Order-disorder phenomena. VIII. Acoustic attenuation and dispersion in NH4 Cl

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Abstract
The attenuation coefficient a of longitudinal ultrasonicwaves propagating in the [100] direction of an NH4 Cl single crystal has been measured as a function of temperature at 1270, 1500, 1700, and 2656 bar. The elastic constant c 11 corresponding to this wave has also been measured at 1500 and 2656 bar. By combining these data with a variety to other ultrasonic and hypersonic information about NH4 Cl it is possible to determine the dynamical behavior near the order‐disorder transition in terms of the variation in the relaxation strength and the relaxation time. At a given value of the reduced temperature t=|T−T λ |/T λ the acoustic velocity and attenuation in the ordered phase are almost identical at all pressures, but the behavior is different in the disordered phase at 1 atm and at the ``tricritical'' pressure of ∼ 1500 bar. In particular, the relaxation rate 1/τ (in units of 108 sec−1) is 414 t 0.60 at 1 atm and is 123 t 0.88 at 1500 bar.