Due to issues with Pb toxicity, there is an urgent need for high performance Pb-free alternatives to Pb-based piezoelectric ceramics. Although pure BaTiO₃ material exhibits fairly low piezoelectric coefficients, further designing of such a material system greatly enhances the piezoelectric response by means of domain engineering, defects engineering, as well as phase boundary engineering. Especially after the discovery of a Ba(Zr_0.2Ti_0.8)O₃–x(Ba_0.7Ca_0.3)TiO₃ system with extraordinarily high piezoelectric properties (d₃₃ > 600 pC/N), BaTiO₃-based piezoelectric ceramics are considered as one of the promising Pb-free substitutes. In the present contribution, we summarize the idea of designing high property BaTiO₃ piezoceramic through domain engineering, defect-doping, as well as morphotropic phase boundary (MPB). In spite of its drawback of low Curie temperature, BaTiO₃-based piezoelectric materials can be considered as an excellent model system for exploring the physics of highly piezoelectric materials. The relevant material design strategy in BaTiO₃-based materials can provide guidelines for the next generation of Pb-free materials with even better piezoelectric properties that can be anticipated in the near future.

We designed a Pb-free pseudo-binary system, Ba(Sn_0.12Ti_0.88)O₃-x(Ba_0.7Ca_0.3)O₃ (BTS-xBCT), characterized by a phase boundary starting from a tricritical triple point of a paraelectric cubic phase, ferroelectric rhombohedral, and tetragonal phases. The optimal composition BTS-30BCT exhibits a high piezoelectric coefficient d₃₃ ∼ 530 pC/N at room temperature. In view of the recent report of high piezoelectricity in another Pb-free system BZT-BCT (Liu and Ren, Phys. Rev. Lett. 103, 257602 (2009)), which possesses a similar tricritical triple point in the phase diagram, it seems that forming a suitable phase boundary starting from a tricritical triple point could be an effective way to develop high-performance Pb-free piezoelectrics.

There is an urgent demand for high performance Pb-free piezoelectrics to substitute for the current workhorse, the lead zirconate titanate (PZT) family. Recently, a triple point (also tricritical point) type morphotropic phase boundary (MPB) in Pb-free Ba(Zr_0.2Ti_0.8)O₃-x(Ba_0.7Ca_0.3)TiO₃ system has been reported that shows equally as excellent piezoelectricity as soft PZT at room temperature (Liu and Ren⁶). In the present study, we measured a full set of elastic, piezoelectric, and dielectric properties for the MPB composition, Ba(Zr_0.2Ti_0.8)O₃-50(Ba_0.7Ca_0.3)TiO₃ (BZT-50BCT), by using a resonance method. The resonant method gives piezoelectric properties d₃₃ = 546 pC/N, g₃₃ = 15.3 × 10⁻³ Vm/N, electromechanical coupling factor k₃₃ = 65%, and the elastic constant $s_{33}^E$ = 19.7 × 10⁻¹² m²/N, $c_{33}^E$ = 11.3 × 10¹⁰ N/m², which are close to the properties of soft PZT (PZT-5A). Furthermore, the piezoelectric coefficients (k₃₃, d₃₃), the ferroelectric properties (coercive field, remnant polarization), and the elastic constants ($s_{33}^D$ , $s_{33}^E$ , $c_{33}^D$ , $c_{33}^E$ ) were also determined as a function of temperature ranging from −50 to 60°C. Our results show that the properties are optimal around MPB temperature (room temperature) and decrease with deviations from the MPB temperature. Nevertheless, the piezoelectric coefficient d₃₃ can maintain an appreciable level of 93 pC/N even at −50°C. The high piezoelectric properties can be ascribed to the low polarization anisotropy as well as the elastic softening at MPB.

In this letter, we use transmission electron microscopy to study the microstructure feature of recently reported Pb-free piezoceramic Ba(Zr_0.2Ti_0.8)O₃-x(Ba_0.7Ca_0.3)TiO₃ across its piezoelectricity-optimal morphotropic phase boundary (MPB) by varying composition and temperature, respectively. The domain structure evolutions during such processes show that in MPB regime, the domains become miniaturized down to nanometer size with a domain hierarchy, which coincides with the d₃₃-maximum region. Further convergent beam electron diffraction measurement shows that rhombohedral and tetragonal crystal symmetries coexist among the miniaturized domains. Strong piezoelectricity reported in such a system is due to easy polarization rotation between the coexisting nano-scale tetragonal and rhombohedral domains.

We report a large piezoelectric Pb-free system—Ba(Ti_0.8Hf_0.2)O₃-(Ba_0.7Ca_0.3)TiO₃, designed upon a triple-point morphotropic phase boundary (TMPB) idea. The system shows anomalies of both ac (dielectric and piezoelectric) and dc (P-E hysteresis) properties around TMPB, especially the d₃₃ achieving ∼550 pC/N at room temperature. Moreover, the detected non-zero thermal hysteresis around triple point shows a first order transition nature and non-isotropic polarization state, as well as verified by our theoretical Landau-type deduction, thus being considered as an important factor to influence the piezoelectricity along TMPB. Our work may stimulate the study on triple-point-related critical phenomena in other ferroic systems.