Cr³⁺ Doped Al₂O₃ Obtained by Non-Hydrolytic Sol-Gel Methodology

This paper reports the synthesis and characterization of Cr³⁺-doped alumina by the sol-gel non-hydrolytic methodology. The resulting sample was treated at different temperatures. X-ray diffraction revealed that the ruby phase emerged in the sample treated at 1100 ºC, which was later confirmed by absorption bands correspondent to Cr³⁺ ions allowed transitions at ⁴A₂ → ⁴T₁, ⁴T₂, and forbidden at ⁴A₂ → ²T₁, ²E, observed by diffuse reflectance UV-Vis. The luminescence spectroscopy showed the intensity band at 694 nm in red region, characterized of the Cr³⁺ ion. The peaks at 702 and 705 nm correspond to N₁ and N₂ lines, respectively, which arose from the second and fourth nearest-neighbor exchange-coupled pairs of chromium(III) ion, respectively, ascribed to high chromium(III) concentration. The Cr³⁺ cluster formation was observed in electron paramagnetic resonance signal as discussed in this work. Nuclear magnetic resonance evidenced that the ²⁷ Al symmetry changed in the samples treated between 900 and 1100 ºC.

Keywords:
ruby; EPR; NMR; sol-gel; luminescence