Piezoelectricity and long-range order in NH4Cl

Abstract

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/T_c) with σ=335. This anomalous behavior is essentially identical to that observed previously for the second‐harmonic scattering intensity.