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 VO. Mg2SnO4 exhibits long-lasting luminescence at two different wavelength regions, peaking at ∼498 and ∼755 nm. A Sn-VO-Sn defect plus a Mg vacancy VMg away from the VO generates the empty midgap states, σSn-Sn and σSn-Sn*, localized at the Sn-VO-Sn defect, while an oxygen vacancy VO between adjacent Sn4+ and Mg2+ sites creates a filled midgap state Sn2+ (5s2 lone pair) lying below the σSn-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 σSn-Sn* level as the trapping level for a photogenerated electron.