This review discusses about the crystal structure, chemical bonding, and the electronic band structure of tetrahedrite materials. Also, this review outlines the effect of different doping elements on the thermoelectric properties of tetrahedrite materials.

Tetrahedrite compounds Cu(12-x)Mn(x)Sb4S13 (0 ≤x≤ 1.8) were prepared by solid state synthesis. A detailed crystal structure analysis of Cu10.6Mn1.4Sb4S13 was performed by single crystal X-ray diffraction (XRD) at 100, 200 and 300 K confirming the noncentrosymmetric structure (space group I4[combining macron]3m) of a tetrahedrite. The large atomic displacement parameter of the Cu2 atoms was described by splitting the 12e site into a partially and randomly occupied 24g site (Cu22) in addition to the regular 12e site (Cu21), suggesting a mix of dynamic and static off-plane Cu2 atom disorder. Rietveld powder XRD pattern and electron probe microanalysis revealed that all the Mn substituted samples showed a single tetrahedrite phase. The electrical resistivity increased with increasing Mn due to substitution of Mn(2+) at the Cu(1+) site. The positive Seebeck coefficient for all samples indicates that the dominant carriers are holes. Even though the thermal conductivity decreased as a function of increasing Mn, the thermoelectric figure of merit ZT decreased, because the decrease of the power factor is stronger than the decrease of the thermal conductivity. The maximum ZT = 0.76 at 623 K is obtained for Cu12Sb4S13. The coefficient of thermal expansion 13.5 ± 0.1 × 10(-6) K(-1) is obtained in the temperature range from 460 K to 670 K for Cu10.2Mn1.8Sb4S13. The Debye temperature, Θ(D) = 244 K for Cu10.2Mn1.8Sb4S13, was estimated from an evaluation of the elastic properties. The effective paramagnetic moment 7.45 μB/f.u. for Cu10.2Mn1.8Sb4S13 is fairly consistent with a high spin 3d(5) ground state of Mn.

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Electrical resistivity (ρ), magnetoresistance (MR), magnetization, thermopower and Hall effect measurements on the single crystal Gd2PdSi3, crystallizing in an AlB2-derived hexagonal structure are reported. The well-defined minimum in ρ at a temperature above Néel temperature (TN = 21 K) and large negative MR below ∼ 3TN , reported earlier for the polycrystals, are reproducible even in single crystals. Such features are generally uncharacteristic of Gd alloys. In addition, we also found interesting features in other data, e.g., twostep first-order-like metamagnetic transitions for the magnetic field along [0001] direction. The alloy exhibits anisotropy in all these properties, though Gd is a S-state ion.The observation of large negative magnetoresistance (MR) above respective magnetic transition temperatures in some polycrystalline Gd (also Tb, Dy) alloys is of considerable interest. 1-5 Among the Gd alloys, we have studied the transport properties of the compound, Gd 2 PdSi 3 , crystallizing in an AlB 2 -type structure, which has been found to show unusual nature. While this compound orders antiferromagnetically at (T N =) 21 K, there is unexpectedly a distinct minimum in the temperature dependent electrical resistivity (ρ) at about 45 K. This minimum disappears by the application of a magnetic field (H), thereby resulting in large MR in the vicinity of T N [Ref. 3]. These properties are also characteristic of Ce/U-based Kondo lattices, but uncharacteristic of Gd systems, considering that the Gd-4f orbital is so deeply localised that it cannot exhibit the Kondo effect. Though magnetic-polaronic effect (even in metallic environments) has been proposed in references 1-5 as one of possible mechanisms behind this large MR, its origin is not clear yet. It, however, appears that short-range correlation as a magnetic precursor effect may be the primary ingredient 5 for the origin of the resistivity minimum above T N and negative MR. The importance of such findings is obvious from similar recent reports from other groups 6-10 and among these the observation of ρ minimum and resultant colossal magnetoresistance (CMR) in a pyrochlore-based oxide, Tl 2 Mn 2 O 7 [Ref. 7,8], has attracted recent attention. In view of the importance of the observations on polycystals of this Gd compound, we considered it important to confirm the findings on the single crystals. With this primary motivation, we have investigated ρ, MR, thermopower (S), Hall-effect and magnetization behavior on single crystals of Gd 2 PdSi 3 , and these results are presented in this article.Single crystals of Gd 2 PdSi 3 have been prepared by the Czochralsky pulling method using a tetra-arc furnace in an argon atmosphere. The single-crystalline nature has been confirmed using back scattering x-ray technique. The ρ, MR and Hall effect (employing a magnetic field of 15 kOe) measurements have been performed by a conventional DC four-probe method down to 1.2 K; the MR and Hall effect measurements have also been performed as a function of H at 4.2 K. The magnetic m...