Advances in ultrasonic and electronic technology have made it practica
l to make elastic wave measurements with wavelengths which are near th
e optical range, i.e. in the micro meter range. Ultrasonic interferome
try has been adapted to a diamond anvil cell and thus expands the tool
s available for material property measurements under high pressure and
temperature. Sound waves with a frequency of 1 GHz were propagated th
rough an H2O sample in a diamond anvil cell. Starting at room temperat
ure and a pressure of approximately 1.2 GPa a single crystal of ice VI
was heated until it was completely liquid. Good signal transmission w
as observed in the solid, two phase and liquid states. While the sampl
e was in the two phase region the velocity of propagation of the phase
boundary between the solid and the liquid and its roughness were obse
rved. At a propagation velocity of 2 mu s(-1) the phase boundary is sm
ooth on the order of 1 mu. Compressional wave travel time measurements
at similar to 2.5 GPa through a similar to 200-mu thick garnet sample
illustrate the capability of the technique for solids. No frequency d
ispersion in the travel time is observed for this sample from 600 MHz
to 1.3 GHz.