Electronic structure of stoichiometric and non-stoichiometric epitaxial FeTiO_3+δ films

Well-crystallized epitaxial FeTiO_3+δ films were prepared on the sapphire substrate by reactive sputtering technique. A wide range of oxygen nonstoichiometry from FeTiO₃ to FeTiO_3.5 was successfully controlled by controlling the sputtering conditions. All films had uniform structure without phase separation even for the highly oxidized FeTiO_3.5 film. Structures, magnetic and electronic properties of the FeTiO_3+δ films were seriously influenced by the oxygen nonstoichiometry. The nearly stoichiometric FeTiO₃ films had a R structure, while the nonstoichiometric FeTiO_3.5 had a R c structure. With increasing the oxygen nonstoichiometry, the valence states of Fe ions in FeTiO_3+δ were examined to be changed from Fe²⁺ to Fe³⁺ by means of both Mössbauer and XPS spectroscopy. The Ti 2p core-level XPS spectra clearly confirmed the valence states of Ti⁴⁺ ions in nonstoichiometric FeTiO_3.5. However the Ti ions in stoichiometric FeTiO₃ suggested the abundant electron density than the conventional Ti⁴⁺ ions due to the Fe²⁺ to Ti ⁴⁺ intervalence charge transfer.