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.