We here report the substitution effects of the smaller Ca for the bulky Ba in the (Ba1-xCax)(Ti-1-yZry)O3 perovskite oxides for two systems (Ba1-xCax)TiO3 with y=0 and (Ba1-xCax)(Ti0.9Zr0.1)O3 with y=0.1. Ca off-centering was found to play a critical role in stabilizing the ferroelectric phase and tuning the polarization states in both systems. It was demonstrated that the atomic displacement due to Ca off-centering in the bulky Ba-sites in the perovskite structure provides an effective approach to compensate for the reduction of ferroelectricity due to chemical pressure, which allows to keep the Curie point nearly constant in the (Ba1-xCax)TiO3 system and increase the Curie point in the (Ba1-xCax)( Ti0.9Zr0.1)O3 system. It was commonly observed that the Ca off-centering effects lead to the shift of the rhombohedral–orthorhombic and orthorhombic–tetragonal phase transitions toward lower temperatures and the ferroelectric stability of the tetragonal phase, resulting in the occurrence of quantum phase transitions with interesting physical phenomena at low temperatures in the (Ba1-xCax)TiO3 system and remarkable enhancement of electromechanical coupling effects around room temperature in the (Ba1-xCax)(Ti0.9Zr0.1)O3 systems over a wide range of Ca-concentrations. These findings may be of great interest for the design of green piezoelectric materials.
- Ca off–centering
- perovskite oxides
- phase transition
- quantum effects
- electromechanical coupling effects
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 investigations on the mono-domain of a single crystal by high energy synchrotron x-ray radiation show that BaTiO3 has a
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 (
2. Effects of Ca substitution in BaTiO3
In the early years of 1960, Mitsui and Westphal investigated the influence of Ca substitution on the phase transitions in BaTiO3. 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), 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-
2.2. Dielectric behaviors
Figure 5 shows the temperature dependence of the dielectric permittivity of a (Ba1-
As is well known, the dielectric response in displacive-type ferroelectrics is dominated by phonon dynamics, particularly the soft-mode behavior. Lyddane–Sachs–Teller (LST) relationship predicts that the dielectric permittivity is inversely proportional to the soft-mode frequency. As a step toward understanding the temperature independence of the dielectric response of (Ba1-
2.3. Phase diagram and quantum phase transitions
From the temperature change of dielectric permittivity of a (Ba1-
One important finding is that ferroelectric–ferroelectric quantum phase transitions occur in (Ba1-
as opposed to the classical relationship,
(b) The inverse dielectric susceptibility varies with temperature as
for the quantum mechanical limit instead of the classical Curie law
Our phase diagram clearly shows that the
To examine point (b), we have analyzed the temperature variation of the inverse dielectric susceptibility of
Figure 8 shows two typical examples: one for
2.4. Ca off-centering predicted from first principles calculations
As mentioned above, the ionic radius of Ca is approximately 16% smaller than that of Ba. The substitution of Ca for Ba will absolutely result in the shrinkage of the perovskite unit cell. As shown in Fig. 9(a), both
In contrast, for the case of hydrostatic pressure, at the same level of unit-cell reduction, the Curie point is reduced to ~180 K, which is greatly lower than the ~400 K of pure BaTiO3 (Fig. 9(c)). The hydrostatic pressure gradually reduces the Curie point, leading to the complete disappearance of ferroelectricity in BaTiO3 at a level of 5% reduction of the unit cell. Although the chemical substitution of the smaller Ca for the bulky Ba also leads to the reduction of the unit cell, the effects of chemical pressure on the ferroelectricity in the (Ba1-
Apparently, the reduction of the unit cell by the chemical pressure shrinks the oxygen octahedron in the perovskite structure, resulting in the reduction of available space for a Ti off-centering shift in the oxygen octahedron. Since the ferroelectricity is derived from the Ti-shift in the oxygen octahedron in the perovskite structure of BaTiO3, it is naturally expected that chemical-pressure-induced reduction of the unit cell in the (Ba1-
To examine the idea of Ca off-centering in the bulky Ba sites, we performed first principles calculations for this system.[7, 24] Since Ca substitution tends to stabilize the tetragonal structure, we focused on the calculations in this structure to get information about Ca displacement. The results are summarized in Fig. 10. As shown in Fig. 10(a), a Ti shift along the  direction leads to the stability of the tetragonal phase in BaTiO3 (
2.5. Polarization and strain responses
For many technical applications, understanding the physical properties of a ceramics sample is of great importance. Figure 11 shows the variation of polarization, bipolar-, and unipolar-field-induced strains with the Ca substitution in the (Ba1-
For the strain response under an electric field, with the exception of BaTiO3 (
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 demonstrates 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
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-
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
At room temperature, BCTZO with
3.3. Phase evolution with temperature
To understand the phase evolution in the BCTZO system, we have measured the temperature variation of dielectric permittivity for different Ca concentrations. The results are summarized in Fig. 14. As reported in many researches, three successive phase transitions are not easy to distinguish for BCTZO with
The phase diagrams as functions of Ca concentration and unit cell volume are shown in Fig. 15(a) and (b), respectively. For comparison, a phase diagram of (Ba1-xCax)TiO3 is also shown in the figure. There are several similarities between the BCTZO and (Ba1-
However, there are also some differences between BCTZO and (Ba1-
3.4. Polarization and strain responses under an electric field
On the other hand, the great enhancement of strain responses under an electric field is clearly observed for Ca-substituted ceramics as shown in Fig. 16(b) and (c). For example, the electric-field-induced strain for
Ca off-centering was demonstrated to play a critical role in stabilizing the ferroelectric phase and tuning the polarization states in a (Ba1-
We thank Prof. Shin-ya Koshihara & Dr. T. Shimizu of the Tokyo Institute of Technology, Mr. T. Kosugi & Prof. S. Tsuneyuki of the University of Tokyo, and Mr. Y. Kamai of the Shizuoka University for their collaboration in this work. We also thank the support from the Collaborative Research Project of the Materials and Laboratory, Tokyo Institute of Technology, and KAKENHI (15H02292 and 26620190).
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