RESUMEN
A series of Ca9-x-yCe(PO4)7:xEu2+,yMn2+ phosphors were synthesized by a high-temperature solid-state reaction method. The as-prepared samples were characterized by XRD and EDX measurements, which showed that Eu2+ and Mn2+ could be efficiently doped into the host. Ce3+ acts concurrently as activator and sensitizer in Ca9Ce(PO4)7, and the energy transfer mechanisms between Ce3+/Eu2+ and Ce3+/Mn2+ in Ca9Ce(PO4)7 were validated and proven to be a resonant type via dipole-quadrupole and dipole-dipole interactions, respectively. Besides, there is also energy transfer from Eu2+ to Mn2+ ions. The host, Ca9Ce(PO4)7, emits blue-white light and Ca9Ce(PO4)7:xEu2+,yMn2+ phosphors emit blue-green through white to orange-red light under near-ultraviolet radiation as a result of tuning the ratio of Eu2+/Mn2+. Ca9Ce(PO4)7:0.04Eu2+,0.08Mn2+ emits white light with CIE coordinates (0.333, 0.310), a CCT of 5446 K, and a high CRI of 81. The energy transfer efficiency between Ce3+ and Mn2+ increases significantly with temperature. These results reveal that Ca9Ce(PO4)7:Eu2+,Mn2+ may be a potential candidate for white light-emitting phosphors.
RESUMEN
Controlling the occupation sites of Mn2+ emitters by multiple-cation substitution, the Li2ZnGe3O8:Mn2+ phosphor could be optionally tuned in the green to NIR region. For the Mn2+ single doped Li2ZnGe3O8 phosphor, the tetrahedral coordinated Mn2+ (ZnO4 site) affords a green emission, and the octahedrally coordinated Mn2+ (ZnO6 site) shows an NIR (832 nm) emission. Interestingly, the Li2ZnGe3O8 host has three cation crystallographic sites, in which the octahedrally coordinated Mn2+ could exhibit a red emission by occupying GeO6 sites. The different luminescence centers for Mn2+ have been demonstrated using time-resolved emission spectra (TRES), excitation spectra and the decay curves. However, how to selectively regulate these fluorescence emissions corresponding to the different occupation sites is critical. In this paper, a common effect between the group's transition and energy transfer makes it possible to enhance the green emission (ZnO4 site) continuously and to restrain the NIR emission (ZnO6 site) through increasing the Zn/Li ratio in the Li2ZnGe3O8:Mn2+ phosphor. When Zn2+ is substituted by bigger ions from Ca to Sr and Ba, the local environments of Mn2+ around ZnO6 are influenced which leads to part of the Mn2+ emitters occupying the Ge4+ site, and it has the most enhanced effect on the Mn red-emission (GeO6 site). In general, we show that the spectral property of Mn2+ in different occupation sites could be efficiently regulated within the Li2ZnGe3O8 host, and provide a method for photoluminescence tuning.
RESUMEN
Blue emitting phosphor is urgently needed in commercial application of white light emitting diodes for the reason that the existing ones lack proper emission and have poor thermal stability. Herein, we report two methods for Sr0.8Ca0.2Al2Si2O8:Eu2+ phosphors that were performed to optimize the luminescence properties and thermal stability based on structural reconstruction. One is partial substitution of Al3+-Sr2+ by Si4+-Na+ forming Sr0.77-xNaxCa0.2Al2-xSi2+xO8:0.03Eu2+ (x = 0-0.7); the other is altering the proportion of Ca2+ and Sr2+, forming Sr0.97-yCayAl2Si2O8:0.03Eu2+ (y = 0-0.97) solid solution. The phosphor's structural evolution, variation of the Eu2+ local environment, luminescence and thermal quenching were explored in detail. As a result, for Sr0.77-xNaxCa0.2Al2-xSi2+xO8:0.03Eu2+ (x = 0-0.7) series, (Al/Si)O4 tetrahedrons are enlarged and Eu-O polyhedrons are compressed; the emission gets red-shifted from 418 nm to 428 nm under 350 nm excitation; the thermostability of Sr0.97-yCayAl2Si2O8:0.03Eu2+ is improved with the incorporation of Si4+-Na+ pair. For Sr0.97-yCayAl2Si2O8:0.03Eu2+ (y = 0-0.97) series, the emission peak shows a large shift from 420 nm to 440 nm, which is attributed to a comprehensive result of nephelauxetic effect and crystal field splitting. Furthermore, thermal stability is improved by the coexistence of Ca2+ and Sr2+. White light emitting diodes fabricated by combining the as-prepared phosphor with commercial phosphors exhibit an excellent color rendering index Ra of 90.1 and a temperature of 4459 K with CIE coordinates of (0.357, 0.356). These results indicate that the as-developed composition series are promising for commercial application.