Understanding the structure of phase change (PC) materials in their amorphous and metastable crystalline phase and their working mechanisms is of major importance.

While many groups have focused their attention on the structure of the crystalline or amorphous states, we start from studies of the liquid state of germanium tellurides. Combining experiments on liquids (thermodynamics, neutron scattering, EXAFS) and computer simulations (RMC, first principles Molecular Dynamics) we end up with a model emphasizing the role of electronically driven distortions of the local octahedral order in the peculiar behavior of these liquids (see references).

We will present DFT based molecular dynamics simulations of Ge₂Sb₂Te₅ alloys in crystalline, liquid and amorphous states. The interpretation of these results in the same framework leads to a simple and original picture that helps understanding the structure and properties difference between the crystalline and amorphous phases, thus paving the way for the development of new phase change alloys.

Raty J.-Y., Godlevsky V., Ghosez P., Bichara C., Gaspard J.-P., Chelikowsky J. R.,

Evidence of a reentrant Peierls distortion in liquid GeTe. Phys. Rev. Lett., 85, -9-, 1950-3, 2000.

Bergman C., Bichara C., Gaspard J.-P., Tsuchiya Y. Experimental investigation of the water-like density anomaly in liquid Ge₁₅Te₈₅ eutectic alloys. Phys. Rev. B67, 104202, 2003.

Coulet M.-V., Testemale D., Hazemann J.-L., Gaspard J.-P., Bichara C.,

A Reverse Monte Carlo analysis of the local order in liquid Ge_.15 Te_.85 alloys combining neutron scattering and X-Ray Absorption Spectroscopy. Phys. Rev. B, 72, 174209, 2005.

Bichara C., Johnson M., Raty J.-Y.,

Temperature-induced density anomaly in Te-rich liquid germanium tellurides: p versus sp³ bonding?

Phys. Rev. Lett., 95, 267801, 2005.

C. Bichara, M. Johnson, J.-P. Gaspard

Octahedral structure of liquid GeSb₂Te₄ alloy : first principles molecular dynamics study

Phys. Rev. B 75, 060201R, 2007.

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1. Distortions and low symmetry environments in liquids :  alkalis at high pressure.