The Converse Piezoelectric Effect in Mixed Crystals Isomorphous with Rochelle Salt

The temperature dependence of the piezoelectric strain constant d14 was investigated in mixed crystals isomorphous with Rochelle salt. Crystals oriented for utilization of Voigt's theory of the converse piezoelectric effect in the hemihedral class of the orthorhombic system were grown at constant temperature from supersaturated solutions of Rochelle salt containing isomorphous impurities C4H4O6Na NH4·4H2O or C4H4O6Na Tl·4H2O and measurements of the strain resulting from an applied electric field were made in the range 10 to 35°C. Hysteresis and saturation occur in the range of large piezoelectric activity where d14 evaluated from the straight portion of the strain curve is approximately 5 × 10−4 in both the pure salt and mixed crystal, a value several times as large as that reported from measurements of the direct effect in Rochelle salt. The critical temperature for a mixed crystal lies below that of pure Rochelle salt (25+273°K) and the shift observed compares favorably in magnitude with the value calculated from the dipole theory of dielectrics. The region in which d14 decreases to about 10−5 covers 3°C. The value of d14 diminishes further as the temperature is raised. d14 is defined as twice the change in length per cm length of a narrow crystal cut at 45° with the Y and Z axes when unit electrostatic field is applied in the X direction.