Combustion synthesis and nontrivial luminescence properties of nanosized δ-Al2O3 doped with Cr3+ ions

Powder samples of Cr3+-doped δ*-Al2O3 with particles sizes of 10–70 nm were obtained by combustion method. It is shown for the first time that Cr3+ ions in the δ*-Al2O3 occupy octahedral sites in the bulk and at the surface of the nanosized particles. The photoluminescence spectra of Cr3+ in δ*-Al2O3 consist of several relatively narrow lines (R-lines luminescence) in the 680–720 nm range, which are due to the 2Eg→ 4A2g transitions. The local structure around Cr3+ ions inside the nanoparticles is shown to differ from that of Cr3+ located in the surface layer, which results in essential differences in the full width at half maximum values (14 cm−1 vs. 39 cm−1 at 77 K) and in red shift of the R-lines with increasing temperature from 77 to 293 K (20 cm−1 vs. ~11 cm−1). It was found that the Cr3+-doped δ*-Al2O3 exhibits some striking features, including an unexpectedly high crystal-field strength at the interior Cr3+ site and an unusually high concentration of surface Cr3+ centers. Possible reasons for these features are discussed. The results obtained are anticipated to be useful for the monitoring of processes involved in the preparation of ceramics and composites based on Al2O3:Cr3+. [Display omitted] •The luminescent properties of Сr3+ in nanosized δ*-Al2O3 are analyzed.•Two types of Сr3+ related centers (regular and surface) are observed.•In the 77–293 K range, all the Cr3+centers show emissions of the 2Eg.→4A2g type.•Structural disorder degree as a notable characteristic of Al2O3 polymorphs.