Improvement of thermal stability and luminescence in Ca6Ce2Na2(PO4)6F2:Mn2+/Tb3+via the substitution of Ca with Sr/Ba for structural modification

Abstract

A series of Ca_6−x−ySr_xBa_yCe₂Na₂(PO₄)₆F₂ (CMCNPF, M = Sr²⁺/Ba²⁺), CMCNPF:0.1Mn²⁺ and CMCNPF:0.1Tb³⁺ phosphors were synthesized by a high temperature solid-state method. In order to improve the luminescence properties of the phosphors, Sr²⁺ and Ba²⁺, which are in the same main group as Ca²⁺, are used to partially substitute Ca²⁺ in the host. The excited state electrons of Ce³⁺ and Mn²⁺ are located in the outermost layer of the ions and are easily influenced by the external crystal field. Therefore, when substituted for cations, the emission peaks of Ce³⁺ and Mn²⁺ tend to shift, while that of Tb³⁺ remains unchanged. For the peak shift of Ce³⁺, this may be considered from the two aspects of nephelauxetic effect and crystal field splitting. However, the low-temperature spectra and lifetimes of the two emission peaks show that the shift of the Ce³⁺ emission peak is only influenced by the Ca₁ lattice. In addition, the blueshift of the luminescence center Mn²⁺ is influenced by the average bond length of the polyhedron and the symmetry of the polyhedron. Regulating the composition of the cations can improve the symmetry of the polyhedron occupied by the luminescence center, increase the rigidity of the crystal structure, and improve the thermal stability of the luminescence center to a certain extent.