Phonons and fractons in sol-gel alumina: Raman study

Abstract

We report Raman scattering from the boehmite, γ-, δ- and α-phases of the alumina gel. Samples are characterized by transmission and scanning electron microscopy, X-ray diffraction and density measurements. The main Raman line in the boehmite phase is red-shifted as well as asymmetrically broadened with respect to that in the crystalline boehmite, signifying the nanocrystalline nature of the gel. Raman signatures are absent in the γ- and δ-phases due to the disorder in cation vacancies. We also show that low frequency Raman scattering from the boehmite phase resembles that from a fractal network, characterized in terms of fraction d^~. Taking Hausdorff dimension D of the boehmite gel to be 2.5 (or 3.0), the value of d^~ is 1.33±0.02 (or 1.44±0.02), which is close to the theoretically predicted value of 4/3.