Modulating Optical Properties through Cation Substitution: Composition-Property Relationships in $M_3^I$$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: $M_{2.97}^I$Eu${}_{0.015}$$M^{III}$P${}_3$O${}_9$N [$M^I$=Na, K; $M^{III}$=Al, (Al${}_{0.75}$Ga${}_{0.25}$), (Al${}_{0.5}$Ga${}_{0.5}$), (Al${}_{0.25}$Ga${}_{0.75}$), Ga, (Ga${}_{0.75}$In${}_{0.25}$), (Ga${}_{0.5}$In${}_{0.5}$)] to understand how the narrow-emitting photoluminescence in K${}_3$AlP${}_3$O${}_9$N:Eu${}^{2+}$ can evolve during host structure cation substitution. Photoluminescence measurements at low temperature (15 K) support that Eu${}^{2+}$ replaces three crystallographically independent Na${}^{+}$ sites in Na${}_{2.97}$Eu${}_{0.015}$AlP${}_3$O${}_9$N, similar to the parent K${}^{+}$ phosphor, but substituting Ga${}^{3+}$ and In${}^{3+}$ for Al${}^{3+}$ leads to a change in Eu${}^{2+}$ site preference, narrowing the full-width-at-half-maximum (fwhm) of the emission peak. The chromatic stability and photoluminescence quantum yield are also enhanced with higher Ga${}^{3+}$ content in the host but not with In${}^{3+}$. Thermoluminescence analysis indicates the relationship between trap states and the enhanced quantum yield with Ga${}^{3+}$ leads to the series' best performance. The analysis of the $M_{2.97}^I$Eu${}_{0.015}$$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.