Evidence of mixed-valence hydrated europium-chloride phase in vacuum by means of optical and electronic spectroscopies

Studying the optical and electronic properties of commercial hydrated EuCl₃ salts, we took advantage of the manifestation of the unfilled 4f valence shell in photoluminescence and core-level photoemission spectroscopies to unambiguously identify contributions from both Eu³⁺ and Eu²⁺ configurations. Lanthanide ion salt exhibits sharp spectral lines in good agreement with Eu³⁺ oxidized state. Surprisingly, the photoemission spectrum of europium 4d core level presents a singular asymmetric shape component attributed to Eu²⁺ species, thus revealing a significant amount of divalent europium. X-ray absorption spectroscopy confirmed the presence of Eu²⁺ species in proportions that depend on the origin of the salt and on the applied material processing.