It is well known, that the magnesium aluminate (MgAl2O4) spinels are characterized by some disorder between the cation sites leading to the form Al(MgAl)O4 [1]. Such behavior takes a place both in natural and synthetic spinels, affecting strongly optical properties in case doping with chromium ions. The natural Cr3+:MgAl2O4 crystal being most often of ruby color, is characterized by a narrow band luminescence associated with the 2E ? 4A2 phosphorescence assisted by vibronic side bands, whereas the synthetic Cr3+:MgAl2O4 spinel (olive color) shows the broad band luminescence characteristic for the intermediate crystal field where both the phosphorescence 2E ? 4A2 and fluores- cence 4T2 ? 4A2 transitions appear. The optical properties of Cr3+ doped MgAl2O4 synthetic spinels were reported by several authors [2–10]. The luminescence properties of Cr3+ doped natural MgAl2O4 mineral were investigated by Czaja and Mazurak [12]. The luminescence of Cr3+ ions in cordierite glass nucleating MgAl2O4 nanocrystals were studied by Rosi et al. [13].
The MgAl2O4 spinels doped with lanthanide and transition metal ions form interesting class of phosphors [14] due to their high mechanical strength and thermal resistance. Recently, the solgel technique was successfully applied for synthesis of nanosized MgAl2O4 spinels [15]. In the present work we report the optical properties of Cr3+-doped MgAl2O4 nanocrystalline spinels. In par- ticular the effect of grain size of nanocrystals on luminescence properties of Cr3+:MgAl2O4 was investigated.