Microscopic Model of Anomalous Low-Frequency Excitations in Inorganic Glasses: Molecular Dynamics Study of BeF2

Abstract

In molecular-dynamics simulations of Be${\mathrm{F}}_2$ glass, it is observed that a few percent of the ions are capable of displacements of the order of 1 Å in 10 ps at temperatures lower than 300 K. It is shown that these ions are located at well-defined defect sites in the glass. The activation energy for such motion is between 0.03 and 0.2 eV. Ions making large displacements often return to their starting points after a few picoseconds (periodic motion). It is suggested that such mobile ions are responsible for the low-temperature anomalous properties of glass.