Changes in the chemical state of monocrystalline SrTiO₃ surface by argon ion bombardment

High-resolution X-ray photoemission spectroscopy (XPS) of single crystalline SrTiO₃, SrTiO₃:La and SrTiO₃:Nb fractured in situ in ultrahigh vacuum (UHV) with Ar⁺ bombardment revealed that the bombardment caused reduction of Ti⁺⁴, Sr⁺², La⁺³ and Nb⁺⁵ to the lower valence states at surfaces. Formation of Ti⁺³ state can be ascribed to the partial occupation of the conduction band; some electrons transfer into Ti 3d band due to the lack of O 2p states. Similarly, the partial occupation of the La 5d orbitals due to the lack of O 2p states was observed for the bombarded SrTiO₃:La surface. Loss of surrounding oxygen due to the bombardment also resulted in the states of Nb⁺³ and Sr⁰ at the surfaces. To our knowledge, there was no report on reduction of Sr⁺², La⁺³ and Nb⁺⁵ in SrTiO₃, SrTiO₃:La and SrTiO₃:Nb.