VARIATION OF INFRARED ABSORPTION SPECTRA IN THE SYSTEM BI2 AL4-XFEXO9 (X=0-4), STRUCTURALLY RELATED TO MULLITE

Abstract

The crystal structure of Bi2Al4-xFex O9 compounds (x=0-4) has striking similarities with the crystal structure of mullite. A complete substitution of Al by Fe3+ in both octahedral and tetrahedral sites is a particular structural feature. The infrared (IR) spectra of the Bi2 M4O9 compounds (M = Al, Fe3+) are characterised by three band groups with band maxima in the 900-800, 800-600 and 600-400 cm1 region. Based on the spectroscopic results obtained from mullite-type phases, the present study focuses on the composition-dependent analysis of the 900-800 cm1 band group, which is assigned to Al(Fe3+) - O stretching vibrations of the corner-sharing MO4 tetrahedra. The Bi2Al4O9 and Bi2 Fe4O9 endmembers display single bands with maxima centred at 922 and 812 cm-1, respectively. Intermediate Bi2Al4-xFexO9 compounds exhibit a distinct splitting into three relatively sharp bands, which is interpreted in terms of ordering effects within the tetrahedral pairs. Thereby the high-energy component band of the band triplet relates to Al-O-Al conjunctions and the low-energy component band to Fe-O-Fe conjunctions. The intermediate band is assigned to stretching vibrations of Al-O-Fe or Fe-O-Al configurations of the corner-sharing tetrahedral pairs. Bands in the 800-600 cm-1 range are assigned to low-energy stretching vibrations of the MO4 tetrahedra and to M-O-M bending vibrations of the tetrahedral pairs. Absorptions in the 600-400 cm-1 range are essentially determined by M-O stretching modes of the M cations in octahedral coordination. © Springer-Verlag 2006.