Europium Valence in Pressurized EuO.

The Eu valence in EuO under compression has been debated since the first observation of a pressure-induced isostructural volume collapse by Jayaraman five decades ago. Spectroscopic studies at the Eu L_{3} edge suggested a valence change from divalent to trivalent due to promotion of charge from the half-filled 4f shell to empty 5d states, while density functional theory investigations have reported both support for and against such a valence change. The Eu N_{4,5} edges (4d→4f excitations) are highly sensitive to the 4f occupancy and thus to the Eu valence. Using x-ray Raman scattering, we measure the Eu N_{4,5} edges of EuO up to 66 GPa and observe no significant change in spectral shape indicative of a modification of the 4f occupation. Our results therefore support a robust Eu^{2+} state across the isostructural transition at 35 GPa and the subsequent B1-B2 transition at 44 GPa.