Atomic scale foundations of a new phase in network-forming glasses and liquids – PHASABINIT

CONTEXT: The recent discovery of an intermediate phase between the well-known flexible and rigid phases in network glasses has opened a new field of investigation in glass science with potential interesting applications. Indeed, this phase displays a certain number of remarkable properties such as the absence of ageing in the glassy state, the absence of stress at a microscopic level and space-filling properties. A certain number of questions have emerged both from the experimental and theoretical viewpoint. The present project plans to address those questions by combining experimental and theoretical methods in order to elucidate the origin of the intermediate phase. Specifically, we will use First-Principles Molecular Dynamics and Raman and Infrared spectroscopy to study the liquid and the glassy state in the three phases of interest: flexible, intermediate, stressed rigid. Such kind of studies have never been undertaken but we do believe that they are very timely as interest in the origin and possible applications of the intermediate phase are growing very rapidly. PROJECT DESCRIPTION: The project is by nature interdisciplinary and the members have a well-known expertise in the field of either molecular simulations (classical or ab initio), spectroscopy of liquids and glasses, and Phillips-Thorpe rigidity transitions. We plan to study by simulations and experimentally two particular systems: a binary Ge-Se and a ternary Ge-As-Se which both display an intermediate phase. The partners (3) belong to LPTMC (University Paris 6), IPMCS (Strabsourg), and CEMHTI (Orléans). The duration of the project is three years with well-identified objectives and tasks. We will investigate liquid and amorphous structures with system sizes of 120 and 480 atoms. The latter system size which lies at the limit of what can be used in massive parallel computer, will be used to study in detail finer compositional changes close to the boundaries of the intermediate phase. It will furthermore imply the use of a new density functional theory (DFT) scheme that has never been applied in chalcogenides. Experimental and theoretical studies will be driven in parallel with close interaction between partners. EXPECTED RESULTS: With this project, we expect that the structure and the dynamics of model-chalcogenides will be characterized in terms of the mechanical nature of their network backbone: flexible, stressed, intermediate thus providing an origin into the nature of the intermediate phase. In the liquid phase, we will provide on the basis of numerical simulations the elucidation of the measured vibrational spectra, an issue never adressed before in liquid chalcogenides. The present research field (rigidity transitions, network glasses) appears to be very useful for applications because more and more chalcogenides are used in memory cells, optical data storage, photo-chemical etching,etc. Although some of these techniques are now used routinely, a certain number of questions remain based on how these techniques/performances could be improved, especially under compositional changes. In providing generic answers based on the nature of the chalcogenide molecular network, we will be able to contribute to this technological aspects. HISTORY AT THE ANR: The present project has been already submitted in february 2008 within the same Program ANR-Blanc Chimie. It has been ranked 3rd on the complementary list.