Lattice Dynamics of Argon at 4°, 40°, and 77°K

Abstract

Neutron scattering techniques have been used to measure phonon dispersion relations in a 1-ml single crystal of 36Ar. Data were taken along the 100 crystallographic direction for the transverse mode at 4°, 40°, and 77°K. The measurements were performed on a free-standing crystal in equilibrium with its own vapor pressure. Preliminary analysis of the results yields mean shifts of the frequency of the transverse mode of 7% at 40°K and 21% at 77°K, relative to the 4°K value. These frequency shifts appear to decrease slightly with increasing wave vector. The mean Grüneisen parameter for this mode between 4° and 77°K is then 3.2±0.4 if it is assumed that the frequency shifts are due to the change in volume alone. Good agreement is found between the experimental frequency shifts and those predicted using a classical lattice dynamical model.1