Long-wave UVA radiation excited warm white-light emitting NaGdTiO₄: Tm³⁺/Dy³⁺/Eu³⁺ ions tri-doped phosphors: Synthesis, energy transfer and color tunable properties

NaGdTiO₄ (NGT) phosphors doped with different activator ions (Tm³⁺, Dy³⁺, and Eu³⁺) were synthesized by a conventional solid-state reaction method in an ambient atmosphere. These phosphors were characterized by scanning electron microscope images, transmission electron microscope images, X-ray diffraction patterns, Fourier transform infrared spectra, and photoluminescence spectra. All the samples were crystallized in an orthorhombic phase with a space group of Pbcm (57). In Tm³⁺/Dy³⁺ ions co-doped samples, white-light emission was observed under near-ultraviolet (NUV) excitation. In addition, the energy transfer between Tm³⁺ and Dy³⁺ ions was proved to be a resonant type via an electric dipole-dipole mechanism and the critical distance of energy transfer was calculated to be 19.91 Å. Furthermore, Tm³⁺/Dy³⁺/Eu³⁺ ions tri-doped NGT phosphors demonstrated warm white-light emission by appropriately tuning the activator content, based on the principle of energy transfer. These NUV wavelength excitable phosphors exhibit great potential as a single-phase full-color emitting phosphor for white light-emitting diode applications.