Simulation of crystallization in Ge 2 Sb 2 Te 5 : A memory effect in the canonical phase-change material

Crystallization of amorphous ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$ (GST) has been studied using four extensive (460 atoms, up to 4 ns) density functional/molecular dynamics simulations at 600 K. This phase change material is a rare system where crystallization can be simulated without adjustable parameters over the physical time scale, and the results could provide insight into order-disorder processes in general. Crystallization is accompanied by an increase in the number of $ABAB$ squares $(A:\text{Ge},\text{Sb};B:\text{Te})$, percolation, and the occurrence of low-frequency localized vibration modes. A sample with a history of order crystallizes completely in 1.2 ns, but ordering in others was less complete, even after 4 ns. The amorphous starting structures without memory display phases $(g1\text{ns})$ with subcritical nuclei (10--50 atoms) ranging from nearly cubical blocks to stringlike configurations of $ABAB$ squares and $AB$ bonds extending across the cell. Percolation initiates the rapid phase of crystallization and is coupled to the directional $p$-type bonding in metastable GST. Cavities play a crucial role, and the final ordered structure is distorted rock salt with a face-centered cubic sublattice containing predominantly Te atoms. We comment on earlier models based on smaller and much shorter simulations.