Thomas Armbruster

Microporous VSH-13Na of composition Na₂(VO)(Si₄O₁₀)·3H₂O was synthesized under mild hydrothermal conditions and studied by single-crystal X-ray diffraction at room temperature and 398 K. Its vanadosilicate framework, consisting of sheets of silicate tetrahedra connected by vanadyl-type square-based pyramids, closely resembles that of the mineral cavansite, Ca(VO)(Si₄O₁₀)·4H₂O. Due to the disorder in the orientation of the short apical vanadyl groups, the topological symmetry of VSH-13Na was originally described in space group Imma. However, when analysing the systematic absences in our dataset, only the 2₁ screw axis along b was strictly fulfilled suggesting monoclinic space group P12₁1. The resulting structure in P2₁ with a = 14.364 (4), b = 9.134 (2), c = 10.373 (3) Å, β = 90.056 (7)°, V = 1360.9 (7) ų was interpreted as a case of allotwinning of two polytypes with topologically idealized orthorhombic symmetry: A (∼62%) with antiparallel orientation of the vanadyl groups in adjacent (100) layers and B (∼38%) with all vanadyl groups in adjacent layers oriented in the same way. At 398 K, the structure of VSH-13Na became fully dehydrated and adopted the unit-cell parameters a = 12.584 (16), b = 9.525 (13), c = 9.696 (14) Å, β = 90.10 (4)°, V = 1162 (3) ų (space group P2₁). Release of H₂O caused severe contraction of T—O—T angles and the unit-cell volume decreased by ∼15%. Despite their structural similarity, the VSH-13Na framework seems to be more flexible upon dehydration compared with cavansite, whose structure collapsed before removal of the last H₂O molecule. Thus, the presence of monovalent or divalent extraframework cations plays a key role in the dehydration process of natural and synthetic vanadosilicates.

Single-crystal X-ray data were collected in steps of 25°C up to 200° C on a natural sample of zemannite, Mg_0.5 [ZnFe³⁺ (TeO₃)₃ ]· 4.5H₂ O. Ab initio simulations were performed to determine the hydrogen-bonding network to get better insight into the mechanism of the dehydration process. At room temperature, zemannite is hexagonal with cell parameters a = 9.4301(12) Å, c = 7.6692(10) Å, V = 590.63(13) ų. The structure was refined in the non-centrosymmetric space group P 6₃, instead of P 6₃/m. Ab initio simulations suggested an ordered Fe/Zn distribution at the octahedral sites that resulted in agreement with P 6₃ symmetry. The dehydration starts at 100°C and proceeds with continuous loss of the non-Mg-bonded H₂ O up to 125°C. At 150°C, the interstitial water is lost leading to 3H₂O pfu. The polyhedral framework does not undergo significant modifications. The unit-cell volume slightly decreases from 590.63(13) ų at room temperature to 577(2) ų at 200°C.