Phase transition behavior in ferroelectric BaTi_0.8Zr_0.2O₃: Evidence of polar cluster reorientation above Curie temperature

We study the phase transition behavior of the ferroelectric BaTi_0.8Zr_0.2O₃ in the paraelectric region above the Curie temperature. The investigation of the phase transition using caloric, dielectric, and elastic measurements indicates that the ferroelectric transition at T_c = 292 K is continuous and displays weakly relaxor characteristics. The nonlinear scaling of entropy and polarization, as well as the temperature dependencies of dielectric and elastic properties, indicates the presence of local structures in the paraelectric phase. The non-zero remnant polarization is measured up to a characteristic temperature T* ∼ 350 K. This temperature coincides with the temperature where the dielectric constant deviates from the Curie-Weiss law and is identified as the coherence temperature T*, associated with the formation of static polar nanostructures. Finally, direct current field cooling in the paraelectric phase using fields smaller than the coercive field leads to an elastic response and remnant piezoelectricity below T*, attributed to the re-orientation of polar nanostructures. The observed remnant effect, along with the temperature dependence of the piezoelectric effect and its time dependence below and above T*, is consistent with increased coherence and slower dynamics of these structures on cooling, leading to symmetry-disallowed remnant piezoelectricity due to glassy behavior below T*.