Elastic softening near the phase transitions in (1 − x)Bi_1/2Na_1/2TiO₃-xBaTiO₃ solid solutions

Phase transitions in sol–gel-derived (1 − x)(Bi_1/2Na_1/2TiO₃)-xBaTiO₃ (0 not ≦ x not ≦ 0.06) solid solutions were investigated via dynamic mechanical, dielectric, and ferroelectric analyses. Structural phase transition was observed from both noncentrosymmetric ferroelectric to nonpolar (antiferroelectric) forms (FE-AFE) at temperatures ranging from ∼120 °C to ∼210 °C, and from antiferroelectric to paraelectric forms (AFE-PE) at temperatures ranging from ∼250 °C to ∼320 °C for 0 ≦ x ≦ 0.06. The former transition is accompanied by a significant and broad elastic softening or soft-mode process, and the latter is of typical diffused type. The dynamic mechanical scaling exponent and critical scaling exponent are used to characterize these two phase transitions, respectively. The temperature-dependent polarization showed an abnormal trend unlike other reported ferroelectrics, confirming the presence of two different phases: FE and AFE. The d₃₃ piezoelectric constant increases with increasing x up to the morphotropic phase boundary. It is suggested that the dynamic mechanical scaling exponents could be used to characterize the mobility of the polar domains and the elastic softening processes, which are closely related to abnormal pyroelectric properties and piezoelastic hardening behaviors. (paper)