Elastic anomaly in (NH4)2SO4

Measurements of the elastic compliance and electrostrictive constant of a (NH4)2SO4 crystal, together with the dielectric constant and spontaneous polarization, have been made. It is observed that there are anomalies in the elastic compliances and that the electrostrictive constants are of the ordinary order of magnitude. A phenomenological theory has been developed to analyse the experimental results, where an order parameter is introduced and its couplings with polarization and stress are taken into account. The anomalous temperature dependence observed in the dielectric constant, spontaneous polarization, spontaneous strain, and elastic compliance is explained in terms of the temperature dependence of the order parameter. The only exception is the shear component for which a further investigation might be necessary.