Oxygen-Vacancy-Induced Midgap States Responsible for the Fluorescence and the Long-Lasting Phosphorescence of the Inverse Spinel Mg(Mg,Sn)O4

Samples of inverse spinel Mg2SnO4 were prepared using a ceramic method, their phosphorescence phenomenon was probed by optical measurements, and its cause was explored on the basis of density functional theory calculations for model structures of Mg2SnO4 with oxygen vacancies V-O. Mg2SnO4 exhibits long-lasting luminescence at two different wavelength regions, peaking at similar to 498 and similar to 755 nm. A Sn-V-O-Sn defect plus a Mg vacancy V-Mg away from the V-O generates the empty midgap states, sigma(Sn-Sn) and sigma(sn-sn)*, localized at the Sn-V-O-Sn defect, while an oxygen vacancy V-O between adjacent Sn4+ and Mg2+ sites creates a filled midgap state Sn2+ (5s(2) lone pair) lying below the igsn.sn level. The long-lasting luminescence at two different wavelength regions and the up-conversion photostimulated luminescence observed for undoped Mg2SnO4 are well explained by considering the sigma(sn-sn)* level as the trapping level for a photogenerated electron.