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)

Abstract

Cr³⁺ 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 via⁴A₂ → ⁴T₂ transition of Cr³⁺, 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 Cr³⁺ 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 Cr³⁺ 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 Cr³⁺ ions with neighboring antisite defects in the mechanism of persistent luminescence exhibited by Cr³⁺ doped AB₂O₄ spinel compounds.