Site Occupation and Spectral Assignment in Eu²⁺-Activated β-Ca₃(PO₄)₂-Type Phosphors: Insights from First-Principles...

Eu²⁺-activated β-Ca₃(PO₄)₂-type phosphors have attracted significant experimental interest for applications in solid-state lighting because of the presence of multiple cation sites, which is highly desirable for site engineering of activator emission. However, the site occupation and associated spectral assignment of dopant Eu²⁺, and hence the mechanism behind the site-regulated emission tuning, still remain elusive. Herein, we perform a systematic first-principles study on Eu²⁺-doped Ca₃(PO₄)₂, Ca₁₀M(PO₄)₇ (M = Li, Na, K), and Ca₃(PO₄)₂:Mg by combining density functional theory and multiconfigurational ab initio calculations. The results show that, among the isovalent Eu_Ca substitutions in Ca₃(PO₄)₂, the occurrence probability correlates positively with the size of the substituted site, which is, nevertheless, weakened by the incorporation of codopant Mg²⁺ ions. In the presence of aliovalent Eu_M substitutions as in Ca₁₀M(PO₄)₇, the site-size-controlled preference is neutralized by the requirement for charge compensation, and the effect becomes more pronounced with an increase of the M⁺ ionic size. On this basis, the emission spectra of the phosphors are interpreted with respect to the substituted sites and the mechanism behind the composition dependence of the emission color is consistently elucidated, as is also verified by a comparison between the calculated 4f → 5d transition energies and experimental excitation spectra. Our results provide a new perspective on the site preference of Eu²⁺ in β-Ca₃(PO₄)₂-type compounds and may also serve as a theoretical guideline on the structure-property relationship for the design of other Eu²⁺-activated phosphors.