Modulating Optical Properties through Cation Substitution: Composition-Property Relationships in MI₃MIIIP_3O_9$N:Eu$²⁺$ (MI=Na, K; MIII=Al, Ga, In)

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: MI2.97Eu0.015MIIIP_3O_9$N [$M^I$=Na, K; $MIII$=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$_3O_9$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$_3O_9$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 $M^I2.97$Eu$0.015MIIIP₃O₉N series offers insight into the potential method for modulating optical properties with cation substitution in the host structure.