Phases in the system Sc2O3-Al2O3-TiO2-SiO2

The project aims at the detailed characterisation of phases and phase relations in the important, but very insufficiently studied ceramic system Sc2O3 (scandium oxide)-Al2O3-TiO2-SiO2. Almost all the phases in this system have significant technological applications but the Sc-phases are poorly characterised, and a complete understanding of the interaction of Sc2O3 with the other phases is missing. Scandium belongs to the so-called `rare earth` elements [i.e., the elements from lanthanum (La) to lutetium (Lu), as well as yttrium (Y) and scandium (Sc)], compounds of which are gaining continuously increasing importance in both materials sciences and geosciences (mineralogy). The complex, currently only very insufficiently known system RE2O3-Al2O3-TiO2-SiO2 (RE = rare earth element) is of particular interest as it includes technologically important materials (in fact, a large variety of ceramic RE-compounds with outstanding chemical and high- temperature stabilities, and useful optical, mechanical and electrical properties). The majority of these RE- compounds have flexible atomic arrangements identical to that of several minerals (e.g., apatite, perovskite, pyrochlore, fluorite, magnetoplumbite). Within the project the system Sc2O3-Al2O3-TiO2-SiO2 will be characterised in detail. The presently unknown crystal structures of several poorly characterised Sc-alumosilicates and -titanates will be determined, and the incorporation of Sc and/or Ti atoms in the technologically important ceramics mullite (aluminium silicate) and Al2TiO5 will be characterised. Furthermore, phase relations in the whole system will be determined at specific contents of Al2O3. Samples will be prepared by the high-temperature flux method (growth of well-developed single crystals from special melts) and by ceramic techniques (synthesis of polycrystalline samples composed of one or more phases). The crystal structure, chemical composition, and stability of the synthesised phases and their stability relations with adjacent phases will be characterised using a combination of specialised diffraction methods (using X-ray and neutron radiation), spectroscopic techniques (Raman scattering and infrared absorption), chemical analyses and high-temperature thermoanalytical methods. Probable Sc-Al and Sc-Ti substitutions will be the subject of specific investigations. It is anticipated that the results from the project will help to find new technological applications in some of the innovative fields involving Sc- (and RE-) compounds.

A detailed characterisation of phases and phase relations in the important, but very insufficiently studied ceramic system Sc2 O3 (scandium oxide)-Al2 O3 -TiO2 -SiO2 was carried out. Almost all the phases in this system have significant technological applications but the Sc-phases are poorly characterised, and a complete understanding of the interaction of Sc2 O3 with the other phases is missing. Well-developed single crystals of a large number of scandium compounds were grown at medium-high temperatures (< 1200C) from special melts and by chemical transport using chlorine gas. The crystal structures of the compounds were determined in detail, and their stabilities, chemical compositions and spectroscopic properties were characterised. Several novel and unusual scandium compounds could be prepared which have potential applications in some innovative fields (e.g., luminescent materials and lasers, zeolite-like materials). The project`s results will also help to improve the manufacturing of some nitride and silicide high-performance ceramics, in the manufacturing of which scandium oxide is used as a sintering aid.