Design and energy transfer mechanism for single-phased Gd₂MgTiO₆: Bi³⁺, Eu³⁺ tunable white light-emitting phosphors

In recent years, numerous efforts have been made to develop single-phased white light-emitting phosphors for the near-UV region to solve problems of color reabsorption and ratio regulation between different phosphors. In this work, we have designed Bi³⁺- and Eu³⁺-codoped single-phased Gd₂MgTiO₆ phosphors to achieve tunable white light emission based on multi-luminescence center energy transfer. The structural analysis showed that all the samples were crystallized as a monoclinic double perovskite with the P21/n symmetry space group (No. 14), with HRTEM images showing clear lattice fringes between the lattice planes. The single Bi³⁺-doped Gd₂MgTiO₆ sample exhibits two obvious emission peaks at 417 and 508 nm, which correspond to a characteristic ³P₁ → ¹S₀ transition for the Bi³⁺ ions under near-UV excitation due to two types of Bi³⁺ emission centers, with their relative emission intensity depending closely on the value of the excitation wavelength. In this case, a suitable choice of excitation wavelength can achieve tunable emission for Gd₂MgTiO₆: Bi³⁺ between blue and green. Eu³⁺ is codoped into Gd₂MgTiO₆ as a red emission component and shows sharp emission lines that correspond to the characteristic ⁵D₀ → ⁷F_J (J = 1, 2, 3, and 4) transitions of Eu³⁺ ions. Energy transfer in Gd₂MgTiO₆: Bi³⁺, Eu³⁺ has been confirmed by the electric dipole–dipole (d–d) interaction from Bi³⁺ to Eu³⁺. Our experiments show that it is straightforward to create tunable white light emission by adjusting the excitation scheme and Eu³⁺ concentration. Moreover, a schematic for the energy transfer mechanism and simplified spectral levels based on Bi³⁺ and Eu³⁺ ions has also been established.