First-principles study of piezoelectric (Ba,Ca)(Ti,Zr)O3 solid solutions

High-performance piezoelectrics are key components of various smart devices and, recently, it has been discovered that (Ba,Ca)(Ti,Zr)O3 (BCTZ) solid solutions show large electromechanical response. Nevertheless, the... [ view full abstract ]

High-performance piezoelectrics are key components of various smart devices and, recently, it has been discovered that (Ba,Ca)(Ti,Zr)O₃ (BCTZ) solid solutions show large electromechanical response. Nevertheless, the microscopic origin of such feature is still unclear and theoretical characterizations of BCTZ remain very limited. Accordingly, we present here a first-principles study of the structural and dynamical properties of different compositions of (Ba,Ca)(Ti,Zr)O₃ solid solutions and related parents compounds in order to identify the microscopic mechanisms tuning the ferroelectric properties of the system. Specifically, we focus on the distinct effects arising from the Ca and Zr doping in the (Ba,Ca)TiO₃ and  Ba(Ti,Zr)O₃ parents binary-system, respectively. When going from BaTiO₃ to CaTiO₃ in (Ba,Ca)TiO₃, the main feature is a gradual transformation from A-site to B-site ferroelectricity due to steric effects that largely determines the behavior of the system. In particular, for low Ca-concentration we found out an overall weakened B-driven ferroelectricity that produces the vanishing of the energy barrier between different polar states and results in an isotropic polarization. A sizable enhancement of the piezoelectric response results from this scenario. When going from BaTiO₃ to BaZrO₃ in Ba(Ti,Zr)O₃, in contrast, the behavior is  dominated by cooperative Zr-Ti motions and the local electrostatics. In particular, low Zr-doping produces the further stabilization of the R3m-phase. Then, the system shows the tendency to globally reduce the polar distortion with increasing Zr-doping. Nevertheless, ferroelectricity can be locally preserved in Ti-rich regions. Therefore, the mixing of (Ba,Ca)TiO₃ and Ba(Ti,Zr)O₃ with low concentration of Ca and Zr, like in the interesting composition-range of BCTZ, allows the system to experience different polar states with different P orientation separated by low energy barrier. Further investigations are in progress to verify the effect of such energy landscape on the piezoelectric response.