Thiospinel

In general, spinel and other chalcogen-based thiospinels with Z=8 leads to the total unit cell content of 8 and 16 (8 a and 16 d sites) cations and 32 anions in the 32 e site corresponding to the formula A8B15O32. In spinels, the AX4 tet-rahedra share comers with BXg octahedra and the octahedra are Hnked together by sharing edges (Fig. 15.1). 

Fig. 15.1 Typical arrangement of octahedra (15d, dark) and tetrahedra (8a, light) in thiospinels structure. Balls represent Sulfur atoms in the 32e site.

The synthesis of quaternary thiospinels, like Ag2FeSn3Sg [5], Ag2MnSn3Sg [6], and Cu2NiSn3Sg [7], has been achieved by reacting metals along with sulfur powder in an evacuated silica tube. Pure phases of these compounds were obtained at 750 °C, 670 °C and 750 °C respectively. 

Apart from the above-mentioned ternary thiospinels, more complex spinels (quaternary thiospinels) belonging to the formula CugM4Sni2S32 or Cu2MSn3Sg (M = Ni, Fe, Co, Mn) are also known in the literature for Li-ion intercalation [14]. 

Recently lithium insertion in quaternary thiospinels like (Cug.giGeDg.ggjga [Fe4Sni2]i6dS32 and (Cu4.xGen3o+x)8a[Co4Sni2]i6dS32 have been reported [15, 16]. It is believed that the vacancies in the tetrahedral 8 a sites result from a topotac-tic substitution of four Cu ions by one Ge ion. 

The electrical resistivity of the Si-doped quaternary thiospinel, Cus.sSiDi.s Fe4Sni2S32 has been measured, in the temperature range 100 K to 300 K. It was found that it behaves like a semiconductor from room temperature down to 100 K. From the log p ys 1/T plot (see inset of Fig. 15.4) the band gap is found to be 0.107 eV in the temperature range (170 -300 K). The room-temperature resistivity is around 3.1x10 Q-cm (Fig. 15.4). 

These studies show that the thiospinel structure is quite flexible with opportunity for cation vacancies at the 8 a site. Our investigation on such cation-deficient thiospinels is significant in that it shows that additional vacancies are possible in the 8 a site. Most of the cation-deficient compounds known earlier (predominantly copper compounds) were obtained by extraction of Cu by using various oxidizing reagents. These studies show that such cation-deficient quaternary thiospinels can also be obtained by direct solid-state reactions. 

Behret H, Binder H, Sandstede G (1975) Electrocatalytic oxygen reduction with thiospinels and other sulphides of transition metals. Electrochim Acta 20 111-117... 

Vaughan D. J., Bnrns R. G. and Bnrns V. M. (1971). Geochemistry and bonding of thiospinel minerals. Geochim. Cosmochim. Acta. 35 365-381. 

Complexes with S-Donor, Se-Donor, or Te-Donor Ligands. The interatomic distances and valence distributions in the chromium thiospinels have been reviewed.151 Several mixed-metal sulphides, selenides, and tellurides containing chromium(m) have been reported this year, as detailed in Table 2. 

Vaughan, D. J. Craig, J. R. (1985) The crystal chemistry of iron-nickel thiospinels. Amer. Mineral., 70, 1036-43. 

Vaughan, D. J. Tossell, J. A. (1980b) Electronic structures of thiospinel minerals Results from MO calculations. Amer. Mineral., 66, 1250-3. 

Thiospinel group Spinel-type (cubic) Linnaeite (CojS ), polydymite (NiiSJ, greigite (Fe,S4) violarite (FeNi2S4), etc. 

Table 6.5. Electrical and magnetic properties" of thiospinel minerals... 

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