Preparation and structure of Fe-containing aluminosilicate thin films.

In attempts to fabricate model systems of Fe-containing aluminosilicates, we studied the incorporation of iron into silicate and aluminosilicate bilayer films grown on Ru(0001). Structural characterization was performed by low energy electron diffraction, X-ray photoelectron spectroscopy, infrared reflection-absorption spectroscopy and scanning tunneling microscopy. The experimental results show that even at low concentrations Fe does not randomly substitute Si(Al) cations in the silicate framework, but segregates into a pure silicate (aluminosilicate) phase and an Fe-silicate phase which is formed by an FeO(111)-like layer underneath a silicate layer. At high Fe/(Si + Al) molar ratios, the resulting films showed two phases depending on the annealing temperature. In both phases, the surface exposes a silicate layer and the bottom layer is dominated by FeO. The Al ions seem to be present in the bottom layer at relatively low oxidation temperatures, but segregate as alumina clusters at the surface at higher temperatures. The results suggest that the formation of in-frame Fe species in silicalites and zeolites is thermodynamically unfavourable. This study provides further steps towards the rational design of model systems for studying surface chemistry of a wide class of layered minerals.