Dehydration of microporous vanadosilicates: the case of VSH-13Na

Abstract

Microporous VSH-13Na of composition Na 2 (VO)(Si 4 O 10 )Á3H 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 squarebased pyramids, closely resembles that of the mineral cavansite, Ca(VO)-(Si 4 O 10 )Á4H 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 P12 1 1. The resulting structure in P2 1 with a = 14.364 (4), b = 9.134 (2), c = 10.373 (3) Å , = 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 , = 90.10 (4) , V = 1162 (3) Å 3 (space group P2 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.

The chromatic symmetry of twins and allotwins

“…Danisi & Armbruster (2018) reported a complex allotwin in the microporous vanadosilicate VSH-13Na, with composition Na 2 (VO)-(Si 4 O 10 )Á3H 2 O. Vanadosilicate VSH-13Na.…”