In the ferroelectric mixed-oxide system Ba(Ti1-xZr x)O3 (BTZ), an increase in Zr content results in the crossover from the ferroelectric MA-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), MA, 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 Tc and Tm, which are, respectively, a transition temperature of the direct (PE→MA) transition and a maximum temperature of the real dielectric permittivity for the relaxor. During cooling from the paraelectric state above Tm 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 Tm for 0.29 ≤ x ≤ 0.40 could be, as a result, identified as an assembly of polar nanometer-sized domains, presumably with monoclinic symmetry.
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