Eu²⁺ Site Preferences in the Mixed Cation K₂BaCa(PO₄)₂ and Thermally Stable Luminescence

Site preferences of dopant Eu²⁺ on the locations of K⁺, Ba²⁺, and Ca²⁺ in the mixed cation phosphate K₂BaCa(PO₄)₂ (KBCP) are quantitatively analyzed via a combined experimental and theoretical method to develop a blue-emitting phosphor with thermally stable luminescence. Eu²⁺ ions are located at K2 (M2) and K3 (M3) sites of KBCP, with the latter occupation relatively more stable than the former, corresponding to emissions at 438 and 465 nm, respectively. KBCP:Eu²⁺ phosphor exhibits highly thermal stable luminescence even up to 200 °C, which is interpreted as due to a balance between thermal ionization and recombination of Eu²⁺ 5d excited-state centers with the involvement of electrons trapped at crystal defect levels. Our results can initiate more exploration of activator site engineering in phosphors and therefore allow predictive control of photoluminescence tuning and thermally stable luminescence for emerging applications in white LEDs.