Chapter 71 X-ray absorption and emission spectra

Novel Valence Transition in Elemental Metal Europium around 80 GPa

Valence transition could induce structural, insulator-metal, nonmagnetic-magnetic and superconducting transitions in rare-earth metals and compounds, while the underlying physics remains unclear due to the complex interaction of localized 4f electrons as well as their coupling with itinerant electrons. The valence transition in the elemental metal europium (Eu) still has remained as a matter of debate. Using resonant x-ray emission scattering and x-ray diffraction, we pressurize the states of 4f electrons in Eu and study its valence and structure transitions up to 160 GPa. We provide compelling evidence for a valence transition around 80 GPa, which coincides with a structural transition from a monoclinic (C2/c) to an orthorhombic phase (Pnma). We show that the valence transition occurs when the pressure-dependent energy gap between 4f and 5d electrons approaches the Coulomb interaction. Our discovery is critical for understanding the electrodynamics of Eu, including magnetism and high-pressure superconductivity.

Magnetic properties and electronic structures of intermediate valence systems CeRhSi2 and Ce2Rh3Si5

The crystal structures and the physical (magnetic, electrical transport and thermodynamic) properties of the ternary compounds CeRhSi(2) and Ce(2)Rh(3)Si(5) (orthorhombic CeNiSi(2)- and U(2)Co(3)Si(5)-type structures, respectively) were studied over wide ranges of temperature and magnetic field strength. The results revealed that both materials are valence fluctuating systems, in line with previous literature reports. Direct evidence for valence fluctuations was obtained by means of Ce L(III)-edge x-ray absorption spectroscopy and Ce 3d core-level x-ray photoelectron spectroscopy. The experimental data were confronted with the results of ab initio calculations of the electronic band structures in both compounds.

Correlated electrons in the Eu-exchanged Preyssler anion [EuP(5)W(30)O(110)](n-)

The Eu-encrypted Preyssler anion, [EuP5W30O110]n-, demonstrates unusual electronic properties as a function of applied potenial. The Preyssler anion itself, when exchanged with a nonredox active trivalent ion such as La3+, is electroactive in acidic solution, reversibly accepting up to 10 electrons under reducing conditions. Encrypted Eu, which is trivalent at rest potential, is reduced concomitantly with the framework. The details of this reduction are probed with Eu L3-XANES, coulometry, and magnetic susceptibility. The results are interpreted in terms of a bonding interaction between the Eu f-states and the W-O framework band, facilitated by the crossing of the framework Fermi level with the localized states. Such behavior has been previously observed in intermediate-valent and correlated-electron systems. The observation of such behavior in an isolated cluster opens the door for new avenues of research into correlated-electron behavior.