Surface core-level shifts of barium observed in photoemission of vacuum-fracturedBaTiO3(100)

When clean, vacuum-fractured surfaces of ${\mathrm{BaTiO}}_3$ are analyzed using ultraviolet and x-ray photoelectron spectroscopy (UPS and XPS), more than one set of barium core levels is observed. Sputtering the surface with ions removes lower-binding-energy intensity within the barium line shapes in UPS spectra while little change is noted in XPS. Sputtering of the surface also causes band bending and the creation of band-gap surface states of Ti 3d character. From a comparison of UPS and XPS Ba 4d line shapes from a vacuum-fractured surface, the lower-binding-energy component is assigned to barium with 12-fold oxygen coordination representative of bulk stoichiometry; the higher-binding-energy intensity originates from undercoordinated barium in the region of the sample surface. Such a negative chemical shift of core levels with increased coordination has been observed in the oxidation of metallic barium as well as in the high-temperature superconductor systems that contain barium. Reasonable values for the energy-dependent attenuation length of electrons in ${\mathrm{BaTiO}}_3$ are derived from the relative intensities of bulk and surface components using a simple model of electron attenuation.