Electric field induced short range to long range structural ordering and its influence on the Eu⁺³ photoluminescence in the lead-free ferroelectric Na_1/2Bi_1/2TiO₃

Abstract

Eu+3 was incorporated into the lattice of a lead-free ferroelectric Na_1/2Bi_1/2TiO₃ (NBT) as per the nominal formula Na_0.5Bi_0.5−xEuxTiO₃. This system was investigated with regard to the Eu⁺³ photoluminescence (PL) and structural behaviour as a function of composition and electric field. Electric field was found to irreversibly change the features in the PL spectra and also in the x-ray diffraction patterns below the critical composition x = 0.025. Detailed analysis revealed that below the critical composition, electric field irreversibly suppresses the structural heterogeneity inherent of the host matrix NBT and brings about a long range ferroelectric state with rhombohedral (R3c) distortion. It is shown that the structural disorder on the nano-scale opens a new channel for radiative transition which manifests as a new emission line branching off from the main 5D0→7F0 line along with a concomitant change in the relative intensity of the other crystal field induced Stark lines with different J values. The study suggests that Eu⁺³ luminescence can be used to probe the relative degree of field induced structural ordering in relaxor ferroelectrics and also in high performance piezoelectric alloys where electric field couples very strongly with the lattice and structural degrees of freedom.