The hydrocarbon-bearing clathrasil chibaite and its host-guest structure at low temperature.

The natural sII-type clathrasil chibaite [chemical formula SiO·(,), where 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 [ = 7348.9 (17) Å, = 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 [2/, = 7251.0 (17) Å, ' = 23.7054 (2), ' = 13.6861 (11), ' = 23.7051 (2) Å, β' = 109.47°]. The transformation matrix from the cubic to the monoclinic system is ' = (½ 1 ½ / ½ 0 -½ / ½ -1 ½). The 2/ 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 [5]-type cages (one crystallographically unique in , four different in 2/) entrap the hydro-carbon species (CH, CH, CH, -CH). The [56]-type cage was found to be unique in both structure types. It contains the larger hydro-carbon mol-ecules CH, CH and -CH.