Structural characterization of mullite-type compounds

Research project P 14578 Structural characterization of mullite-typ compounds Anton BERAN 27.11.2000 The aluminium silicate mullite (3Al 2O3.2SiO2) is one of the most important ceramic phases and has great importance in the field of advanced materials because of its excellent thermo-mechanical properties. Compounds with mullite-type structure also exist in germanate systems (Al2O3-GeO2 and Ga2O3-GeO2), in the systems of the alkali aluminates (Al2O3-Me2O, Me = Na, K) and in the system of the aluminium borates (Al2O3-B2O3). Structural investigations of these mullite isotypes are interesting from the aspect of crystal chemistry and spectroscopy. On the one hand the principles of formation of silicon-free mullite structures, depending on the chemical composition of the phases, can be studied, on the other hand, IR and Raman spectroscopic investigations of the substituted mullites, promise a strongly improved band assignment of spectra of "ordinary" mullite and non-crystalline mullite precursor phases, which are important for the production of advanced ceramics. Within the project mullite-isotypic compounds will be synthesized and characterized in the systems of the aluminium germanates, gallium germanates, aluminium borates and alkali aluminates. X-ray diffraction (XRD), FTIR and Raman spectroscopy will be used as the main characterization methods. Further structural information can be achieved by UV- VISspectroscopy of doped mullites. The project will be carried out in close cooperation with the Institute for Materials Research of the German Aerospace Center (DLR) in K61n. Additional methods for the characterization of the Si02-firee mullites are provided by the cooperation partner, including nuclear magnetic resonance spectroscopy (NMR), analytic transmisson electron microscopy (ATEM) and neutron diffraction. Structural details, especially order/disorder of cations on tetrahedral lattice sites, distribution of oxygen vacancies and distortion of coordination polyhedra will be investigated. The mullites in the systems Al2O3Na2O and Al2O3-B2O3 are metastable intermediate phases of the technically important compounds ß-Al2O3 and 9Al2O3.2B2O3.