Luminescence properties of multi-site Eu2+ emission in Ca9MgNa(PO4)7 phosphors with cation substitution

A series of Eu 2+ -activated β-Ca 3 (PO 4 ) 2 -type Ca 9 MgNa(PO 4 ) 7 phosphors were successfully synthesized via high-temperature solid-state reaction. The substitution effects of Zn 2+ for Mg 2+ and Li + for Na  +  on the local coordination environments and luminescence characteristics of Eu 2+ were systematically investigated. The results demonstrate that Eu 2+ ions predominantly occupy the Ca1/Ca2, Ca3, and Na sites in Ca 9 MgNa(PO 4 ) 7 , corresponding to emission peaks at blue(1), blue(2), and green, respectively. Zn 2+ substitution for Mg 2+ induces lattice expansion, which reduces the occupancy probability of Eu 2+ at Ca1/Ca2 sites while lowering the formation energy of Ca3 sites, thereby significantly enhancing Ca3 and Na + site emissions but reducing Ca1/Ca2 site intensity. Subsequent Li + substitution for Na + causes contraction of the Li/NaO 6 polyhedra, reducing the capacity of the Na sites to accommodate Eu 2+ . Concurrently, the adjacent Ca(3)O 9 polyhedra undergo expansion, promoting preferential Eu 2+ occupancy at the Ca3 sites and consequently enhancing the emission intensity at blue(1).