The correlation between structure and electrical properties of lead-free ͑1−x͒͑Bi 1/2 Na 1/2 ͒TiO 3 -xBaTiO 3 ͑BNT-100xBT͒ polycrystalline piezoceramics was investigated systematically by in situ synchrotron diffraction technique, combined with electrical property characterization. It was found that the morphotropic phase boundary ͑MPB͒ between a rhombohedral and a tetragonal phase evolved into a morphotropic phase region with electric field. In the unpoled material, the MPB was positioned at the transition from space group R3m to P4mm ͑BNT-11BT͒ with optimized permittivity throughout a broad single-phase R3m composition regime. Upon poling, a range of compositions from BNT-6BT to BNT-11BT became two-phase mixture, and maximum piezoelectric coefficient was observed in BNT-7BT. It was shown that optimized electrical properties are related primarily to the capacity for domain texturing and not to phase coexistence.

The domain morphology and crystal structure of (1−x)(Bi1/2Na1/2)TiO3xBaTiO3 lead-free piezoelectric ceramics were systematically studied with transmission electron microscopy for compositions x=0.04through 0.11. It was found that the ceramics with compositions x<0.06 display a R3csymmetry with ferroelectric domains of ∼100 nm forming complex structures at room temperature. Only nanodomains with faint contrast were observed in the compositions of 0.07≤x≤0.09. The presence of weak 1/2 (ooe)superlattice diffraction spots and absence of 1/2 (ooo) ones (o stands for odd and e stands for even miller indices) seem to suggest a P4bm symmetry at room temperature. The morphotropic phase boundary composition x=0.06 showed mixed R3c and P4bm phases. Large lamellar ferroelectric domains with P4mm symmetry were found to dominate in the ceramic of x=0.11. The observed domain structure correlates extremely well with the frequency dispersion of dielectric constant at room temperature and a new concept "relaxor antiferroelectric" was proposed to describe the dielectric behavior of compositions 0.07≤x≤0.09. These results are summarized in a phase diagram for unpoled ceramics in the (1−x)(Bi1/2Na1/2)TiO3xBaTiO3binary solid solution system.

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Giant electric-field-induced strain of 0.70%, corresponding to a d33 * value of 1400 pm V(-1) , is observed in a lead-free (Bi1/2 Na1/2 )TiO3 -based polycrystalline ceramic. This is comparable to the properties of single crystals. An in situ transmission electron microscopy study indicates that the excellent performance originates from phase transitions under the applied electric fields.

A comprehensive review on the latest development of the antiferroelectric ferroelectric phase transition is presented. The abrupt volume expansion and sudden development of polarization at the phase transition has been extensively investigated in PbZrO 3 -based perovskite ceramics. New research developments in these compositions, including the incommensurate domain structure, the auxetic behavior under electric fields in the induced ferroelectric phase, the ferroelastic behavior of the multicell cubic phase, the impact of radial compression, the unexpected electric field-induced ferroelectric-to-antiferroelectric transition, and the phase transition mechanical toughening effect have been summarized. Due to their significance to lead-free piezoelectric ceramics, compounds with antiferroelectric phases, including NaNbO 3 , AgNbO 3 , and (Bi 1/ 2 Na 1/2 )TiO 3 , are also critically reviewed. Focus has been placed on the (Bi 1/2 Na 1/2 )TiO 3 -BaTiO 3 solid solution where the electric field-induced ferroelectric phase remains even after the applied field is removed at room temperature. Therefore, the electric field-induced antiferroelectric-to-ferroelectric phase transition is a key to the poling process to develop piezoelectricity in morphotropic phase boundary (MPB) compositions. The competing phase transition and domain switching processes in 0.93(Bi 1/2 Na 1/2 )TiO 3 -0.07BaTiO 3 are directly imaged with nanometer resolution using the unique in situ transmission electron microscopy (TEM) technique. KeywordsAntiferroelectric ceramics, phase transition, domain structure, polarization and strain, ferroelastic deformation, lead-free piezoelectrics, in situ TEM Disciplines Ceramic Materials | Electromagnetics and Photonics | Metallurgy CommentsThis is the peer reviewed version of the following article: Journal of the American Ceramic Society 94, 4091-4107 (2011 AbstractA comprehensive review of the latest development on the antiferroelectric  ferroelectric phase transition is presented. The abrupt volume expansion and sudden development of polarization at the phase transition has been extensively investigated in PbZrO3-based perovskite ceramics. New research developments in these compositions, including the incommensurate domain structure, the auxetic behavior under electric fields in the induced ferroelectric phase, the ferroelastic behavior of the multicell cubic phase, the impact of radial compression, the unexpected electric field-induced ferroelectric-to-antiferroelectric transition, and the phase transition mechanical toughening effect have been summarized. Due to their significance to lead-free piezoelectric ceramics, compounds with antiferroelectric phases, including NaNbO3, AgNbO3, and (Bi1/2Na1/2)TiO3, are also critically reviewed. Focus has been placed on the (Bi1/2Na1/2)TiO3-BaTiO3 solid solution where the electric field-induced ferroelectric phase remains even after the applied field is removed at room temperature.Therefore, the electric field-induced antiferroelectric-to-ferroelectric phase tr...

The phase transitions in unpoled lead‐free (1−x)(Bi1/2Na1/2)TiO3–xBaTiO3 (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 Td, 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 Td revealed by the dielectric characterization. With further increasing temperature, no structural change was observed for both compositions across TRE, where the dielectric frequency dispersion vanishes, and Tm, where the dielectric permittivity reaches maximum. The tetragonal‐to‐cubic transition is diffuse and takes place in a broad temperature window well above both TRE and Tm. These results of structural phase transitions are summarized in a phase diagram with its composition range covering the morphotropic phase boundary (MPB).

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Most antiferroelectric ceramics are modified from the prototype PbZrO 3 by adding Sn and Ti in conjunction with small amount of Nb or La to optimize their properties. These modifiers introduce unique nanoscale structural feature to the ceramics in the form of incommensurate modulations. It was shown previously that the modulation is strongly responsive to a change in chemical composition or temperature. However, its response to an electric field, the driving force in real applications, has not been explored before. In the present work the dynamic evolution of the incommensurate modulation during the electric field-induced antiferroelectric-to-ferroelectric transformation was observed with an in situ transmission electron microscopy (TEM) technique. The results indicate that the incommensurate modulation exists as a transverse Pb-cation displacement wave. The wavelength was found to be quite stable against external electrical stimuli, in sharp contrast to the dramatic change under thermal stimuli reported previously. It is suggested that the appeared incommensurate modulation is an average effect of a mixture of two commensurate modulations. The electric field-induced antiferroelectric-to-ferroelectric transformation proceeds with aligning the Pbcation displacements, which resembles the process of 90° reorientation and 180° reversal in normal ferroelectrics.Most antiferroelectric ceramics are modified from the prototype PbZrO 3 by adding Sn and Ti in conjunction with small amount of Nb or La to optimize their properties. These modifiers introduce unique nanoscale structural feature to the ceramics in the form of incommensurate modulations. It was shown previously that the modulation is strongly responsive to a change in chemical composition or temperature. However, its response to an electric field, the driving force in real applications, has not been explored before. In the present work the dynamic evolution of the incommensurate modulation during the electric field-induced antiferroelectric-toferroelectric transformation was observed with an in situ transmission electron microscopy ͑TEM͒ technique.The results indicate that the incommensurate modulation exists as a transverse Pb-cation displacement wave. The wavelength was found to be quite stable against external electrical stimuli, in sharp contrast to the dramatic change under thermal stimuli reported previously. It is suggested that the appeared incommensurate modulation is an average effect of a mixture of two commensurate modulations. The electric field-induced antiferroelectric-to-ferroelectric transformation proceeds with aligning the Pb-cation displacements, which resembles the process of 90°reorientation and 180°reversal in normal ferroelectrics.