Phase Transitions in Optical Media
XAFS measurements of structure

We have used XAFS (x-ray absorption spectroscopy) to investigate the structure of the phase-change media Ge2Sb2Te5.

Rewritable phase-change media are now a massive industry, but although they are in wide use, there remains much to be know about these versitile materials as typified by chalcogenide alloys lying along the pseudobinary-tieline between GeTe and Sb2Te3. In this paradigm, information is written as (more reflective) amorphous marks on an (metastable crystalline background. In a recordable phase-change medium, amorphous spots are written using short and high-power laser pulses, while crystallization is induce by longer, lower-power laser pulses. Some facinating properties arise: amorphous to crystalline transitions occur on the order of 100 ns and there is a large change reflectivity between the amorphous (n=4.21+1.89i) and crystalline phases (n=4.56+4.23i @650 nm). What is happening?

See Nature Materials Paper>>

phase transitiion animation

Umbrella Flip

In short, in the metastable, distorted rocksalt structure, the Te atoms form in a well organized fcc like structure, while the Ge and Sb atoms along with about 20% vacancies live in the other fcc lattice (two interlocking fcc lattice form the rocksalt structure). The Ge atoms are located off center and have thus two sets of three bonds. Three shorter bonds (fat

fat bonds in the picture) and three longer bonds (thin bonds) exist. Laser excitation (heat + light) preferentialy break the longer Ge bonds and Ge switches from its original (six fold) octahedrally coordinated position to a (four fold) coordinated position. As you may recall, the group IV element, Ge is much happier being tetrahedrally coordinated.

These large changes do not go unnoticed in the electronic structure and a relatively large change in reflectivity results (as can be seen in the change in the dielectric constant.

So, two positions -- octahedral and tetrahedral -- an umbrella flip. The switching between these two states is what characterizes the reversibility of this system.

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