Local structure of low-temperature γ-Al₂O₃ phases as determined by the luminescence of Cr³⁺ and Fe³⁺

Abstract

The g and g* phases of alumina synthesized from crystalline boehmite and pseudoboehmite were shown to strongly differ in some physicochemical characteristics. Photoluminescence of Cr 3+ and Fe 3+ as luminescent probes with their natural impurity concentrations was detected in the tested alumina samples and the local structure of the samples was examined. The crystal field strength and Racah parameters in g-Al 2 O 3 phases were determined. The ratio of octahedral and tetrahedral cationic positions was equal to 1 for g-Al 2 O 3 synthesized from crystalline boehmite, and 0.3 for that obtained from pseudoboehmite. The effect of residual OH groups and weakly bound molecular water on the local structure of g-Al 2 O 3 phases was revealed. The luminescence of Cr 3+ was detected in subsurface structures of each alumina; defects produced by nanocrystallites were shown to affect the luminescence.

Photoluminescence of Cr3+ in nanostructured Al2O3 synthesized by evaporation using a continuous wave CO2 laser

“…5, curve b), which correspond to the 4 T 1 / 4 A 2 and 4 T 2 / 4 A 2 transitions. 8,23 The nanostructured Al 2 O 3 condensate not modied with chromium has PL in a range of 680-710 nm. Its PL intensity is much weaker as compared to that of Cr 3+ in a-Al 2 O 3 (Fig.…”

“…Our earlier studies of the starting g-Al 2 O 3 showed that PL with l max ¼ 698 nm is caused mostly by octahedrally coordinated Cr 3+ ions (uncontrolled impurity) in the g-Al 2 O 3 matrix with the corresponding nonuniformly broadened 2 E / 4 A 2 transition. 8 The concentration of Cr 3+ ions in the samples of a-Al 2 O 3 target obtained from g-Al 2 O 3 was 10 À4 wt%, as estimated by the PL method. 8 Thus, it would be natural to expect PL of Cr 3+ ions to occur in the samples of Al 2 O 3 condensate produced by laser evaporation, even without the deliberate doping of the starting target.…”

“…8 The concentration of Cr 3+ ions in the samples of a-Al 2 O 3 target obtained from g-Al 2 O 3 was 10 À4 wt%, as estimated by the PL method. 8 Thus, it would be natural to expect PL of Cr 3+ ions to occur in the samples of Al 2 O 3 condensate produced by laser evaporation, even without the deliberate doping of the starting target.…”

“…[2][3][4][5][6][7] Nanostructured Al 2 O 3 can be synthesized by different methods, for example. [8][9][10] In recent years, laser methods have been intensely developing, [11][12][13][14][15][16][17][18] especially the ones that use continuous wave CO 2 laser. 14,15 The application of this type of laser for evaporation has some advantages: monodispersity of the resulting powders and possibility to control the size of nanocrystallites from a few to a few tens of nanometers.…”

Mechanochemical synthesis of α‐Al2O3‐Cr3+ (Ruby) and χ‐Al2O3

“…It is known that the binary system Al 2 O 3 ‐Cr 2 O 3 forms a complete solid solution by the octahedral substitution of Al 3+ for Cr 3+ . The resulting solid solution is commonly known as ruby . Ruby is known as a gemstone; however, it has remarkable applications in physics, optoelectronics, spintronics, fluorescent target for biomolecular imaging, and lasers .…”