Optical absorption by design in a ferroelectric: co-doping in BaTiO3

Abstract

The combination of significant polarization and large light absorption is a challenge in ferroelectric materials. Here we report the optical absorption of BaTiO₃ codoped with trivalent and pentavalent transition metal ions replacing the Ti⁴⁺ cation, i.e., BaTi_1−x(X_1/2Y_1/2)_xO₃ (X = Sc, Mn, Fe, Co; Y = Nb, Ta; with x = 0.075). We investigated the optical absorption behavior as a function of different pairs of co-dopants as well as physical properties such as ferroelectricity and piezoelectricity. X-Ray diffraction and Raman spectroscopy measurements show that the tetragonality and polar coherence length decrease after co-doping, which affects the ferroelectric properties. It was found that the onset of absorption can decrease down to 1.5 eV, especially for the (Co³⁺, Nb⁵⁺) co-doped samples, which simultaneously exhibit higher polarization than with the other co-doped ions. The measured photoconductivity confirms that the (Co³⁺, Nb⁵⁺) doping has higher absorption than others. Our density functional theory calculations show that codoping inserts intragap levels which are responsible for lowering the energy of optical absorption, which is in full agreement with our experimental observations. This work opens new perspectives for the use of ferroelectrics in optoelectronic devices.