Controlling disorder in host lattice by hetero-valence ion doping to manipulate luminescence in spinel solid solution phosphors

Phosphor materials have been rapidly developed in the past decades. Developing phosphors with desired properties including strong luminescence intensity and long lifetime has attracted widespread attention. Herein, we show that hetero-valence ion doping can serve as a potent strategy to manipulate luminescence in persistent phosphors by controlling disorder in the host lattice. Specifically, spinel phosphor Zn(Ga_1−xZn_x)(Ga_1−xGe_x)O₄:Cr is developed by doping ZnGa₂O₄:Cr with tetravalent Ge⁴⁺. Compared to the original ZnGa₂O₄:Cr, the doped Zn(Ga_1−xZn_x)(Ga_1−xGe_x)O₄:Cr possesses significantly enhanced persistent luminescence intensity and prolonged decay time. Rietveld refinements show that Ge⁴⁺ enters into octahedral sites to substitute Ga³⁺, which leads to the co-substitution of Ga³⁺ by Zn²⁺ for charge compensation. The hetero-valence substitution of Ga³⁺ by Ge⁴⁺ and Zn²⁺ enriches the charged defects in Zn(Ga_1−xZn_x)(Ga_1−xGe_x)O₄:Cr, making it possible to trap large amounts of charge carriers within the defects during excitation. Electron paramagnetic resonance measurement further confirms that the amount of Cr³⁺ neighboring charged defects increases with Ge⁴⁺ doping. Thus charge carriers released from defects can readily combine with the neighboring Cr³⁺ to produce bright persistent luminescence after excitation ceases. The hetero-valence ion doping strategy can further be employed to develop many other phosphors and contributes to lighting, photocatalysis and bioimaging.