The temperature dependence of the piezoelectric constantd14of NH4Cl has been measured in the vicinity of the orderndash;disorder phase transition. An analysis of data obtained in the ordered phase yields bgr;=0.134 for the critical exponent associated with the order parameter. Above the transition temperature, there is an anomalous rsquo;rsquo;tailrsquo;rsquo; of nonzerod14values that indicates a persistence of longhyphen;range order in the rsquo;rsquo;disorderedrsquo;rsquo; phase. The values ofd14in this tail are well described by the empirical expressiond14=consttimes;exp(minus;sgr;T/Tc) with sgr;=335. This anomalous behavior is essentially identical to that observed previously for the secondhyphen;harmonic scattering intensity.
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