Chemical Ordering in Liquid and Supercooled Ge2Sb2Te5 Phase-Change Materials

초록

The origin of chemical ordering in liquid and supercooled liquid Ge2Sb2Te5 (GST) was investigated using ab initio molecular dynamics (AIMD) simulations. Bond dynamics were analyzed via continuous (tau(con)) and intermittent (tau(int)) lifetimes. The intermittent lifetime (tau(int)) reveals that chemically ordered Ge-Te and Sb-Te bonds are the most stable, although tau(con) exhibits a stability anomaly. The faster increase of tau(int) for these bonds upon cooling explains the overall chemical ordering. A novel ordering mechanism was identified through the analysis of bond separation dynamics. Te-Te 'wrong' bonds exhibit a unique dynamic instability, breaking and separating much faster than any other bond type, which actively drives the system towards chemical order. A correlation between lifetime and bond strength, as calculated by the Integrated Crystal Orbital Hamilton Population (ICOHP), supports these dynamic findings. Chemical ordering shows a positive correlation with medium-range structural order, evidenced by the instability of 4-fold rings containing wrong bonds. This study provides a detailed dynamic origin for ordering in liquid GST, highlighting the role of Te-Te bond relaxation.

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