Piezoelectricity and long-range order in NH4Cl

The temperature dependence of the piezoelectric constant d14 of NH4Cl has been measured in the vicinity of the order–disorder phase transition. An analysis of data obtained in the ordered phase yields β=0.134 for the critical exponent associated with the order parameter. Above the transition temperature, there is an anomalous ’’tail’’ of nonzero d14 values that indicates a persistence of long-range order in the ’’disordered’’ phase. The values of d14 in this tail are well described by the empirical expression d14=const×exp(−σT/Tc) with σ=335. This anomalous behavior is essentially identical to that observed previously for the second-harmonic scattering intensity.