Present-day multimedia strongly relies on re-writable phase-change optical memory. We have found that Ge2Sb2Te5 (GST), the material of choice in DVD-RAM, does not possess the conventional rock-salt structure as previously believed but that Ge and Sb atoms are displaced from the ideal rocksalt positions. Amorphization of both GeTe and GST results in a significant shortening of covalent bonds and a decrease in the mean-square relative displacement concomitant with a drastic change in short-range order. The order-disorder transition in GeTe and GST is primarily due to a flip of Ge atoms from octahedral positions into tetrahedral positions without rupture of strong covalent bonds. The driving force for the flip are strained Ge-Te bonds in the crystalline phase. It is this nature of the transformation that ensures large changes in reflectivity, fast disk performance and repeatable switching over millions of cycles.
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