**2. Effects of Ca substitution in BaTiO3**

**1. Introduction**

106 Ferroelectric Materials – Synthesis and Characterization

electric ceramics.[5-9]

PZN-PT8% 3m[111]

electromechanical coupling factor *k*.[1-4]

[001]

enhancement of electromechanical responses.[6-9]

There is increased interest in developing green piezoelectric materials in the field of electronics due to environmental concerns regarding the Pb-toxicity in commercially used lead-based Pb(Zr,Ti)O3 (PZT) piezoelectric ceramics. As listed in Table 1 (Ref. 1-4), BaTiO3 single crystals have the highest piezoelectric coefficients among single crystals of lead-free piezoelectrics. Although the reported values of its piezoelectric coefficient vary somewhat, recent investiga‐ tions on the mono-domain of a single crystal by high energy synchrotron x-ray radiation show that BaTiO3 has a *d*<sup>33</sup> value of 149±54 pm/V at least at the level of lattice distortion (Fig. 1).[4] The large piezoelectric response makes BaTiO3 a promising material for novel green piezo‐

**Crystal Point group** *d33* **(pC/N)** *ε<sup>33</sup> k (%)* quartz 32 2.3(*d*11) 4.6 10 (*X-*cut) ZnO 6mm 10.6 11 41 (*k*33) LiNbO3 3m 6 30 17 (*Z-*cut) PbTiO3 4mm 19.3 121 64(*k*33) BaTiO3 4mm 149 168 65(*k*33)

> 84 2500

Piezoelectricity is the ability of a single crystal with non-centrosymmetry (with the exception of point group 432) to develop an electric charge proportional to a mechanical stress or to produce a deformation proportional to an electric field. The piezoelectricity in BaTiO3 is a direct result of its ferroelectricity, originating from the Ti atomic displacement in the oxygen octahedron of the ABO3 perovskite structure[10, 11] (Fig. 2). As can be inferred from the depiction of the variation of the dielectric permittivity with temperature in Fig. 2, there are two challenging issues that remain to be solved for BaTiO3: (1) the temperature instability of physical properties around room temperature due to the tetragonal (*T*)-orthorhombic (*O*) phase transition; and (2) its relatively lower Curie point of ~400 K in comparison with leadbased piezoelectrics. A-site substitution of Pb for Ba is able to increase the Curie point; however, such an approach is undesirable for green piezoelectrics. Principally, A-site and/or B-site substitution can be used to modify the ferroelectricity of BaTiO3. Here, we show that Asite substitution of Ca for Ba in the Ba-based perovskite oxides can lead to a variety of interesting phenomena: (1) the dramatic improvement of temperature stability of its physical properties, (2) the occurrence of quantum fluctuation at low temperatures, and (3) remarkable

**Table 1.** Typical piezoelectric crystals and their piezoelectric (*d*33 or *d*11) & dielectric (*ε*33) constants, and

1000 5000 0.39(*k*33) 0.94(*k*33)

> In the early years of 1960, Mitsui and Westphal investigated the influence of Ca substitution on the phase transitions in BaTiO3.[12] Using the ceramic samples, they established a phase diagram of (Ba1-xCax)TiO3 in the composition range of x<0.25 mole for temperatures higher than ~100 K. One interesting finding is that the Curie point remained nearly unchanged within the studied composition range. Such behavior is unexpected when considering that CaTiO3 is paraelectric and that the ionic radius of Ca (~1.34Å) is smaller than that of Ba (~1.60Å),[13] which would lead to the shrinkage of the unit cell and a reduction in ferroelectricity of systems with substitution of Ca for Ba. To gain new insight into the role of Ca substitution in Ba-based perovskite oxides, we re-examined the (Ba1-xCax)TiO3 system using single crystal samples, which allowed us to observe the intrinsic phenomena of the system.[6-8, 14]

### **2.1. Crystal growth**

To obtain a single crystal of (Ba1-*x*Ca*x*)TiO3, we used the floating zone (FZ) technique (Fig. 3(a)) that allowed us to grow a single crystal with high purity. According to the reported phase equilibria in the system (1-*x*)BaTiO3-*x*CaTiO3 (Fig. 4), only the crystal with a congruent melting composition (*x*=0.27 report by DeVries and Roy[15] or *x*=0.227 report by Kuper et al.[16]) can be directly grown from the melt. Surprisingly, using the FZ technique, we could grow a single crystal of (Ba1-*x*Ca*x*)TiO3 with a perovskite structure for a wide composition range of 0.02 ≤ *x* ≤ 0.34 with a high growth rate of 20 mm/h.[6] Fig. 3(b) shows a rod of crystal obtained by this method. It was found that the crystal can be stably grown under an atmosphere of Ar or N2 gas. The crystal was yellowish but transparent. The Laue X-ray diffraction patterns clearly indicated that the obtained (Ba1-*x*Ca*x*)TiO3 crystal had a perovskite structure.

**Figure 3.** (a)A schematic drawing of the floating zone (FZ) technique used to grow the (Ba1-*x*Ca*x*)TiO3 single crystal. (b) Photograph of the (Ba1-*x*Ca*x*)TiO3 single crystal grown by the FZ technique and its Laue X-ray back diffraction patterns along the [001]c direction of the perovskite structure.

**Figure 4.** Phase equilibria in the system (1-*x*)BaTiO3-*x*CaTiO3 reported by (a) DeVries and Roy[15] and (b) Kuper et al.[16]
