Structural studies on synthetic A₂₋ₓ[M₂(TeO₃)₃]·nH₂O phases (A = Na, K, Rb, Cs; M = Mn, Co, Ni, Cu, Zn) with zemannite-type structures

Abstract

During a systematic study on formation conditions of new compounds with zemannite-type structures, crystals of ten new oxidotellurate(IV) phases were grown under hydrothermal conditions, partially by employing a drastic reduction of the water content. The crystal structures of the obtained phases were determined by single-crystal X-ray diffraction. Na₂[Ni₂(TeO₃)₃]·2.5H₂O, K₂[Ni₂(TeO₃)₃]·H₂O, K₂[Zn₂(TeO₃)₃]·2H₂O, Rb₁.₂₅[Co₂(TeO₃)₃]·1.5H₂O and Rb₁.₂₄[Mn₂(TeO₃)₃]·2H₂O exhibit a unit-cell with hexagonal symmetry (Z = 2, a ≈ 9.3 Å, c ≈ 7.7 Å), in which most of the other compounds with a zemannite-type structure are known to crystallize. Relative to this unit-cell, K₂[Cu₂(TeO₃)₃]·2H₂O exhibits a twofold superstructure, K₂[Co₂(TeO₃)₃]·2.5H₂O a twofold superstructure with an additional incommensurate modulation, Na₂[Cu₂(TeO₃)₃]·1.5H₂O a threefold superstructure, and Rb₁.₅[Mn₂(TeO₃)₃]·1.25H₂O and Cs[Mn₂(TeO₃)₃]·H₂O a fourfold superstructure. Disorder of the alkali metal cations and crystal water molecules in the channels as well as variable water contents complicate modelling and structure refinement.