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

The title compounds were prepared from the elements in the stoichiometric ratio at 800 °C under exclusion of air. Tl6Si2Te6 crystallizes in the space group P1¯, isostructural with Tl6Ge2Te6, with a=9.4235(6)Å, b=9.6606(7)Å, c=10.3889(7)Å, α=89.158(2)°α=89.158(2)°, β=96.544(2)°β=96.544(2)°, γ=100.685(2)°γ=100.685(2)°, V=923.3(1)Å3 (Z=2Z=2). Its structure is composed of dimeric [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 μV K−1 in case of Tl6Si2Te6 and +150 μV K−1 in case of Tl6Ge2Te6 at 300 K, and the electrical conductivities are 5.5 and 3 Ω−1 cm−1, respectively.

The isostructural tellurides Tl6Si2Te6 and Tl6Ge2Te6 contain SiIII and GeIII in dimeric [Si2Te6]6− and [Ge2Te6]6− units, respectively. Both materials are black semiconductors with high Seebeck coefficients and reasonable electrical conductivities, despite their molecular character.Figure optionsDownload as PowerPoint slide