In situ Transmission Electron Microscopy Study on the Phase Transitions in Lead-Free (1-x)(Bi1/2Na1/2)TiO3-xBaTiO3 Ceramics

The phase transitions in unpoled lead-free (1-x)(Bi1/2Na1/2) TiO3-xBaTiO(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 P4bm 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).