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

IUCrJ

Effenberger, H.S.¹  Scheidl, K.S.²  Yagi, T.³ 

[1]Geochemical Research Center, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

[2]Institut fr Mineralogie und Kristallographie, Universitt Wien, Althanstrasse 14, Wien A-1090, Austria

[3]National Museum of Nature and Science, 4-1-1 Amakubo, Tsukuba, Ibaraki 305-0005, Japan

关键词: CHIBAITE;    CLATHRASILS;    HYDROCARBONS;    RAMAN SPECTROSCOPY;    X-RAY DIFFRACTION;    LOW TEMPERATURE;   

DOI  :  10.1107/S2052252518009107

学科分类：数学（综合）

来源: International Union of Crystallography

【 摘 要 】

The natural sII-type clathrasil chibaite [chemical formula SiO2·(M12,M16), where Mx denotes a guest mol­ecule] 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 Fd{\overline 3}m [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 Fd{\overline 3}m model. As suggested from band splitting observed in the Raman spectra, the [512]-type cages (one crystallographically unique in Fd{\overline 3}m, four different in A2/n) entrap the hydro­carbon species (CH4, C2H6, C3H8, i-C4H10). The [51264]-type cage was found to be unique in both structure types. It contains the larger hydro­carbon mol­ecules C2H6, C3H8 and i-C4H10.

【 授权许可】

CC BY