Crystal structure, electronic structure and physical properties of the new low-valent thallium silicon telluride Tl6Si2Te6 in comparison to Tl6Ge2Te6

摘要

The title compounds were prepared from the elements in the stoichiometric ratio at 800 degrees C under exclusion of air. Tl6Si2Te6 crystallizes in the space group PT, isostructural with Tl6GeTe6, with a = 9.4235(6) angstrom, b = 9.6606(7) angstrom, c = 10.3889(7)angstrom, alpha = 89.158(2)degrees, beta = 96.544(2)degrees, gamma = 100.685(2)degrees, V = 923.3(l) angstrom(3) (Z = 2). Its structure is composed of dirneric [Si2Te6](6-) units with a Si-Si single bond, while the Tl atoms are irregularly coordinated by five to six Te atoms. Numerous weakly bonding Tl-Tl contacts exist. Both title compounds are black semiconductors with small band gaps, calculated to be 0.9 eV for Tl6Si2Te6 and 0.5 eV for Tl6Ge2Te6. The Seebeck coefficients are +65 mu V K-1 in case of Tl6Si2Te6 and + 150 mu V K-1 in case of Tl6Ge2Te6 at 300 K, and the electrical conductivities are 5.5 and 3 Omega(-1) cm(-1), respectively. (c) 2006 Elsevier Inc. All rights reserved.