Abstract
Atomic mean-square displacements for frequencies above 1011 Hz have been determined from neutron time-of-flight data in the glassy, liquid and crystalline phase of selenium. As in many other substances, the difference between the values in the disordered and the ordered phase shows a strong increase with increasing temperature, setting in at about the glass temperature. The inverse of that difference turns out to be linearly related to the logarithm of the viscosity. The relation holds from below the glass transition temperature up to above the melting temperature, covering twenty decades in the viscosity.