Synthesis and properties of mullite-type (Bi1−xSrx)2(M11−yM2y)4O9−x (M = Al, Ga, Fe)

Mullite-type A2M4O9 phases (M = Al, Ga, Fe), representing promising oxygen conducting materials for solid oxide fuel cells (SOFCs), were synthesized using the glycerine- and the EDTA/citric acid synthesis method. For strontium-doped material pure phases could be obtained only by washing the samples after the heating in both synthesis methods. Temperature dependent investigations were carried out to show the influence of the metal atoms on the structural stability and thermal expansion coefficients. Whereas the Sr-free phases show a quasi linear thermal expansion behavior in all three directions up to their incongruent melting points, a discontinuity in the measured range is observed for the investigated strontium doped dibismuth-nonaoxotetrametallate(III) caused by the decomposition into Bi2M4O9, strontium metallates and bismuthoxide. Big single crystals were only observed for the Sr-free compound, of which the structure of Bi2(Ga0.45Fe0.55)4O9 will be presented here in the Bärnighausen tree corresponding mullite-type setting.