Phase transitions of the clathrasil dodecasil 3C (MTN) for the guest molecules pyrrolidine and t-butylamine

New silica clathrate minerals that are isostructural with natural gas hydrates

“…In contrast, the cubic and tetragonal chibaite phases coexist at the same temperature and pressure. This coexistence most likely reflects variations in the guest molecules in chibaite, as the space group symmetry of MTN-type clathrasil is also dependent on guest molecules 26 and chibaite contains various hydrocarbons at least up to C≤4. The coexistence of DOH-type mineral with chibaite indicates that there were even larger hydrocarbons at the time of their crystallization because DOH-type clathrasil crystallizes when there are large guest molecules that cannot be incorporated into the MTN-type framework 32 .…”

“…At high temperatures, the space groups of melanophlogite and MTN-type clathrasil have the highest symmetries of Pm3 _ n and Fd3 _ m, respectively 29,30 . They undergo symmetry-lowering on distortion of the frameworks with a decrease in temperature or an increase in pressure 26,29,31 . In contrast, the cubic and tetragonal chibaite phases coexist at the same temperature and pressure.…”

“…Therefore, an average structure of the cubic and tetragonal chibaite was refined by the Rietveld method 25 . Two structural models with space group symmetry Fd3 _ or I4 1 /a were used because they are reported as space groups of room-temperature phases of MTN-type clathrasil 7,26,27 . Although there are other possible tetragonal space groups for MTN-type clathrasil 26,28 , I4 1 /a is the highest symmetry space group belonging to the Laue group I4/m.…”

“…Two structural models with space group symmetry Fd3 _ or I4 1 /a were used because they are reported as space groups of room-temperature phases of MTN-type clathrasil 7,26,27 . Although there are other possible tetragonal space groups for MTN-type clathrasil 26,28 , I4 1 /a is the highest symmetry space group belonging to the Laue group I4/m. The atomic coordinates of quartz and DOH-type mineral phases were not refined, but were fixed at those reported in previous studies because their diffraction peaks are very weak.…”

The Cubic/Tetragonal Phase Transition in D3C–THF: an Optical and X-ray Powder Diffraction Study

“…NE, Glienicker Str. 100, D-14109 Berlin, Germany force the crystallization of D3C structures, which are tetragonal (t-butylamine) or orthorhombic (pyrrolidine) at ambient conditions (8).…”