Crystal structure and luminescence studies of microcrystalline GGG:Bi3+ and GGG:Bi3+,Eu3+ as a UV-to-VIS converting phosphor for white LEDs

The paper contains detailed crystal structure and photoluminescence studies of the Bi³⁺ and Bi³⁺-Eu³⁺ co-doped Gd₃Ga₅O₁₂ (GGG) garnet. The studied materials have been synthesized in the form of microcrystalline powders by the common solid-state reaction method. Precise crystal structure parameters were derived from the X-ray powder diffraction data. Photoluminescence studies were done by the steady-state and time-resolved techniques in the 4.2–400 K temperature range. The studied materials show no UV luminescence caused by electronic transitions of a Bi³⁺ ion. The observed luminescence in visible is interpreted by excitons localized around Bi³⁺ ions. In particular, two emission bands located at 2.55 eV (486 nm) and 2.05 eV (605 nm) at 4.2 K were revealed. The main emission band at 2.55 eV is attributed to the triplet relaxed state of excitons localized around single (regular) Bi³⁺ ions, whereas the weaker band at 2.05 eV is related to some associates of a single Bi³⁺ ion with a crystal lattice defect. The performed studies for Bi³⁺-Eu³⁺ co-doped samples indicate that the energy transfer from Bi³⁺ to Eu³⁺ is radiative and is caused by reabsorption of the Bi-related exciton emission by Eu³⁺ ions. The studied materials were also characterized as a single-phase converting phosphor for near-UV excited white LEDs. The possibility of the emission color tuning from blue to orange-red at UV excitation by the way of changing of Bi³⁺ and Eu³⁺ concentrations was shown.