New insights into the crystal chemistry of zemannite: Trigonal rather than hexagonal symmetry due to ordering within the host-guest structure

Abstract

The microporous crystal structure of zemannite, Mg(H₂O)₆[Zn²⁺Fe³⁺(TeO₃)₃]₂·nH₂O, n ≤ 3, was re-investigated based on single-crystal X-ray diffraction data measured at 298 ± 0.5 K, 200 ± 1 K and 100 ± 3 K. So far, zemannite was described in space group P6₃ exhibiting a pronounced pseudosymmetry (P6₃/m). All refinements confirm the [Zn²⁺Fe³⁺(TeO₃)₃]¹⁻ framework topology with the extra-framework constituents (Mg atoms and H₂O molecules) being located within the channels along [001]. Measurements on a sample from the type locality revealed the unexpected occurrence of 00l reflections with l = 2n + 1, which clearly violate the 6₃ screw-axis symmetry. The minor but significant intensities of the low-order 00l reflections are assigned to the small differences in the scattering power between the Fe and Zn atoms; thus, the Zn and Fe cations are partly ordered between crystallographically distinct sites within the framework. In addition, the low symmetry allows a full order of the extra-framework atoms for the first time. A series of comparative refinement models were performed in the space groups P6₃/m, P6₃, P\(\overline{6}\), and P3. A fully ordered arrangement of the extra-framework guest atoms confirms the earlier postulated theoretical structure model with a hexahydrated Mg²⁺ ion besides additional interstitial H₂O molecules. The final refinements in space group P3 yield R1 ≤ 0.025 for the entire data sets measured at the distinct temperatures (2θ_max = 101.4°, MoKα radiation). The polarity of the arrangement in the channels is restricted to individual domains of equal twin fractions related by a mirror plane parallel to (0001).