Influence of Na on the structure of Bi4Ti3O12 films deposited by liquid-delivery spin MOCVD

Thin Na-substituted Bi4Ti3O12 films were grown by the liquid-delivery spin metal-organic chemical vapor deposition (MOCVD) method with different concentrations of sodium bis(trimethylsilyl)amide [Na(TMSA)] as Na precursor. At a substrate temperature of 600°C the original Aurivillius structure was preserved, however high resolution x-ray diffraction (HRXRD) studies indicate that the Na-substituted phase exhibits a slightly smaller lattice parameter compared to the pure Bi4Ti3O12 phase. From additional x-ray photoemission spectroscopy (XPS) results, we have concluded that monovalent Na+ ions have been incorporated on Bi3+ sites in the perovskite units. The proposed charge compensation for this aliovalent substitution is explained by a shift of the valence state of Bi3+ ions in the vicinity of the incorporated Na+ ions from 3+ to 5+. Due to the small ionic radius of the Bi5+ ions, the incorporation efficiency amounts to a few atomic percent only.