Ordering and structural changes at the glass–liquid transition

• The structure of amorphous materials involves MRO with dynamic fractal structures.
• Amorphous materials are homogeneous at scales exceeding correlation length.
• Supercooled melts have a fractal geometry of bonds.
• Glasses have a 3-D bonding system.

Ordering types of amorphous solids and structural changes which occur at glass–liquid transition are discussed focusing on configuron percolation theory (CPT) of glass transition. The glass transition temperature can be calculated using bond thermodynamic parameters e.g. enthalpy Hd and entropy of formation Sd. Explicit equations have been derived to assess Hd and Sd from available data on viscosity of amorphous materials using the CPT viscosity equation. A universal equation for the variable activation energy of viscous flow Q(T) has been found. The glass–liquid transition is accompanied by formation of a percolation macroscopic cluster made up of broken bonds – configurons – which are dynamic in nature. The characteristic linear size of dynamic clusters formed is given by correlation length which universally depends on formation Gibbs free energy of configurons Gd = Hd − TSd and becomes macroscopic at glass transition. Fractal-type medium range order (MRO) is revealed at correlation length sizes and homogeneous and isotropic disordered state (DS) characteristic for macroscopic sizes larger than the correlation length. The reduction of topological signature (Hausdorff dimensionality) of disordered bonding lattice from 3 for glass to fractal Df = 2.4 − 2.8 for melt is the main signature change to explain the drastic changes of material behaviour at glass transition.