A high-pressure cubic-to-tetragonal phase-transition in melanophlogite, a SiO2 clathrate phase

The crystal-structure and the high-pressure behavior of a natural cubic melanophlogite (Sp. Gr. Pm3n at room-conditions), a type-I SiO2 clathrate containing only methane as guest gas, was investigated by X-ray single-crystal diffraction at room-conditions and in situ high-pressure synchrotron powder diffraction with a diamond anvil cell and methanol: ethanol: water = 16:3:1 (MEW) mixture and silicon-oil (Si-oil) as pressure transmitting media up to about 6 GPa.