  We have studied in this work the effect of increasing structural disorder on
the persistent luminescence of Cr$^{3+}$ doped zinc gallate spinel. This
disorder was introduced by progressive substitution of Zn$^{2+}$ by Mg$^{2+}$
ions, and was studied by photoluminescence, X-ray diffraction, extended X-ray
absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES)
and electron paramagnetic resonance (EPR) spectroscopy. It was found that
increasing Mg/Zn substitution decreases the number of Cr$^{3+}$ in undistorted
sites, and increases the number of Cr$^{3+}$ with neighbouring antisite defects
and with neighbouring Cr$^{3+}$ ions (referred as Cr clusters), which in turn
decreases the intensity of persistent luminescence. Both XANES and EPR spectra
could be simulated by a linear combination of Cr$^{3+}$ spectra with three
types of Cr$^{3+}$ environments. The increasing disorder was found to be
correlated with a decrease of the average Cr-O bond length and a decrease of
crystal field strength experienced by Cr$^{3+}$ ions.
