Ultrasonic Study of KH₂PO₄ near the Pressure-Induced Tricritical Point

Abstract

Elastic stiffness c₆₆^E of KH₂PO₄ was measured at 10 MHz simultaneously with the dielectric constant ε_c as a function of temperature at several fixed hydrostatic pressures. As predicted by the Landau theory, no distinct discontinuous change was observed in the elastic Curie constant A(p) and the dielectric Curie constant C(p) of the paraelectric phase around the tricritical point. The elastic Curie constant decreases with pressure as (A(p)⁻¹−A(0)⁻¹)⁄A(0)⁻¹⁄A(0)⁻¹=(0.065±0.014)p. The piezoelectric strain constant b₃₆(p) which is defined by the relation A(p)=b₃₆(p)₂C(p) was found to decrease with pressure as (b₃₆(p)−b₃₆(0))⁄b₃₆(0)=(−0.028±0.008)p.