No abstract

Infrared spectroscopy X-ray diffraction Ab initio simulations a b s t r a c t w-, kand a-alumina phases formed by dehydration of micro-grained gibbsite between 773 and 1573 K are studied using infrared spectroscopy (IR). The structural transitions evidenced by X-ray diffraction (XRD) were interpreted by comparing IR measurements with ab initio simulations (except for the w form whose complexity does not allow a reliable simulation). For each phase, IR spectrum presents specific bands corresponding to transverse optical (TO) modes of Al-O stretching and bending under 900 cm À 1. The very complex w phase, obtained at 773 K, provides a distinctive XRD pattern in contrast with the IR absorbance appearing as a broad structure extending between 200 and 900 cm À 1 resembling the equivalent spectra for g-alumina phase. k-alumina is forming at 1173 K and its rich IR spectrum is in good qualitative agreement with ab initio simulations. This complexity reflects the large number of atoms in the k-alumina unit cell and the wide range of internuclear distances as well as the various coordinances of both Al and O atoms. Ab initio simulations suggest that this form of transition alumina demonstrates a strong departure from the simple pattern observed for other transition alumina. At 1573 K, the stable a-Al 2 O 3 develops. Its IR spectra extends in a narrower energy range as compared to transition alumina and presents characteristics features similar to model a-Al 2 O 3 U Ab initio calculations show again a very good general agreement with the observed IR spectra for this phase. In addition, for both kand a-Al 2 O 3 , extra modes, measured at high energy (above 790 cm À 1 for k and above 650 cm À 1 for a), can originate from either remnant w-alumina or from surface modes.