Taming the optical response via (Ca:Zr) co-doped impurity in c-BaTiO3: A comprehensive computational insight

Abstract

The lead-free Ca and Zr doped BaTiO3 is the focus of different research groups owing to its greater piezoelectricity and superior dielectric properties. To dig out the reason of these superior features at atomic scale, the density functional theory (DFT) based first principles calculations are implemented to study Ca:Zr doped barium titanate (BTO), by using Ultrasoft Pseudopotential (USP) as incorporated in CASTEP. The BTO has been doped with calcium (Ca) and zirconium (Zr) at barium and titanium sites respectively, individually and in co-doped form, to tailor the electronic bandgap. The band structure, density of states (DOS) and optical properties of both the singly doped as well as co-doped BTO structures are presented. A transition of indirect bandgap to the direct one is evident in all cases due to the Brillouin zone folding. In case of Ca-doping (BCTO), the shifting of Fermi level towards the valence band is inferred as converting BTO to a p-type material to some extent, with bandgap decreasing from 1.726eV to 1.714eV. Whereas, considering the case of Zr-doping (BZTO) and Ca:Zr doping (BCZTO), the Fermi level was found to be shifted towards the conduction band making the material an n-type and bandgap increased to the values of 1.776 eV and 2.654 eV respectively. The change in structural properties is observed to affect the optical properties of the material upon doping. The comparative analysis of absorption, refractive index, reflectivity, dielectric constant, and extinction coefficient and energy loss function for pure and doped systems have been presented. The doping changed optical behavior of barium titanate making the material more useful for optoelectronic applications.