K[(CuII,MnII,MnIII)2(TeO3)3]∙2H2O, the first zemannite-type structure based on a Jahn-Teller-distorted framework

Synthetic single crystals of K[(Cu^II,Mn^II,Mn^III)₂(TeO₃)₃]·2H₂O were obtained from an overconcentrated alkaline aqueous solution in the system K₂O-MnO-CuO-TeO₂ under hydrothermal conditions at T ≤ 220 °C. Subeuhedral single crystals have been investigated by means of single-crystal X-ray diffraction. The crystal structure of this new zemannite-type representative adopts a monoclinic twofold superstructure. The doubling of the unit-cell volume is accompanied by a hexagonal-to-monoclinic symmetry reduction, resulting in threefold twinning with individual crystal domains following the space group symmetry P2₁. Refinements of site-occupation factors and the evaluation of bond valences suggest a distribution of di- and trivalent cations at the octahedrally coordinated M sites with a ratio (Cu^II + Mn^II):Mn^III approximating 1:1. Based on arguments about the cation sizes and the individual bond valence sums, a distribution of Cu^II_1–xMn^III_x and Mn^II_x Mn^III_1–x at each two of the four M sites can be assumed with x between ~ 0.14 and ~ 0.50. The K⁺ cations and H₂O molecules inside the channels are located off the central channel axis. In contrast to most other known zemannite-type phases, the extra-framework atoms show full occupancies and are not disordered. The distribution of the channel contents supports the anisotropic deformation of the surrounding framework, which follows the local symmetry reduction as required for the Jahn-Teller distortion of the octahedrally coordinated M sites within the framework. The arrangement of the deformed channels can be understood as the origin of the existing superstructure.