Desheng Fu and Mitsuru Itoh

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http://dx.doi.org/10.5772/61017

### **Abstract**

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 offcentering 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.

**Keywords:** Ca off–centering, BaTiO3, Ba(Zr,Ti)O3, perovskite oxides, ferroelectric, piezoelectric, phase transition, quantum effects, electromechanical coupling effects

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