Distortion induced structural characteristics of Ba2R2/3TeO6 (R = Y, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) double perovskites and their multifunctional optical properties for lighting and ratiometric temperature sensing

Abstract

Group theoretical predictions obtained from Raman and IR spectra emphasize the structures of B-site ordered double perovskites Ba₂R_2/3TeO₆ (R = Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) with ubiquitous BO₆ or B′O₆ octahedral tilting are monoclinic (P2₁/n), which was further confirmed via the Rietveld refinement of XRD patterns. The wide band gaps of these compounds were consistent with their ability to absorb UV radiation and hence their potential to act as photoluminescent host materials. Photoluminescence spectra of Eu³⁺-activated Ba₂Y_2/3TeO₆, irrespective of substitution at A- or B-sites, yielded a dominant magnetic dipole transition at 592 nm and a less intense electric dipole transition at 610 nm when excited at 276 nm suggested the preference of Eu³⁺ ions to occupy Y³⁺ sites. The thermal activation energy of the Ba₂Y_2/3TeO₆:Eu³⁺ phosphor was 0.15 eV and its relative sensitivity in the temperature range 300–500 K was estimated to have a maximum of 0.18% K⁻¹ at 300 K for FIR measurements, which is better than its decay lifetime counterpart. The red shift and the peak broadening of the high-energy Raman modes at elevated temperatures of Ba₂Y_2/3TeO₆:Eu³⁺ are due to enhanced B-site octahedral distortions and are correlated with the thermal quenching of Eu³⁺ emission. Temperature-dependent lifetimes in the range 80–500 K imply intensification of non-radiative relaxation within the limit. Furthermore, the phosphorescence lifetime, color parameters, and JuddOfelt parameters are explained in detail. The multifunctional optical properties of this phosphor can thus be utilized for technological applications, such as solid-state lighting and ratiometric optical thermometry.