Preparation, Characterization, Photoluminescence and Thermoluminescence Studies of Li_2−3xLn_xGeTeO₆ (Ln = Eu³⁺, Tb³⁺; x = 0.0, 0.02, 0.05, 0.075 and 0.1)

Li_2-3xLn_xGeTeO₆ phosphors with doping of Ln³⁺ (Ln = Eu³⁺,Tb³⁺:0.0 < x < 0.1) were successfully prepared. Parent Li₂GeTeO₆ was obtained by the solid-state method, while Ln-doped compositions were synthesized by the facile ion-exchange method. The prepared phosphors were characterized by powder x-ray diffraction (P-XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and Raman spectroscopy techniques. All of the samples were crystallized in rhombohedral structure with space group R3/(146). The photoluminescence (PL) and thermoluminescence (TL) characteristics of Li_2-3xLn_x³⁺GeTeO₆ were studied. The excitation spectra of Eu³⁺/Tb³⁺-doped samples were obtained by monitoring the 611/544 nm emission line. The emission spectra of Eu³⁺/Tb³⁺-doped samples were recorded by excitation at 394/372 nm. The Li_2-3xEu_xGeTeO₆ phosphor exhibited red emission with a maximum intense band at 611 nm. The Li_2-3xTb_xGeTeO₆ phosphor gave a strong emission band at 544 nm in the green range. The variation in emission with dopant concentration was studied. It was found that the concentration quenching was predominant above 0.05 mol.% in both Eu³⁺- and Tb³⁺-doped phosphors, which was demonstrated based on Blasse’s equation and Dexter’s theory. The chromaticity coordinates of Eu³⁺- and Tb³⁺-incorporated phosphors were located at red and green regions, respectively, as per NTSC standards. The colour purity values for 0.05 mol.% of Eu³⁺ (Tb³⁺)-doped Li₂GeTeO₆were 54% and 43%, respectively. The correlated colour temperature (CCT) values of Li_2-3xEu_xGeTeO₆and Li_2-3xTb_xGeTeO₆ were 2763 K and 4562 K, respectively, indicating their suitability for use in warm and cool white-light devices. The parameters of trap-depth/activation-energy, order of kinematics and frequency-factor of the synthesized samples were calculated using the Chen glow curve peak-shape method. The investigation shows Eu³⁺- and Tb³⁺-doped Li₂GeTeO₆ compositions can be utilized as white-LEDs.