In situ Transmission Electron Microscopy Study on the Phase Transitionsin Lead‐Free (1− x )( Bi 1/2 Na 1/2 )TiO 3 – x BaTiO 3 Ceramics

The phase transitions in unpoled lead‐free (1− x )(Bi 1/2 Na 1/2 )TiO 3 – x BaTiO 3 ( x = 0.06 and 0.11) ceramics are investigated using hot‐stage transmission electron microscopy ( TEM ). It is found that large ferroelectric domains in both ceramics start to disappear around T d , the depolarization temperature. After the transition, both compositions exhibit the P 4 bm tetragonal symmetry in the form of nanodomains. The structural transition observed by the in situ TEM experiments seems to be gradual and occurs within a temperature range of several tens of degrees, in contrast to the sharp anomaly at T d revealed by the dielectric characterization. With further increasing temperature, no structural change was observed for both compositions across T RE , where the dielectric frequency dispersion vanishes, and T m , where the dielectric permittivity reaches maximum. The tetragonal‐to‐cubic transition is diffuse and takes place in a broad temperature window well above both T RE and T m . These results of structural phase transitions are summarized in a phase diagram with its composition range covering the morphotropic phase boundary ( MPB ).