, Volume 44, Issue 3, pp 201-209

Phonons and fractons in sol-gel alumina: Raman study

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Abstract

We report Raman scattering from the boehmite,γ-, δ- andα-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 theγ- andδ-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 dimension $\tilde d$ . Taking Hausdorff dimension D of the boehmite gel to be 2.5 (or 3.0), the value of $\tilde d$ is 1.33±0.02 (or 1.44±0.02), which is close to the theoretically predicted value of 4/3.