Enhanced spontaneous polarization in Sr and Ca co-doped BaTiO3 ceramics

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Abstract

The dielectric and ferroelectric properties of (BaxSr1−x)0.77Ca0.23TiO3 ceramics with x=1 to 0.7 were studied and compared with those of BaxSr1−xTiO3 and Ba0.77Ca0.23TiO3 ceramics. It has been found that Sr doping of the Ba0.77Ca0.23TiO3 ceramics causes a drastic decrease of the Curie temperature, just like Sr doping of pure BaTiO3 ceramics, demonstrating a cell volume effect. However, the (BaxSr1−x)0.77Ca0.23TiO3 ceramics with x=0.9 and 0.8 have larger spontaneous polarization than those of the corresponding BaxSr1−xTiO3 and Ba0.77Ca0.23TiO3 ceramics, along with sufficient insulating properties. The enhancement of their polarization was explained by the increase of the lattice parameter c/a ratio due to the lattice distortion and strain developed in the ceramics.

Introduction

Ferroelectric materials have been applied to a large field of applications because of their excellent dielectric, piezoelectric and ferroelectric properties. BaTiO3 was the first perovskite-type ferroelectric material developed and intensively studied, ever since its discovery about 60 years ago [1]. BaTiO3 can form solid solutions with other perovskite ferroelectrics, such as SrTiO3 and CaTiO3. This kind of solid solution has been, and continues to be, of interest for investigation, not only because of their various applications, but also for their interesting dielectric and ferroelectric behaviors [2], [3], [4], [5].

BaTiO3,SrTiO3 and CaTiO3 have different crystal structures and ferroelectric behaviors. They are cubic at high temperatures, but their crystal structures are not identical at room temperature, being tetragonal, cubic and orthorhombic, respectively. BaTiO3 is ferroelectric at room temperature and its Curie temperature (Tc) is around 120 C [6], [7]. On the other hand SrTiO3 and CaTiO3 are paraelectric and undergo no ferroelectric phase transitions [8], [9], [10], [11]. It has been established that SrTiO3 forms continuous ranges of solid solutions with BaTiO3, and the Tc decreases almost linearly with the increase of Sr content. This action is usually ascribed to the small ionic size of Sr2+. The increase of Sr concentration in (Ba,Sr)TiO3 causes a decrease of the unit cell volume of the perovskite crystalline structure, thereby lowering the Tc in the same manner as the hydrostatic pressure [12], [13].

On the other hand, there is a limitation of solubility for the Ca doping of BaTiO3. It has been found that Ca2+ replaces Ba2+ in BaTiO3 to form tetragonal Ba1−xCaxTiO3 solid solutions when x is less than 0.23, which causes a negligible change of the Tc [14]. Ca2+ has a smaller ionic radius than both Ba2+ and Sr2+. However, it has been pointed out that the Ca ions in (Ba,Ca)TiO3 might have greater atomic polarizability, which intensifies the interactions between the Ti ions, and thus compensates for the decrease of Tc originating from the decrease of the unit cell volume.

In this study, we conducted research into the Sr and Ca co-doping effects on the dielectric and ferroelectric properties of the BaTiO3 ceramics, since there is no detailed work on the (Ba,Ca,Sr)TiO3 ceramics, to the best of our knowledge. A comparative study of the microstructure and dielectric and ferroelectric properties between (BaxSr1−x)0.77Ca0.23TiO3 and Ba1−xSrxTiO3 ceramics was performed, with Sr concentrations of x=1,0.9,0.8 and 0.7, respectively.

Section snippets

Experiment

(BaxSr1−x)0.77Ca0.23TiO3 and BaxSr1−xTiO3 ceramics with x between 1 and 0.7 were all prepared by a conventional solid-state reaction technique. For (BaxSr1−x)0.77Ca0.23TiO3 ceramics, high-purity starting powders of BaCO3,TiO2,SrCO3 and CaCO3 were mixed, with the further addition of alcohol, ball-milled for 10 h, then dried and calcined at 1000 C for 2 h in air. After that, they were remixed and pressed into disk-shaped pellets 10 mm in diameter, and finally sintered at 1450 C for 4 h. The raw

Results and discussions

The microstructure of the (BaxSr1−x)0.77Ca0.23TiO3 and BaxSr1−xTiO3 ceramics for x=1,0.9,0.8 and 0.7 is shown in Fig. 1. All the XRD patterns show a polycrystalline perovskite structure without impurity phases: no pyrochlore or secondary phase was observed. This indicates that Sr doping of Ba0.77Ca0.23TiO3 forms single-phase solid solution, similar to the case of Sr doping of pure BaTiO3. At room temperature, the microstructures of (BaxSr1−x)0.77Ca0.23TiO3 and BaxSr1−xTiO3 ceramics are all

Conclusions

Measurements of the structural, dielectric and ferroelectric properties have been carried out of (BaxSr1−x)0.77Ca0.23TiO3 and BaxSr1−xTiO3 ceramics, with different Sr contents of x=1 to 0.7. Sr doping of Ba0.77Ca0.23TiO3 ceramics causes a drastic decrease of the Curie point, just like the Sr doping of pure BaTiO3 ceramics, which was ascribed to the cell volume effect. The (BaxSr1−x)0.77Ca0.23TiO3 ceramics with x=0.9 and 0.8 have larger spontaneous polarizations than those of the corresponding Ba

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