Quansheng Guo

Crystal structure, electronic structure and physical properties of the new quaternary chalcogenides Tl2NdAg3Se4 and Tl2NdAg3Te4

The chalcogenides Tl2NdAg3Q4 (Q = Se, Te) were prepared by heating the elements in stoichiometric ratios under exclusion of air between 973 K and 1073 K. Both compounds adopt new structure types, with Tl2NdAg3Se4 crystallizing in the space group Pnma with a = 15.9937(14) A, b = 4.3420(4) A, c = 14.3676(12) A, V = 997.75(15) A3, Z = 4, while Tl2NdAg3−xTe4 adopts a different structure with related motifs, crystallizing in the noncentrosymmetric space group Cmc21 with a = 4.5968(2) A, b = 16.8620(6) A, c = 44.7131(15) A, V = 3465.8(2) A3, Z = 12, when x = 0.05. In both cases, linear chains of [AgQ4] tetrahedra are interconnected via common corners and edges to build a three-dimensional network, which encompasses the Tl and Nd atoms in linear channels. The differences lie in their three-dimensional connection, resulting in a larger, noncentrosymmetric unit cell of the telluride. Electronic structure calculations indicate semiconducting properties, in accord with the transport property measurements in both cases. A special feature of both materials is the extraordinarily low thermal conductivity.

Thermoelectric properties of Sn- and Pb-doped Tl9BiTe6 and Tl9SbTe6

A variety of substitutions in Tl9BiTe6 and Tl9SbTe6 with Sn and Pb, amounting to 14 different samples, were performed by melting the stoichiometric amounts of elements at 923 K, followed by slow cooling. The pulverized powders were sintered using the hot-pressing technique. All samples were of single phase according to the powder X-ray diffraction patterns. Thermoelectric property measurements were performed to investigate the effects of Sn- and Pb-doping on the electrical conductivity, Seebeck coefficient, and thermal conductivity. Increasing the concentration of the dopants caused increases in electrical and thermal conductivity, while decreasing the Seebeck coefficient. Tl9Bi0.90Pb0.10Te6 and Tl9Bi0.85Pb0.15Te6 exhibited the highest power factor. The changes in lattice thermal conductivity were minor and did not follow a clear trend. Competitive ZT values were obtained for Tl9Bi0.95Sn0.05Te6, Tl9Bi0.95Pb0.05Te6, Tl9Sb0.97Sn0.03Te6, and Tl9Sb0.95Pb0.05Te6, namely 0.95, 0.94, 0.83, and 0.71 around 500 K,...