c Considerable Improvement of Long-Persistent Luminescence in Germanium and Tin Substituted ZnGa2O4

The red long-lasting luminescence properties of the ZnGa2O4:Cr3+ spinel material are shown to be much improved when germanium or tin is substituted to the nominal composition. The resulting Zn1+xGa2-2x(Ge/Sn)(x)O-4 (0 <= x <= 0.5) spinel solid solutions synthesized here by a classic solid state method have been structurally characterized by X-ray and neutron powder diffraction refinements coupled to Ga-71 solid state NMR studies. In contrast to ZnGa2O4:Cr3+ for which long lasting luminescence properties have been reported to arise from tetrahedral positively charged defects resulting from the spinel inversion, our results show that a different mechanism occurs complementary for Zn1-xGa2-2x(Ge/Sn)(x)O-4. Here, the great enhancement of the brightness and decay time of the long lasting luminescence properties is directly driven by the substitution mechanism which creates distorted octahedral sites surrounded by octahedral Ge and Sn positive substitutional defects which likely act as new efficient traps.