Abstract
Attenuation and dispersion of the longitudinal phonons of the [100] propagation in NH 4 Cl were investigated by the Brillouin scattering of the back scattering geometry (∼33 GHz) and by that of the nearly forward (θ= 8^°) scattering (∼2.3 GHz). The velocity obtained from the back scattering shows a temperature dependence almost the same as that from the right angle scattering. The velocity at 2.3 GHz shows an intermediate behavior between those of the ultrasonic measurements and of the back Brillouin scattering. The temperature dependence of the phonon decay rate shows behavior different from that predicted by the classical theory of sound attenuation and dispersion due to relaxation. The increase in the velocity in the ordered phase was interpreted by the compressible Ising model proposed by Garland and Renard.
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