Eu²⁺ Stabilized at Octahedrally Coordinated Ln³⁺ Site Enabling Red Emission in Sr₃LnAl₂O_7.5 (Ln = Y or Lu) Phosphors

Abstract Red spectrum loss in phosphor‐converted light‐emitting diodes (pc‐LEDs) restricts high‐quality warm‐white lighting. Herein, two blue‐light excitable red‐emitting Sr₃LnAl₂O_7.5:Eu (Ln = Y or Lu) phosphors are reported, and red emission originating from an unprecedented substitution model, with Eu²⁺ occupied sixfold octahedrally coordinated lanthanide (Ln³⁺) sites is demonstrated. Site occupancy identification reveals that three different sites are occupied by Eu²⁺ and one distinct site is occupied by Eu³⁺, and the Eu²⁺ stabilized at Ln³⁺ site accompanied by Eu³⁺ selectively occupies at Sr²⁺ site as a charge compensator. An anomalous prolonged Eu²⁺‐photoluminescence decay emission with increasing temperature in the low‐temperature region is demonstrated. This accounts for the trapped electrons, which are thermally released from shallow traps and eventually populate the Eu²⁺ 5d level to form an excited‐state Eu²⁺. The findings help better understand Eu²⁺ occupancy and luminescence. These also provide a new perspective for the exploration of novel Eu²⁺ activated oxide‐based red phosphor for pc‐LEDs.