  The speed of longitudinal sound waves at 7 and 22 MHz has been measured in
liquid, supecooled, and amorphous selenium, including the region around the
glass transition temperature, Tg, near 35 C. In amorphous selenium the speed of
shear waves at 7 MHz was also measured. The experiments were performed with
high purity Se (99.9999%) hermetically sealed in an evacuated quartz ampoule.
Four temperature regions with strongly different relaxation times can be
distinguished between room temperature and the melting point: (1) a glassy
state below Tg, which is stable on the time scale of the experiments, (2) a
glassy state above Tg,, which is metastable on the time scale of the
experiments, (3) a region where homogeneous crystal nucleation occurs, and (4)
a supercooled liquid, which is stable on the time scale of the experiments.
Each region is marked by a change in the slope of the temperature dependence of
the sound velocity. Near the glass transition temperature the velocities of
longitudinal and transverse sound exhibit hysteresis with a step-like drop on
heating and a more continuous rise on cooling. The step-like anomaly in sound
velocity may be a general property of the glass transition.
