Deep red emission in Eu2+-activated Sr4(PO4)2O phosphors for blue-pumped white LEDs

Abstract

The deep red phosphor Sr₄(PO₄)₂O:Eu²⁺, which has an excitation peak around 450 nm for blue LED applications, is reported. This behavior is unusual for most phosphate phosphors. The crystal structure of Sr₄(PO₄)₂O:Eu²⁺ is found to be monoclinic P2₁ and isotypic with Ca₄(PO₄)₂O:Eu²⁺, which also shows deep red emission. Sr₄(PO₄)₂O:Eu²⁺ has a larger lattice volume than Ca₄(PO₄)₂O:Eu²⁺, but their emission and excitation spectra at room temperature are very similar. The key factors are discussed for achieving a large redshift of the 5d levels of Eu²⁺ ion in order to emit red light. In particular, the importance of the anion polarizability and the distortions of the metal coordination polyhedra are discussed, including the effective coordination number. Importantly, Sr₄(PO₄)₂O:Eu²⁺ lacks the yellow emission at 77 K, which is found in Ca₄(PO₄)₂O:Eu²⁺. The differences in thermal quenching behavior for Eu²⁺ dopants in Sr₄(PO₄)₂O:Eu²⁺ and Ca₄(PO₄)₂O:Eu²⁺ are attributed to the degree of auto/photo-ionization due to differences in the band gaps of these compounds. The importance of the large band gap of the host lattice in avoiding non-radiative processes of energy relaxation was confirmed.