Structure-dependent mixed valence of Sm on Cu(1 1 1) studied by XPS and STM

Geometric and electronic structures of adsorbed Sm on Cu(1 1 1) were studied by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). The surface morphology and Sm valency change with the Sm coverage, suggesting that Sm atoms in the islands have a valence of approximately 2.7 and those diffusing are divalent. Heating the adlayers yields surface alloy and close-packed overlayer of Sm depending on the coverage and temperature. Atomically resolved STM images indicate that the surface of the alloy phase is not rigid. The closed packed overlayer of a mixed valence Sm is observed as a uniform atomic species.