In situ Transmission Electron Microscopy Study on the Phase Transitions in Lead-Free (1-x)(Bi_(1/2)Na_(1/2))TiO_3-xBaTiO_3 Ceramics

摘要

The phase transitions in unpoled lead-free (1-x)(Bi_(1/2)Na_(1/2) TiO_3=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).