Dynamics of interface reactions and metastable phasesin mixed aluminate structures

Aluminate structures exhibit phase formation anomalies irrespective of the initialstoichiometry of the oxide powder mixtures. In metal oxide systems (Me2+ or Me3+),multiphases form at different cation to aluminum (Mex+/Al) ratios. These systemscontain tetrahedral ((AlO4)5-) and/or octahedral ((AlO6)9-) polyhedron in the aluminatecrystallography. Large cation size of Me2+ in aluminate systems was claimed to be thereason of the difficulties in diffusion. In the presented work, polymerized complexmethod was applied to synthesize 14 compounds present in the three binary equilibriumphase diagrams of SrO-Al2O3, CaO-Al2O3, and Y2O3-Al2O3. Solid-solution reactions were performed to test the effect of having tetrahedra or octahedra Al-O network inaluminates by mixing carbonate and γ- or α-Al2O3 particles. γ- Al2O3 has dual aluminacoordination and samples prepared by solid-state reactions formed hexaluminates. Instrontium, calcium and yttrium aluminate systems, the first forming phase is 1:1 oxidecompound SA (SrAl2O4), CA (CaAl2O4), and YAP (YAlO3) irrespective of the nominalMex+/Al3+. Cation to aluminum ratio of 1:1 compound forms first with the lowestformation enthalpy. We highlighted the first formation of the 1:1 compound is due tohaving one type of Al3+ tetrahedra or octahedra network.The interface reactions were investigated between the reaction couple of single-crystalsapphire (α-Al2O3) substrates and metal oxides (MeOx). The incipient interphase andchemical distribution of elements are presented with energy dispersive X-rayspectroscopy (EDS) mapping and backscattered electron (BSE) imaging in scanningelectron microscopy (SEM). Interface between the reaction couple will be imaged andanalyzed with a transmission electron microscope (TEM) enhanced with EDS andelectron energy loss spectroscopy (EELS) capabilities. The incipient interphase will beidentified with the help of previously collected Al-L and O-K ELNES edges from Sraluminatecompounds in the binary SrO-Al2O3 system.