means of X-ray crystallography, Mössbauer spectroscopy, and magnetization measurements in order to reexamine the role and the valence state of iron in the Cu 12-x Fe x Sb 4 S 13 compounds.…”