Grain-size effect on Cr3+and F-centres photoluminescence in nanophase MgAl2O4 ceramics

Photoluminescence (PL) spectroscopy of transparent MgAl2O4 spinel ceramics with grain size between 100 and 300 nm was studied at 7 K temperature in the near-IR-VUV range of spectrum with synchrotron radiation excitation. The PL spectra were composed of optical transitions from spatially different regions of the ceramics, which analysis evidenced grain size effect on the emission line-shapes and intensities. In particular, emission of impurity Cr3+ ions, being structured in the crystalline bulk, became broad-band in the grain boundary regions, which was associated with respectively strong and weak local crystalline fields. It was observed that (i) excitons and F centres transfer energy to Cr3+ and (ii) Cr(2Eg)/Cr(4T2g) and F-centres/Cr3+ PL intensity ratios underwent a linear dependence on the grain size.