Photo-dielectric and electric field effect on the structural and phonon properties of calcium-substituted BaTiO3

Abstract

Ferroelectric materials represent a fascinating class of functional systems, uniquely responsive to external stimuli such as light, electric field etc. and opening pathways for new generation applications like optoelectronics. In this study, we have explored the photo-dielectric and electric field effects on the structural and phonon properties of Ca-substituted BaTiO₃ with nominal composition Ba_0.80Ca_0.20TiO₃ (BCTO) polycrystalline sample along with the detailed temperature-dependent ferroelectric properties. The room temperature (RT) X-ray diffraction (XRD) results confirm the coexistence of tetragonal (P4mm) and orthorhombic (Pna2₁) phases. The temperature-dependent dielectric properties evidence two transitions: orthorhombic to tetragonal around 225 K and a tetragonal to cubic transition around 367 K. The dielectric results elucidate the diffused-type tetragonal to cubic phase transition and exhibit the presence of a polar region well above the maxima of the dielectric as also confirmed by the temperature-dependent pyroelectric measurements. The tetragonal to orthorhombic transition is confirmed by temperature-dependent XRD measurements and also supported by temperature-dependent Raman measurements. The isothermal photo-dielectric properties are found to be different in the orthorhombic and tetragonal phases. This difference is due to the different direction of polarization and domain landscapes and related to the change in the dipole reorientations and domain wall motions. The effect of an electric field on the crystal structure is studied by adopting a unique poling protocol, i.e. poling with different electric fields well above the transition temperature. The RT structural analysis of the electric field poled sample illustrates the significant decrease in the diffraction intensity of some Bragg's peaks when the applied field is higher than the coercive field. These results elucidate the electric field-driven change of domain reorientations in BCTO. The many-fold enhancement in the Raman intensity and sharpening of some distinct Raman modes is also observed in the electric field poled sample and explained as an increase in the electric polarizability. These findings highlight the rich microscopic dynamics of BCTO and its potential for optoelectronic devices.