**3. Effects of Ca-substitution in the Ba(Ti,Zr)O3 solid solution**

To confirm the effects of Ca off-centering in Ba-based perovskite oxides, we also performed investigations on the system of Ba(Ti,Zr)O3 solid solutions, which have been intensively studied since the mid-1950s.[25, 26] The most amazing finding in this system is that it dem‐ onstrates very large piezoelectric response, comparable to that of industrial PZT. A high electromechanical coupling factor of 74% and large piezoelectric coefficients of 340 pC/N under a high field were observed in this system.[27, 28] Another interesting thing in this system is that the *O*- or *R*-phase can be tuned to room temperature through controlling Zr substitution, which is of great significance for ferroelectric phase modification. However, as shown in the phase diagram reported by Kell and Hellicar (Fig. 12),[25] the problem of this system is that the Curie point decreases with the increase in the Zr concentration. For Zr concentrations larger than 20 mol%, the Curie point is reduced to a temperature lower than that at room temperature, leading to the disappearance of ferroelectricity in the crystal at room temperature. Here, we show that the Ca off-centering effects mentioned above can also be used to increase the Curie point of Ba(Ti,Zr)O3 and enhance its electromechanical coupling effects through tuning the ferroelectric phase boundaries to room temperature.[9]

**Figure 12.** Phase diagram of Ba(Ti1-xZrx)O3 solid solutions proposed by Kell and Hellicar.[25] For comparison, the phase diagram of (Ba1-*x*Ca*x*)TiO3 is also shown (solid circles).

### **3.1. Sample preparation**

In our study, we selected a composition with a Zr concentration of 10 mol%, at which the three successive phases tend to approach each other as shown in Fig. 12. We prepared the (Ba1-*x*Ca*x*) (Ti0.9Zr0.1)O3 (BCTZO) ceramics by a solid-state reaction approach. Mixtures of BaCO3, CaCO3, ZrO2, and TiO2 were calcined at 1823 K for 3 h. The calcined powders were ground, pressed, and sintered at 1823 K for 5 h. The ceramic pellets were then electroplated with silver for electrical measurements.

### **3.2. Phase formation and structure transformation at room temperature**

At the sintering temperature of 1823 K, a single phase of BCTZO was found to be formed within the composition range of *x* ≤ 0.18 beyond which a non-ferroelectric phase with CaTiO3-type orthorhombic structure occurs and coexist with the BaTiO3-type ferroelectric phase. The phase equilibria of (1-*x*)Ba(Zr0.1Ti0.9)O3-*x*CaTiO3 are very similar to those of (1-*x*)BaTiO3-*x*CaTiO3 reported by DeVries and Roy[15] as shown in Fig. 4(a). However, the solid solution limit of (1 *x*)Ba(Zr0.1Ti0.9)O3-*x*CaTiO3 is approximately half that of (1-*x*)BaTiO3-*x*CaTiO3. This fact indicates that the substitution of Zr for Ti in BaTiO3 will reduce the substitution amount of Ca for Ba.

At room temperature, BCTZO with *x*=0 has a ferroelectric rhombohedral structure as shown in phase diagram of Fig. 12. When Ba is substituted with Ca, the structure of BCTZO at room temperature was found to transform from *R*-phase to *O*-phase, and finally to *T*-phase with the increase in Ca concentration. Figure 13 shows the change of lattice parameters with Ca concentration for the BCTZO system. Similar to the unit cell of (Ba1-*x*Ca*x*)TiO3 (Fig. 9(a)), the unit cell of BCTZO shrinks with the substitution of the smaller Ca for the bulky Ba, and its volume is reduced from 65.21 Å3 for *x*=0 to 63.91 Å3 for *x*=0.18.The ferroelectric lattice distortion

**Figure 13.** Change of the lattice parameters with composition at room temperature for the (Ba1-xCax)(Ti0.9Zr0.1)O3 system.

at room temperature is very small in the BCTZO solid solution. The distortion angles *α* in the *R*-phase and *β* of the monoclinic unit cell in the *O*-phase have deviations of only 0.01° and 0.1° from a right angle, respectively, while the tetragonality *c/a* has a value of 1.005 for x ≥ 0.15 in the *T*-phase.
