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

Abstract

Rare earths and their compounds show many interesting physical phenomena caused by the complex electronic structure related to f electrons. External pressure can affect the hybridization between 5d band electrons and the more atomic-like 4f ones, giving rise to intermediate valence behaviour. Resonant x-ray emission spectroscopy (RXES) has been shown to be a very effective tool for probing the mixed valence ground state under pressure. A RXES experiment detects the x-ray emission that follows decay into a resonantly created core hole. Being a photon-in–photon-out spectroscopic technique, it is not hindered by the presence of the pressure cell. In the case of rare earths we chose to detect the 3d–2p x-ray emission following a 2p–5d resonant excitation process. The divalent and trivalent components of the ground state are separately enhanced because they resonate at different incident photon energies. Very good precision in the determination of even small changes of valence 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 have allowed 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 be discussed.

Skutterudite Results Shed Light on Heavy Fermion Physics

“…The paradox is that despite the valence (v) is intermediate as estimated by high energy spectroscopy 61,62 or Raman measurements, 63 the development of the correlation on cooling will lead to renormalize the electronic and magnetic properties either to the divalent limit (insulator ground state with 2+ magnetic form factor) for v < 2.8 or to the trivalent limit for v > 2.8 (metallic ground state with 3+ magnetic form factor). 58 The same renormalization has been observed for TmSe.…”

“…In the extreme conditions of Sm 2+ or Sm 3+ the only selected configurations of the ground states are respectively a PM insulator and a LRO metal; the isostructural trivalent lanthanum references LaS and LaB 6 are excellent metals with n ∼ 1. The paradox is that despite the valence (v) is intermediate as estimated by high energy spectroscopy 61,62 or Raman measurements, 63 the development of the correlation on cooling will lead to renormalize the electronic and magnetic properties either to the divalent limit (insulator ground state with 2+ magnetic form factor) for v < 2.8 or to the trivalent limit for v > 2.8 (metallic ground state with 3+ magnetic form factor). 58 The same renormalization has been observed for TmSe.…”

Valence and magnetic ordering in intermediate valence compounds: TmSe versus SmB6

“…TmSe [4,5,6,7] as well as SmS [2] and SmB 6 [8] in their low pressure intermediate valent gold phase have a valence near 2.6-2.7. Their trivalent limit will be reached smoothly only at very high pressure above 10 GPa for TmSe and 20 GPa for SmS and SmB 6 [9,10,11] . As will be discussed, the striking point is that for these three systems, the change from insulating to metallic conduction at low temperature occurs when n f ∼ 0.8.…”

Valence and magnetic instabilities in Sm compounds at high pressures

“…The gap closes continuously as pressure is increased and disappears for p > p ∆ , leaving the system in a metallic ground state [5,6]. In the golden phase the valence increases continuously with pressure, with an inflection point at p ∆ (v ∼ 2.7) [7], and the trivalent state is reached only at considerably higher pressure (p 3+ > 10 GPa) [7,8]. Recently a first order transition at p ∆ with the appearance of long-range magnetic order associated with intermediate valence has been evidenced by nuclear forward scattering (NFS) of synchrotron radiation [9] and specific heat [10].…”