Intermediate valence behaviour under pressure: how precisely can we probe it by means ofresonant inelastic x-ray emission?

Abstract

Rare earths and their compounds show many interesting physical phenomena caused by thecomplex electronic structure related to f electrons. External pressure can affect thehybridization between 5d band electrons and the more atomic-like 4f ones, givingrise to intermediate valence behaviour. Resonant x-ray emission spectroscopy(RXES) has been shown to be a very effective tool for probing the mixed valenceground state under pressure. A RXES experiment detects the x-ray emission thatfollows decay into a resonantly created core hole. Being a photon-in–photon-outspectroscopic technique, it is not hindered by the presence of the pressure cell. Inthe case of rare earths we chose to detect the 3d–2p x-ray emission following a2p–5d resonant excitation process. The divalent and trivalent components of theground state are separately enhanced because they resonate at different incidentphoton energies. Very good precision in the determination of even small changes ofvalence is reached. We will present experimental results on ytterbium compounds(Y bAl2 and YbS), as well as recent data on the valence state of SmS in the gold phase, that haveallowed us to unveil the progressive valence change towards the completion of trivalency.The spectral changes and the way to extract the valence from measured spectra will bediscussed.