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According to the nature of the chemical bonds which hold particles together solids can be classified as metals, ionic solids, molecular solids, or covalent network solids. Solids can be either amorphous or crystalline in structure. In the solid state elementary particles, which form the substance (atoms, molecules), are in fixed positions which can be arranged in a repeating pattern in crystalline solids or in a not repeating e.g. disordered in amorphous solids. The structures of crystalline solids are formed of repeating, regular units called unit cells, while the structures of amorphous solids are irregular or random. The simplest repeating unit in a crystal is called a unit cell. Each unit cell is defined in terms of lattice points, e.g. the points in space about which the particles are free to vibrate. The structure of amorphous materials cannot be described in term of repeating unit cells; another way of stating this is that because of its inherent non-periodicity, the "unit cell" of an amorphous material comprises all the atoms within it.
Configuron is an elementary configurational excitation in an amorphous material which involves breaking of a chemical bond and associated strain-releasing local adjustment of centres of atomic vibration. The higher the temperature of an amorphous material the higher the configuron concentration. The higher the content of configurons the lower the viscosity of an amorphous material (see viscosity). Formation of configuron percolating clusters at glass transition temperature explains the glass transition phenomenon. Clustering of configurons above glass transition temperature facilitates the viscous flow in melts (see the viscosity of amorphous materials). Thermodynamic parameters of configurons can be found from viscosity-temperature relationships.
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