Stereochemically Active Lone Electron Pairs in the Chalcogen Oxide-Carbonates Te[CO3]O and Te4O7[CO3] at Elevated Pressures.

Two anhydrous tellurium carbonates, Te[CO3]O and Te4O7[CO3], have been synthesized in laser-heated diamond anvil cells by a reaction of TeO2 with CO2. Te[CO3]O was obtained at higher pressures (40(2) GPa), while Te4O7[CO3] was synthesized at moderate pressures (20(2) GPa). The crystal structures of both compounds were determined from synchrotron single crystal X-ray diffraction data and confirmed by density functional theory-based calculations. For Te[CO3]O the experimental Raman data were accurately reproduced by a Raman spectrum derived from the calculations. Both carbonates belong to the family of sp2-carbonates and are characterized by the presence of trigonal-planar [CO3]2--groups. While the crystal structure of Te[CO3]O is rather simple, Te4O7[CO3] is a layered compound with layers of [CO3]2--groups alternating with complex Te-O-layers. Our DFT-based calculations show that stereochemically active lone electron pairs are present in both crystal structures at elevated pressures. The successful synthesis of Te[CO3]O is a significant enlargement of the crystal chemistry of carbonates, as it demonstrates that anhydrous carbonates of chalcogenide group elements hosting no further cations can be synthesized. Excluding the noble gases, the chalcogenide group was the last main group of the periodic table where such carbonates were not known up to now.