Abstract
The crystalline and amorphous phases of tetrahedrally co-ordinated solids are investigated using a dynamical bond-charge model. In the SiO2 systems the role of the bond charges is played by the oxygen ions. Tetrahedral solids exhibit a structural weakness against deformations of the space angle. This leads to low-frequency transverse acoustic (TA) phonon branches with anomalous anharmonic properties. It is therefore likely that the amorphous phases are related to specific local distortions of the covalent bonds, which cause a softening of the zone boundary TA modes. This approach suggests a new class of amorphous clusters built of 5- and (‘boat’ - type) 6- rings. These clusters are low-entropy or entropy-free configurations of nearly undistorted tetrahedral units with no translational symmetry or several hundred atoms in a unit cell. The properties of amorphous solids are discussed on the basis of our model.
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