Abstract
The search for improved piezoelectric materials is based on the morphotropic phase boundaries (MPB) between ferroelectric phases with different crystal symmetry and available directions for the spontaneous polarization. Such regions of the composition phase diagrams provide the conditions for minimal anisotropy with respect to the direction of the polarization, so that the polarization can easily rotate maintaining a substantial magnitude, while the near verticality of the boundary extends the temperature range of the resulting enhanced piezoelectricity. Another consequence of the quasi-isotropy of the free energy is a reduction of the domain walls energies, with consequent formation of domain structures down to nanoscale. Disentangling the extrinsic and intrinsic contributions to the piezoelectricity in such conditions requires a high level of sophistication from the techniques and analyses for studying the structural, ferroelectric and dielectric properties. The elastic characterization is extremely useful in clarifying the phenomenology and mechanisms related to ferroelectric MPBs. The relationship between dielectric, elastic and piezoelectric responses is introduced in terms of relaxation of defects with electric dipole and elastic quadrupole, and extended to the response near phase transitions in the framework of the Landau theory. An account is provided of the anelastic experiments, from torsional pendulum to Brillouin scattering, that provided new important information on ferroelectric MPBs, including PZT, PMN-PT, NBT-BT, BCTZ, and KNN-based systems.
Highlights
The ferroelectric (FE) material most used in applications is PbZr1−xTixO3 (PZT)
It started to be studied in the early 1950s, when in its x − T phase diagram a nearly vertical boundary between ferroelectric (FE) rhombohedral (R) and tetragonal (T) phases was found [1] and called morphotropic phase boundary (MPB)
A MPB between ferroelectric phases with different available directions for the spontaneous polarization is favorable to a high piezoelectric response for two reasons: because the transverse susceptibilities near the phase transition are enhanced and because have better thermal stability
Summary
The ferroelectric (FE) material most used in applications is PbZr1−xTixO3 (PZT). It started to be studied in the early 1950s, when in its x − T phase diagram a nearly vertical boundary between ferroelectric (FE) rhombohedral (R) and tetragonal (T) phases was found [1] and called morphotropic phase boundary (MPB). This discovery fitted perfectly with the almost simultaneous theoretical analysis of the high piezoelectricity of PZN-PT in terms of rotation of the polarization rather than domain wall motion [9], since the M phase of PZT corresponds to the intermediate, often called “bridging”, state between R and T phases, if the polarization rotates under the application of a strong electric field These findings and interpretation elicited considerable experimental and theoretical activities aimed at perfecting the model and searching materials with improved properties, but soon after a completely different explanation of the same facts was proposed [10]. Already in the prototype PZT, the MPB can be crossed by varying temperature and its near verticality has only the role of increasing the thermal stability of the state with enhanced piezoelectricity This is especially true in Pb-free materials, where the boundaries involving a rotation of the spontaneous polarization are generally normal TPB [22].
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