Experimental Difference between First and Zero Sound in KBr

Long-wavelength acoustic phonons have been studied for each of the $\left[00\mathrm{}\zeta \right]T$, $\left[00\mathrm{}\zeta \right]L$, $\left[0\mathrm{}\zeta \zeta \right]T_1$, and $\left[0\mathrm{}\zeta \zeta \right]L$ branches in KBr at 95 and 463°K by means of inelastic neutron scattering. This study has shown that the elastic constants of KBr determined by inelastic neutron scattering (zero-sound regime) exhibit a different temperature dependence from those determined by ultrasonic techniques (first-sound regime). For example, the change in the zero-sound elastic constant for the $\left[0\mathrm{}\zeta \zeta \right]T_1$ branch between 95 and 463°K is (23±4)%, while the change in the first-sound elastic constant determined by Haussühl using ultrasonic techniques is (35.1±1.2)%. Cowley's theory for first and zero sound shows reasonable agreement with experiment.