# Controlling disorder in the ZnGa$_2$O$_4$:Cr$^{3+}$ persistent phosphor by Mg$^{2+}$ substitution

## TL;DR

This study investigates how increasing structural disorder, induced by Mg$^{2+}$ substitution, affects the persistent luminescence of Cr$^{3+}$ doped zinc gallate spinel, revealing a decrease in luminescence with more disorder.

## Contribution

It introduces a systematic analysis of disorder effects on persistent luminescence in Cr$^{3+}$ doped zinc gallate, combining multiple spectroscopic techniques to elucidate the local environment changes.

## Key findings

- Mg/Zn substitution reduces Cr$^{3+}$ in undistorted sites
- Increases Cr$^{3+}$ with antisite defects and clusters
- Disorder correlates with shorter Cr-O bonds and weaker crystal fields

## Abstract

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.