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

Cr 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, 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 4 A 2 → 4 T 2 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 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 are 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 ZnAl 2 O 4 , 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 ZnGa 2 O 4 and MgGa 2 O 4 . These results highlight the importance of Cr 3+ ions with neighboring antisite defects in the mechanism of persistent luminescence exhibited by Cr 3+ doped AB 2 O 4 spinel compounds.

Order and disorder around Cr3+ in chromium doped persistent luminescent AB2O4 spinels

“…27,33 Varying site occupancy for ZAO compound yielded no cationic inversion hinting to the normal spinel lattice. 17 The Rietveld refined lattice constants and the cationic occupancy are tabulated in Table 1. EXAFS results were presented in our earlier paper and were correlated with optical data. 17 The bond distances of Cr to first Table 1.…”

“…A preliminary comparison shows that the first and second cationic bond distances obtained from EXAFS fitting are almost similar to the XRD bond distances in all compounds. 17 However EXAFS Cr-O bond distances in ZGO and MGO are much smaller compared to their respective XRD Ga-O bond distances. We had proposed in our previous paper that, this reduction in bond lengths is a result of the increased π interaction of Cr and O orbitals due to the presence of antisite defects around Cr 3+ .…”

“…17 The Rietveld refined lattice constants and the cationic occupancy are tabulated in Table 1. EXAFS results were presented in our earlier paper and were correlated with optical data. 17 The bond distances of Cr to first Table 1. For the EXAFS fitting of MGO, two models based respectively on normal spinel and inverse spinel structures were used.…”

“…A strong correlation between Cr-O bond distances and visible light induced NIRPL was shown. 17 This was interpreted to be due to increased hybridization between Cr 3d t 2g orbitals and ligand 2p orbitals due to presence of antisite defects in the near neighbourhood of Cr 3+ ions. The π bonding between Cr and O resulted in decreased crystal field (CF) splitting of the Cr 3d band in spite of decrease in Cr-O bond distance in these Cr 3+ doped AB 2 O 4 spinels.…”

“…23 25 In a previous paper, we had presented EXAFS results along with optical and electron paramagnetic resonance (EPR) studies on Cr 3+ doped AB 2 O 4 spinels (A=Zn, Mg and B=Ga, Al). 17 In there, the crucial role of inversion defects in necessitating the persistent luminescence in spinels specially with visible light excitation was illustrated in detail. A strong correlation between Cr-O bond distances and visible light induced NIRPL was shown.…”