The 1D‐Polymeric (Te₇²⁺)_n in (Te₇)[MF₆]₂ (M=As, Sb, Nb, Ta) by Solvothermal, Microwave Assisted, and Electrochemical Synthesis

Abstract

Reactions of tellurium with metal pentafluorides MF5 (M=As, Sb, Nb, Ta) in liquid SO2 under supercritical conditions at 160 °C yield (Te7)[MF6]2 containing the polymeric Zintl cation (Te72+)n consisting of chair‐shaped Te6 rings connected via single Te atoms. Crystal structure analyses show all four compounds to crystallize isotopically in the (Te7)[AsF6]2 structure type. Monitoring the reactions via Raman spectra reveal that (Te4)[MF6]2 is formed as an intermediate, which reacts with Te finally to (Te7)[MF6]2. Crystal structures of (Te4)[MF6]2 (M=Nb, Ta) were determined. Microwave assisted solvothermal synthesis give analogous results in very short reaction time. All compounds are also accessible via electrochemical oxidation of Te in presence of [MF6]− as electrolyte. (Te7)[AsF6]2 is a semiconductor with σ=10−7 S/cm at 300 K and an energy for the thermally activated electron process of 0.91 eV. Theoretical calculations on the self‐consistent hybrid DFT level confirm that the fundamental band gap is independent from the kind of M. The frontier orbitals are predominantly composed of Te states. Raman bands at 196, 178, 153, and 138 cm−1 are specific for (Te72+)n. Phonon calculations of (Te7)[TaF6]2 confirm three intensive Raman bands at 210, 174 and 163 cm−1. The corresponding normal modes are mainly localized on (Te7)n.