Abstract
The natural sII-type clathrasil chibaite [chemical formula SiO2·(M12,M16), where Mx denotes a guest mol-ecule] was investigated using single-crystal X-ray diffraction and Raman spectroscopy in the temperature range from 273 to 83 K. The O atoms of the structure at room temperature, which globally conforms to space group [V = 7348.9 (17) Å3, a = 19.4420 (15) Å], have anomalous anisotropic displacement parameters indicating a static or dynamic disorder. With decreasing temperature, the crystal structure shows a continuous symmetry-lowering transformation accompanied by twinning. The intensities of weak superstructure reflections increase as temperature decreases. A monoclinic twinned superstructure was derived at 100 K [A2/n, V = 7251.0 (17) Å3, a' = 23.7054 (2), b' = 13.6861 (11), c' = 23.7051 (2) Å, β' = 109.47°]. The transformation matrix from the cubic to the monoclinic system is ai ' = (½ 1 ½ / ½ 0 -½ / ½ -1 ½). The A2/n host framework has Si-O bond lengths and Si-O-Si angles that are much closer to known values for stable silicate-framework structures compared with the averaged model. As suggested from band splitting observed in the Raman spectra, the [512]-type cages (one crystallographically unique in , four different in A2/n) entrap the hydro-carbon species (CH4, C2H6, C3H8, i-C4H10). The [51264]-type cage was found to be unique in both structure types. It contains the larger hydro-carbon mol-ecules C2H6, C3H8 and i-C4H10.
Highlights
Clathrates are inclusion compounds which are built up from topologically sub-nanoporous host frameworks that entrap guest atoms and molecules of suitable size into cage-like voids
Spectra were collected at room temperature (RT) as well as at low temperatures (LT) down to 83 K
The single-crystal investigations of the naturally occurring sIItype hydrocarbon clathrasil, named chibaite, revealed a cubic Fd3m symmetry at RT in accordance with the topology of the MTN-type framework
Summary
Clathrates are inclusion compounds which are built up from topologically sub-nanoporous host frameworks that entrap guest atoms and molecules of suitable size into cage-like voids. Clathrate frameworks consisting of only SiO2 are referred to as clathrasils. They exhibit structural analogs to H2O-ice phases which are observed for several other SiO2 phases (Kamb, 1965). Clathrasils are found in marine sediments related to low-temperature hydrothermal processes of convergent plate boundaries (Momma et al, 2011; Momma, 2014; Likhacheva et al, 2016)
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