Crystal structure, tunable luminescence and energy transfer properties of Na3La(PO4)2:Tb3+,Eu3+ phosphors

Abstract

A series of Tb³⁺ and/or Eu³⁺ doped Na₃La(PO₄)₂ phosphors were successfully synthesized and their crystal structure and photoluminescence (PL) properties were investigated in detail. Double phosphates with the compositions Na₃Tb(PO₄)₂ and Na₃Eu(PO₄)₂ were obtained by the substitution of Tb or Eu for La in the Na₃La(PO₄)₂ host. XRD pattern analysis indicates that these obtained compounds crystallize in the orthorhombic system with the space group Pbc2₁. The crystal structure of the Na₃RE(PO₄)₂ (RE = Tb, Eu) is made up of isolated PO₄ tetrahedra and of sodium and RE atoms arranged in an ordered way. The REO_y polyhedra are isolated from one another, resulting in a high critical concentration of Tb³⁺ or Eu³⁺ activators. Under excitation of near-ultraviolet (NUV) irradiation, Tb³⁺ doped Na₃La(PO₄)₂ shows a blue-greenish emission with a predominant peak at 546 nm, while the emission spectra of Eu³⁺-doped Na₃La(PO₄)₂ exhibits a reddish orange emission due to the ⁵D₀ → ⁷F_J transitions of Eu³⁺ ions. The energy transfer from Tb³⁺ to Eu³⁺ in the Na₃La(PO₄)₂ host is demonstrated by the luminescence spectra and fluorescence decay dynamics. Meanwhile, the emission color of Na₃La(PO₄)₂:Tb³⁺,Eu³⁺ can be tuned from green to red through tuning the Tb³⁺/Eu³⁺ ratio. These results indicate that the Na₃La(PO₄)₂:Tb³⁺,Eu³⁺ phosphor exhibits broadband NUV absorption and green-reddish orange tunable emission, which might serve as a down-converting phosphor for NUV light-emitting diodes.