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

Abstract

Synthetic single crystals of K[(CuII,MnII,MnIII)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 crystallographically by means of single-crystal X-ray diffraction. The crystal structure of this new zemannite-type representative follows 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 on the octahedral M sites with a ratio (CuII+MnII) : MnIII approximating 1 : 1. From arguments about the cation sizes and the individual bond valence sums, a distribution of CuII1–xMnIIIx and MnIIx MnIII1–x at each two of the four M positions can be assumed with a presumed x between ~0.14 and ~0.50. The K+ cations and crystal water 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 site occupations 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.

Crystal chemistry of zemannite-type structures: IV. Wortupaite, the first new tellurium oxysalt mineral described from an Australian locality

“…The issue with the non-centrosymmetric refinement included a Flack parameter close to ½, and the presence of non-positive definite atoms such as the channel Mg 2+ and framework O sites). Additionally, lower symmetry space groups are also possible for zemannite, with recent examples of lower symmetry found in monoclinic twin domains in zemannite-structured K[(Cu 2+ ,Mn 2+ ,Mn 3+ ) 2 (TeO 3 ) 3 ]⋅2H 2 O (Eder et al , 2023b). Refinements in P 2 1 / m and P were also undertaken but did not result in improved statistics for wortupaite.…”

“…The only previously described naturally occurring Ni-bearing Te oxysalt keystoneite, ideally Mg 0.5 Ni 2+ Fe 3+ (TeO 3 ) 3 ⋅4H 2 O, has Ni–O bonds with an average length of 2.085 Å. In addition, several new zemannite-like compounds have recently been reported for the first time, including Ni-bearing Na 2 [Ni 2 (TeO 3 ) 3 ]⋅2.5H 2 O and K 2 [Ni 2 (TeO 3 ) 3 ]⋅H 2 O (Eder et al , 2023a) and the first Jahn-Teller distorted framework in a zemannite-like structure observed in K[(Cu 2+ ,Mn 2+ ,Mn 3+ ) 2 (TeO 3 ) 3 ]⋅2H 2 O (Eder et al , 2023b). The Ni-bearing compounds have Ni–O distances of 2.064 and 2.069 Å, respectively – close to the average 2.06 Å bond length in the wortupaite framework.…”