The importance of inversion disorder in the visible light induced persistent luminescence in Cr³⁺ doped AB₂O₄ (A = Zn or Mg and B = Ga or Al).

Cr(3+) doped spinel compounds AB2O4 with A = Zn, Mg and B = Ga, Al exhibit a long, near infrared persistent luminescence when excited with UV or X-rays. In addition, the persistent luminescence of ZnGa2O4, and to a lesser extent MgGa2O4, can also be induced by visible light excitation via (4)A2→(4)T2 transition of Cr(3+), which makes these compounds suitable as biomarkers for in vivo optical imaging of small animals. We correlate this peculiar optical property with the presence of antisite defects, which are present in ZnGa2O4 and MgGa2O4. By using X-ray absorption fine structure (XAFS) spectroscopy, associated with electron paramagnetic resonance (EPR) and optical emission spectroscopy, it is shown that an increase in antisite defects concentration results in a decrease in the Cr-O bond length and the octahedral crystal field energy. A part of the defects occurs in the close environment of Cr(3+) ions, as shown by the increasing strain broadening of EPR and XAFS peaks observed upon increasing antisite disorder. It appears that ZnAl2O4, which exhibits the largest crystal field splitting of Cr(3+) and the smallest antisite disorder, does not show considerable persistent luminescence upon visible light excitation as compared to ZnGa2O4 and MgGa2O4. These results highlight the importance of Cr(3+) ions with neighboring antisite defects in the mechanism of persistent luminescence exhibited by Cr(3+) doped AB2O4 spinel compounds.