Dynamic critical phenomena in ammonium halide crystals

R. C. Leung, C. Zahradnik, and C. W. Garland
Phys. Rev. B 19, 2612 – Published 1 March 1979
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Abstract

All available ultrasonic and hypersonic data on NH4Br, NH4Cl, and ND4Cl have been analyzed over a wide range of temperature, pressure, and frequency in the disordered cubic phase D, the "ferro-ordered" cubic phase Oc, and the "antiferro-ordered" tetragonal phase OT. The acoustical absorption and dispersion in the disordered phase can be interpreted in terms of a single relaxation time τ that exhibits critical behavior τΔTθ near the order-disorder transition. The resulting values of the critical exponents θ confirm the existence of a master phase diagram for these systems. In the tetragonal regime (NH4Br at 1 atm) the experimental value θ=1.38 for T>Tc corresponds quite well with the critical exponent zν, describing order-parameter relaxation for n=3 fixed points in Hohenberg and Halperin's model A. In the DOcOT triple-point regime (NH4Cl at 1 atm, NH4Br at 1.88 kbar) the experimental value θ=1.0 corresponds to zν given by conventional theory, which is appropriate for model A near higher-order critical points. In the multicritical regime (ND4Cl at 1 atm, NH4Cl at 1.5 kbar, NH4Br at 3.25 kbar) the experimental values of θ(1.1 to 1.2) associated with this DOc multicritical point cannot be easily interpreted in terms of currently available theory. The critical relaxation times were obtained using acoustical relaxation strengths that are independent of temperature, and several pieces of experimental evidence are presented in support of this method of analysis.

  • Received 7 September 1978

DOI:https://doi.org/10.1103/PhysRevB.19.2612

©1979 American Physical Society

Authors & Affiliations

R. C. Leung, C. Zahradnik, and C. W. Garland

  • Department of Chemistry and Center for Materials Science and Engineering Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

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Issue

Vol. 19, Iss. 5 — 1 March 1979

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