Growth of Eu on Pd(111) studied by x-ray and uv photoemission and crystallographic properties as determined by reflection-high-energy-electron-diffraction and x-ray-diffraction studies.

The electronic properties of europium layers deposited on a Pd(111) single crystal (up to 190 \AA{} of europium) have been studied by ultraviolet and x-ray photoemission spectroscopies on both the valence bands and core levels (Eu and Pd). It is shown that, except for very low europium thicknesses, the formation of the Eu/Pd(111) interface is dominated by diffusion processes between europium and palladium. A systematic comparison between the electronic properties of this diffusive interface and the ones of well-defined intermetallic compounds (${\mathrm{EuPd}}_{5}$, ${\mathrm{EuPd}}_{3}$, ${\mathrm{EuPd}}_{2}$, and EuPd) and amorphous ${\mathrm{Eu}}_{\mathit{x}}$${\mathrm{Pd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ alloys, allows one to specify what kind of alloys or compounds formed at the interface as a function of the Eu thickness and/or temperature. Here, benefit is taken from the well-known sensitivity of the Eu valence to local environment. The most interesting result of this study is that these highly disordered Eu/Pd interfaces can be crystallized by heating the layers at moderate temperatures (800--1000 K), or by performing the evaporations of Eu on the Pd(111) substrate held at similar temperatures. In all cases and for all Eu thicknesses, reflection high-energy electron diffraction, x-ray diffraction, and photoemission experiments show evidence of the epitaxy of a trivalent, most likely ${\mathrm{EuPd}}_{3}$, intermetallic compound on the Pd(111) surface. Moreover, the epitaxial growth of Pd(111) on this trivalent compound is possible. It opens the possibility of building a metallic superlattice such as Pd/${\mathrm{EuPd}}_{3}$.