An insight into the preferential substitution and structure repair in Eu2+-doped whitlockite-type phosphors based on the combined experimental and theoretical calculations

Abstract

Due to the wide compositional variability and structural tolerance, Eu²⁺-doped whitlockite β-Ca₃(PO₄)₂ (TCP)-type phosphors with different visible emissions have been realized by manipulating the chemical compositions. However, the site preference of the dopant Eu²⁺ at the multi-cation sites has been a dispute recently. In this study, a combination of experimental and theoretical calculations was applied to identify the preferential occupancy and variable optical behavior of Eu²⁺ through the evolution of Ca_10.5−0.5xK_x(PO₄)₇ (0 ≤ x ≤ 1) isostructures. Both spectral analysis and theoretical calculations reveal that Eu²⁺ prefers to occupy the loose Ca(4) sites and produce a narrow band emission around 420 nm. Moreover, a continuous increase in the K content compels the redistribution of dopants from the Ca(4) into the Ca(3) site, resulting in an abrupt spectral shift to 470 nm. Moreover, a vacancy defect repair effect was proposed for the enhancement in the luminescence of Ca_10.5−0.5xK_x(PO₄)₇:Eu²⁺, in which the incorporation of K⁺ repaired the half-unoccupied Ca(4) sites and benefitted the reduction of Eu³⁺ to Eu²⁺. Moreover, to further prove the significant vacancy repair and dopant redistribution effects, Na⁺- and Li⁺-doped systems were investigated. In addition, together with the reported Sr²⁺- or Gd³⁺-doped systems, the preferential occupancy and saltatory spectral variations of Eu²⁺ in whitlockite-type phosphors were uniformly clarified by cationic modification events.