**1. Introduction**

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‐ electric ceramics.[5-9]


**Table 1.** Typical piezoelectric crystals and their piezoelectric (*d*33 or *d*11) & dielectric (*ε*33) constants, and electromechanical coupling factor *k*.[1-4]

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 enhancement of electromechanical responses.[6-9]

**Figure 1.** Lattice distortion of mono-domain of a BaTiO3 crystal under the application of an electric field.4

**Figure 2.** Change with temperature of the dielectric permittivity of a BaTiO3 single crystal. The schematics of Ti dis‐ placement in the oxygen octahedron of the perovskite structure are also shown.
