Conditions Extrêmes et Matériaux : Haute Température et Irradiation

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K.Vignarooban, P.Boolchand, M.Micoulaut, M.Malki, W.Bresser, 'Rigidity transitions in glasses driven by changes in network dimensionality and structural groupings', Europhysics Lett. 108 56001-56005 (2014) doi:10.1209/0295-5075/108/56001

Calorimetric, Raman and electrical conductivity properties of alkali borates (, Na) are studied as a function of composition and these show the presence of stiffness transitions and an intermediate phase which are driven by a combination of network dimensionality change and usual topological constraint changes. This picture is confirmed by a detailed Raman analysis showing that specific modes of molecular structural groupings dominate the network structure in the intermediate phase. Their evolution shows a one-to-one correspondance with the observed non-reversing heat flow at the glass transition, and are correlated with thresholds in ionic conductivity that allows identifying a flexible phase at high alkali content, whereas the mildly stressed-rigid -rich glasses are driven by the conversion of planar 2D boroxol ring structures into the 3D structural groupings. These findings deeply modify the usual picture of these archetypal glasses, and reveal the very first example of the onset of rigidity tuned by network dimensional conversion.