Dielectric, electromechanical, and elastic properties of Rb$_{1-x}$(NH$_{4}$)$_{x}$H$_{2}$PO$_{4}$ single crystals

The longitudinal dielectric, piezoelectric, and elastic characteristics of ferroelectric RbH$_{2}$PO$_{4}$ and antiferroelectric NH$_{4}$H$_{2}$PO$_{4}$ crystals are calculated in terms of the modified model of proton ordering with allowance for the piezoelectric coupling in an approximation of a four-particle cluster. The results obtained agree well with known experimental data. Along with this, the electromechanical properties of a Rb$_{0.2}$(NH$_{4}$)$_{0.8}$H$_{2}$PO$_{4}$ single crystal undergoing an antiferroelectric phase transition have been studied experimentally over a wide temperature range. The model predictions and the experimental data agree qualitatively.