Insights into the fivefold symmetry of the amorphous Sb-based change materials in the rapid phase change from first principles

In materials science, it is widely believed that the fivefold symmetry in the amorphous phase will suppress crystallization because of its structural incompatibility with the crystal phase. The Sb-based phase change materials dominated by the fivefold rings, however, display the ultrafast growth-dominated phase change from amorphous to crystalline phase and exhibit significant property contrast consequently when they are heated by a laser or electrical pulse. To resolve the paradox, the long-time ab initio molecular dynamics calculation is carried out to simulate the crystallization cycle of the amorphous Ge15Sb85 under non-isothermal condition. The calculation results are in a reasonable agreement with experimental data. In particular, it is found that a transient phase state exists just before the crystallization onset temperature of the amorphous Ge15Sb85, in which the predominant fivefold rings in half-chair conformation undergo rapid conversion into the puckered sixfold rings. It is further demonstrated that such rings conversion via a route of the bond exchange model involves small atomic displacements and results in the structural motif similarity between the transient and the crystalline states. This thus reduces the crystal-amorphous interfacial energy promoting crystal nucleation consequently. The subsequent electronic calculations indicated that such conversion of the dominant rings in the amorphous Ge15Sb85 may be triggered by the increased pp orbital hybridizations and modulated by the diminished sp electron mixing. It is anticipated that the findings presented will provide a stepping-stone for the rational design of the Sb-based phase change materials. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.