Structural properties and phase transitions in a silica clathrate

Melanophlogite, a low-pressure silica polymorph, has been extensively studied at different temperatures and pressures by molecular dynamics simulations. While the high-temperature form is confirmed as cubic, the low-temperature phase is found to be slightly distorted, in agreement with experiments. With increasing pressure, the crystalline character is gradually lost. At 8 GPa, the radial distribution function is consistent with an amorphous state. Like pristine glass, the topology changes, plastic behavior, and permanent densification appear above similar to 12 GPa, triggered by Si coordination number changes. We predict that a partial crystalline and amorphous sample can be obtained by recovering the sample from a pressure of similar to 12-16 GPa. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3532543]