Disorder–Order transition and Metal-to-Metal charge transfer induced rare Orangish-Red emission in Bi3+-Activated double perovskite phosphors

Revealing and regulating the photoluminescence behaviors of Bi³⁺-activated phosphors play a crucial role in the exploration of high-quality photoluminescent materials for phosphor-converted white light-emitting diodes (pc-WLEDs). However, the existing development strategies of Bi³⁺-doped red phosphors are still scarce. Herein, a series of excellent NaLaScNbO₆:Bi³⁺, Li⁺ orangish-red phosphors were synthesized via a high-temperature solid-state reaction. Under 370 nm NUV excitation, the optimal sample emits an orangish-red light with a peak wavelength of 625 nm, full width at half maximum of 123 nm. The A-site disorder–order transition induced by Bi³⁺ doping in NaLaScNbO₆ double perovskite was first confirmed through Rietveld refinements and high-resolution transmission electron microscopy. The co-doped Li⁺ as an alkali flux material can effectively promote the layer ordering of the A-site and considerably elevate the inner quantum efficiency from 38.97 to 76.10 %. This remarkable orangish-red luminescence can be attributed to the metal-to-metal charge transfer (between Bi³⁺ and Nb⁵⁺) in conjunction with structural disorder–order transition. The luminous efficiency of the fabricated pc-WLED is 7.63 lm/W. The fabricated pc-WLED shows a high color rendering index (R_a = 92, R₉ = 92) that can used in warm white pc-LED. The obtained results provide a fresh perspective and valuable insights into the design of Bi³⁺-doped red phosphors, establishing a correlation between disorder–order transition and luminescence phenomena.