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Dehydration of microporous vanadosilicates: the case of VSH‐13Na
Author(s) -
Danisi Rosa Micaela,
Armbruster Thomas
Publication year - 2018
Publication title -
acta crystallographica section b
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.604
H-Index - 33
ISSN - 2052-5206
DOI - 10.1107/s2052520618011423
Subject(s) - crystallography , monoclinic crystal system , orthorhombic crystal system , chemistry , alkali metal , antiparallel (mathematics) , crystal structure , group (periodic table) , tetrahedron , space group , microporous material , x ray crystallography , diffraction , physics , organic chemistry , quantum mechanics , magnetic field , optics
Microporous VSH‐13Na of composition Na 2 (VO)(Si 4 O 10 )·3H 2 O 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)(Si 4 O 10 )·4H 2 O. 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 2 1 screw axis along b was strictly fulfilled suggesting monoclinic space group P 12 1 1. The resulting structure in P 2 1 with a = 14.364 (4), b = 9.134 (2), c = 10.373 (3) Å, β = 90.056 (7)°, V = 1360.9 (7) Å 3 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) Å 3 (space group P 2 1 ). Release of H 2 O 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 H 2 O molecule. Thus, the presence of monovalent or divalent extraframework cations plays a key role in the dehydration process of natural and synthetic vanadosilicates.