Direct observation of the polar state in the relaxor Ba(Ti _1-xZr_x)O₃ by transmission electron microscopy

In the ferroelectric mixed-oxide system Ba(Ti_1-xZr _x)O₃ (BTZ), an increase in Zr content results in the crossover from the ferroelectric M_A-type state with monoclinic symmetry to the relaxor state around x = 0.28. As a result of the crossover, BTZ is identified as the relaxor for x > 0.28. To understand the nature of the relaxor in BTZ, direct observations of the paraelectric (PE), M_A, and relaxor states for 0.17 ≤ x ≤ 0.40 was carried out between 400 K and 87 K by transmission electron microscopy with the help of the failure of Friedel's law. The observations indicated that polar nanometer-sized regions with <110>_PE and <001>_PE components were separately observed in the PE state above both T_c and T_m, which are, respectively, a transition temperature of the direct (PE→M_A) transition and a maximum temperature of the real dielectric permittivity for the relaxor. During cooling from the paraelectric state above T_m in the relaxor, in the relaxor state below it, <001>_PC-component regions locally formed a 180° domain structure, together with no coalescence of <110>_PC regions. It is thus understood that the M _A-to-relaxor crossover is characterized by the complete suppression of the coalescence of polar nanometer-sized regions with <110> _PE components. The relaxor state below T_m for 0.29 ≤ x ≤ 0.40 could be, as a result, identified as an assembly of polar nanometer-sized domains, presumably with monoclinic symmetry.