The importance of inversion disorder in the visible light induced persistent luminescence in Cr sup(3+) doped AB(2)O(4) (A = Zn or Mg and B = Ga or Al)

Abstract

Cr sup(3+) doped spinel compounds AB(2)O(4) 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 ZnGa(2)O(4), and to a lesser extent MgGa(2)O(4), can also be induced by visible light excitation via sup(4)A(2) to sup(4)T(2) transition of Cr sup(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 ZnGa(2)O(4) and MgGa(2)O(4). 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 sup(3+) ions, as shown by the increasing strain broadening of EPR and XAFS peaks observed upon increasing antisite disorder. It appears that ZnAl(2)O(4), which exhibits the largest crystal field splitting of Cr sup(3+) and the smallest antisite disorder, does not show considerable persistent luminescence upon visible light excitation as compared to ZnGa(2)O(4) and MgGa(2)O(4). These results highlight the importance of Cr sup(3+) ions with neighboring antisite defects in the mechanism of persistent luminescence exhibited by Cr sup(3+) doped AB2O4 spinel compounds.