Synthesis, structural, luminescence (Eu³⁺, Sm³⁺) and electron spin resonance (Gd³⁺) studies of Na_2-3xLn_xTiTeO₆ (Ln³⁺ = Eu³⁺, Sm³⁺ and Gd³⁺, 0.0 < x < 0.1) red emitting phosphors for white-light emitting diodes

In this work, Na₂TiTeO₆ (NTTO) and Na_2-3xLn_xTiTeO₆ (Ln = Eu, Sm and Gd, 0 < x < 0.1) were prepared via the well-known solid-state method at 800 ℃ for 16 h. All the materials were thoroughly characterized by Powder X-ray diffraction (PXRD), Scanning Electron Microscopy (SEM), Fourier Transform Infrared spectroscopy (FT-IR), and Energy Dispersive Spectroscopy (EDS). Both NTTO and its rare-earth doped samples were crystallized in a rhombohedral lattice with the space group R 3. The FT–IR and Raman spectra of these phosphors reveal characteristic bands associated with Te–O–Te vibrations in TeO₃ and TeO₄ groups. The room temperature (30 ℃) photoluminescence (PL) emission and excitation spectra of Na_2-3xLn_xTiTeO₆ (Ln = Eu, and Sm) and Electron Spin Resonance (ESR) spectra of Na_2-3xGd_xTiTeO₆ were analyzed in detail. The composition Na_1.85Eu_0.05TiTeO₆ exhibits the highest emission intensity with a strong peak located at 610 nm corresponding to transition of ⁵D₀ → ⁷F₂ when excited at 394 nm (⁷F₀ → ⁵L₆). The Sm³⁺ doped NTTO phosphor gave an intense emission peak at 599 nm corresponding to the transition of ⁴G_5/2 → ⁶H_7/2 for x = 0.05 mol% when excited at 402 nm (⁶H_5/2—⁴F_7/2). We found that dipole–dipole interactions are accountable for concentration quenching in both phosphors when Dexter’s theory was invoked. The CIE color coordinates of Na₂TiTeO₆:5 mol% Eu³⁺ (0.5043, 0.3608) and Na₂TiTeO₆:5 mol% Sm³⁺ (0.3665, 0.2989) phosphors are located in the red and light orange red color regions respectively. The Gd³⁺ incorporated Na₂TiTeO₆ samples gave weak “U” type ESR spectra and analyzed.