The attenuation coefficient α of longitudinal acoustic waves propagating along the [100] direction in ammonium chloride has been reinvestigated as a function of frequency from 10 to 60 Mc/sec and of temperature from 215° to 300°K. Special emphasis was given to 10‐Mc/sec data at temperatures very close to the order—disorder transition, and hysteresis was observed. These attenuation data are consistent with the behavior expected from a compressible Ising model which predicts a mechanical instability in the immediate vicinity of the transition temperature.
REFERENCES
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K. F. Herzfeld and T. A. Litovitz, Absorption and Dispersion of Ultrasonic Waves (Academic Press Inc., New York, 1959), Sec. 31.
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D. ter Haar, Elements of Statistical Mechanics (Rinehart Publishers, New York, 1958), pp. 254–273.
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In fact, (and ) should vary slightly with temperature since thermal expansion causes the volume to change even away from the transition point. This refinement does not add any qualitatively new features, and for the sake of clarity it will not be made. Indeed, it would be desirable to study the effect of pressure on the attenuation so that experimental data could be deduced for α vs T at a constant volume.
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D. G. Thomas and L. A. K. Staveley, J. Chem. Soc. 1951, 1420.
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© 1966 American Institute of Physics.
1966
American Institute of Physics
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