Ferroelectric state in lead-free mixed-oxide system (1 − x)Na_0.5Bi_0.5TiO₃–xBaTiO₃ having high Ba contents

Abstract

The ferroelectric state in a simple-perovskite mixed-oxide system (1 − x)Na_0.5Bi_0.5TiO₃–xBaTiO₃ (NBT–xBT) for 0.30 ≤ x ≤ 1.0 was investigated primarily via in situ transmission electron microscopy observation. Dielectric permittivity measurements reveal that the NBT–xBT samples for 0.30 ≤ x < 0.80 exhibited hysteresis with a temperature width of approximately 20–50°C. This result suggests that a successive phase transition was present and that the ferroelectric state at room temperature was not a simple tetragonal symmetry. In situ transmission electron microscopes observation reveals that the ferroelectric state in the BT-rich end had an M_C-type monoclinic symmetry with 〈201〉_C polarization vectors in {100}_C planes. The ferroelectric M_C state was characterized by a 79° (pseudo 90°) domain with a (110)_C twin structure and 180° domain. Thus, the continuous change at room temperature from the ferroelectric tetragonal state in BT for x = 1.0 to the monoclinic M_C state for x = 0.30 can be explained by the introduction of a 〈100〉_C component perpendicular to the original [001]_C polarization vector in the tetragonal system.