Piezoelectric materials have attracted much attention for a wide range of applications, including electrical, mechanical and medical devices, and, for the last three decades, high-performance, environmentally friendly (i.e., Pb-free) piezoelectric materials have been an important research target. The piezoelectric constant (d₃₃) can be maximized at the Curie temperature (T_C) [1]; this is because materials can be in their softest state due to the coexistence of two different phases. The depth analysis of microstructure at T_C has shown the existence of numerous nano-ordered interfaces, and the structure-gradient epitaxial interfaces are much softer compared to non-interfacial regions [2]. The material softness (elastic compliance) at room temperature is inversely related to T_C, but Pb(Zr,Ti)O₃ (PZT) with morphotropic phase-boundary composition is a very soft material, despite its high T_C of 350 ˚C, due to numerous heteroepitaxial interfaces at room temperature [3]. Moreover, Pb(Zn_1/3Nb_2/3)O₃-PbTiO₃ (PZN-PT) single crystals with engineered domain configurations have shown ultrahigh d₃₃ values, over 2,000 pC/N, owing to their highly dense domain walls [4]. Thus, high-performance, environmentally friendly (without Pb) piezoelectric materials should be composed of nano-ordered ferroelectric domains with fixed (static) electric polar vectors (interface engineering). First, we focused on the development of perovskite-type BaTiO₃-Bi(Mg_1/2Ti_1/2)O₃ (BT-BMT, Pb-free) relaxor-like materials (T_C over 200 ˚C) composed of polar nanoregions with flipping (dynamic) polar vectors [5]. The origin of the polar vectors is the off-centering of Bi³⁺ ions because of (1) the coexistence of large Ba²⁺ and small Bi³⁺ ions at the same A-site position in perovskite-type ABO₃, and (2) strong covalent bonding between Bi³⁺ and O^2- ions. To fix their dynamic polar vectors, nano-ordered electric or stress bias fields are required. We proposed the hypothesis that the introduction of three or more B-site ions with different valences and sizes in ABO₃ can lead to nano-ordered bias fields because of the heterogeneous distribution of the various B-site ions in nano-sized regions, even though the distribution is homogeneous at the macroscopic scale. Therefore, we introduced BiFeO₃ (BF; T_C of 830 ˚C) in the BT-BMT system so that there were three cations with different valences and sizes [Mg²⁺ (72 pm), Fe³⁺ (64.5 pm) and Ti⁴⁺ (60.5 pm)] at the B-site. As a result, the studied BT-BMT-BF materials exhibited nanodomains, with sizes of only a few nm, and static polar vectors. The ultrahigh lattice strain of around 0.15 % under an applied electric field of 50 kV/cm is related to their softness. For more information, please see our recent publication in Communications Materials: Piezoelectricity in perovskite-type pseudo-cubic ferroelectrics by partial ordering of off-centered cations.

References

[1] M. Budimir et al., J. Appl. Phys. 94, 6753 (2003).

[2] T. Tsuji et al., Appl. Phys. Lett. 87, 071909 (2005).

[3] T Asada et al., Phys. Rev. B 75, 214111 (2007).

[4] S. Wada et al., Ferroelectrics 221, 147 (1999).

[5] S. Wada et al., J. Appl. Phys. 108, 094114 (2010).