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Pressure-induced structural transformation of clathrate Ge$_{136}$ via an ultrafast recrystallization of an amorphous intermediate

We study the pressure-induced structural transformation of Ge$_{136}$ clathrate by ab initio molecular dynamics and metadynamics. The system under pressure first undergoes amorphization followed by an ultrafast recrystallization to the $β$-tin structure on the time scale of 30 ps. The initial pressure-induced amorphization of clathrate is triggered by high pressure while the subsequent fast recrystallization to $β$-tin is driven by low temperature. Interestingly, the amorphous intermediate is still diffusive even at room temperature, in spite of very strong undercooling, making the ultrafast recrystallization possible. The system provides an explicit example of structural transformation between two crystalline phases proceeding via non-crystalline intermediate. Upon fast decompression of the amorphous structure with incipient crystalline order the recrystallization is blocked and the system instead proceeds to the tetrahedral LDA amorphous phase.