Glass transition criterion and plastic deformation of glass Damba S. Sanditov 1, 2, * , Michael I. Ojovan 3,4, ** , Migmar V. Darmaev 1 1 Banzarov Buryat State University, Ulan-Ude 670000, Russia 2 Institute of Physical Materials Science, Siberian Branch of the Russian Academy of Sciences, Ulan-Ude 670047, Russia * e-mail: sanditov@bsu.ru 3 Department of Materials, Imperial College London, SW7 2AZ, United Kingdom 4 Department of Radiochemistry, Lomonosov Moscow State University, Moscow 119991, Russia ** e-mail: m.ojovan@imperial.ac.uk, m.i.ojovan@gmail.com Abstract We develop the notion that amorphous substances undergo reversible configurational structural changes accompanied by local expansion and compression (atom delocalization) near the glass transition temperature. They are similar in nature to configurational changes in the structure of glasses in the case of reversible frozen (plastic) deformation and its thermally stimulated relaxation. We assume that the glass-liquid transition is associated with the process of atom delocalization caused by bond breaking and formation of elementary excitations e.g. configurons. We discuss the possibility of detection of configuron formation and atom delocalization near glass transition based on temperature dependence of X-rays or neutron first sharp diffraction (pair distribution function) minimum. Keywords: viscosity, softening of glass, frozen deformation, atom delocalization, configuron, pair distribution function 1. Introduction The nature of structural changes accompanying vitrification and glass-liquid transitions is not yet fully clear and discussed in many recent publications, see, for example, Refs. [1–6]. In this paper, using delocalized-atom model previously developed [2, 7, 8] we compare glass softening with reversible frozen deformation of glassy solids, which makes it possible to gain certain information about glass-liquid transition and vitrification processes. In addition, we discuss a possible scenario of liquid–glass transition based on switching off the creep triggering mechanism: atom delocalization. Our approach is based on analysis of experimental data on behavior of amorphous materials near the glass transition. 2. Glass-liquid transition as a consequence of atom delocalization Atom delocalization in silicate glasses is an ultimate displacement of the oxygen atom in a Si– O–Si bridge due to local low-activation-energy deformation of the silicon–oxygen network of