Eu2+-Eu3+ Co-doping provoking polyhedron-cluster-based emission structural unit evolution triggering phases transition and blue-green-red multimode luminescence emission

Abstract

A series of Ca2-xSiO4:xEu2+/Eu3+ and Ca2-2xSiO4:xEu2+/Eu3+, xM (M = Na+, K+) phosphors were prepared by co-doping Eu2+-Eu3+ in Ca2SiO4 solid solution for selectively locating Eu2+-Eu3+ ions deriving polyhedron-distorted emitting structure units’ evolution. During β, γ1,2-C2S matrix phase transitions, the structure and luminescence properties were analyzed by XRD, diffuse reflection, PL and luminescence thermal stability. The results show that Eu2+/Eu3+ occupying 6 Ca2+ sites forms 10 emission structural units in the matrix and the phase transition of γ1-Ca2-xSiO4(x = 0.0001–0.0004)→β-Ca2-xSiO4 (x = 0.0005–0.04)→β, γ2-Ca2-xSiO4 mixed phase (x = 0.08–0.32) is generated by the extrusion of structural units. Ultraviolet–visible diffuse reflectance spectra confirm the band gap difference caused by structural unit rotation. According to the bond energy method, the priority occupation order for the Eu2+/3+ ions was determined. The energy transfer of Eu2+→Eu3+ occurs when the structural units 3–10 of β, γ2-Ca2-xSiO4 mixtures coexist. After co-doped M+ as charge compensator, the luminescence intensity/thermal stability of Ca2-2xSiO4:xEu3+, xM(M = Na+, K+) were improved by 33.88 %/57.12 %, and 5.1 %/6.6 % than that of Ca2-xSiO4:xEu3+, respectively. These characteristics indicate that phosphors have broad application prospects in the field of solid-state lighting.