Modulating Optical Properties through Cation Substitution: Composition-Property Relationships in $M^I_3$$M^{III}$P$_3$O$_9$N:Eu$^{2+}$ ($M^I$=Na, K; $M^{III}$=Al, Ga, In)

Abstract

Developing phosphors with narrow photoluminescence emission peaks and high chromatic stability holds significant importance in light-emitting diode (LED) display technologies, where a wide color gamut is essential to achieve the Rec. 2020 specifications. This research focuses on the optical properties of a solid solution: $M^I_{2.97}$Eu$_{0.015}$$M^{III}$P$_3$O$_9$N [$M^I$=Na, K; $M^{III}$=Al, (Al$_{0.75}$Ga$_{0.25}$), (Al$_{0.5}$Ga$_{0.5}$), (Al$_{0.25}$Ga$_{0.75}$), Ga, (Ga$_{0.75}$In$_{0.25}$), (Ga$_{0.5}$In$_{0.5}$)] to understand how the narrow-emitting photoluminescence in K$_3$AlP$_3$O$_9$N:Eu$^{2+}$ can evolve during host structure cation substitution. Photoluminescence measurements at low temperature (15 K) support that Eu$^{2+}$ replaces three crystallographically independent Na$^+$ sites in Na$_{2.97}$Eu$_{0.015}$AlP$_3$O$_9$N, similar to the parent K$^+$ phosphor, but substituting Ga$^{3+}$ and In$^{3+}$ for Al$^{3+}$ leads to a change in Eu$^{2+}$ site preference, narrowing the full-width-at-half-maximum (fwhm) of the emission peak. The chromatic stability and photoluminescence quantum yield are also enhanced with higher Ga$^{3+}$ content in the host but not with In$^{3+}$. Thermoluminescence analysis indicates the relationship between trap states and the enhanced quantum yield with Ga$^{3+}$ leads to the series' best performance. The analysis of the $M^I_{2.97}$Eu$_{0.015}$$M^{III}$P$_3$O$_9$N series offers insight into the potential method for modulating optical properties with cation substitution in the host structure.