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Electronic structures and associated properties of the d- and f-band transition metal compounds

d- and f-band metal and compounds, with unfilled outer electrons, constitute a fascinating class of materials which are of great importance in materials science, solid-state physics, and planetary science. The electronic structures and associated changes in physical properties of the d- and f-band metals and compounds under extreme pressure-temperature conditions, such as hcp-Fe, Fe2O3, SmX (X=S, Se, Te), and YbNi2Ge2 and YbPd2Si2 have been studied using an array of recently developed synchrotron-based techniques, namely inelastic X-ray scattering spectroscopies (IXS), including resonant and non-resonant inelastic X-ray spectroscopy with energy resolution ranging from 1 meV to 1 eV. Important milestone results are highlighted below: Mott Insulator-Metal Transition in Fe2O3: A particular example for using multi-IXS techniques is manifested in our recent study on Fe2O3, a classic ferric oxide, at high pressures. Fe2O3 has been reported to undergo structural, electronic, magnetic, and metal-insulator transitions under high pressures, yet the sequence of these transitions and their consequences on the properties of Fe2O3 remain unclear. Through these multi-IXS studies, our results show that the structural transition occurs before the magnetic collapse, and that the structural transition has significant effects on the physical properties of Fe2O3 such as phonon density of states and sound velocities (Figure 1). We are now working with theorists (Dr.J. Tse) to better interpret these new observations. Pressure-induced mixed-valence behavior in SmX (X=S, Se, Te): Among the samarium monochalcogenide series SmX (X=S,Se,Te), SmS has received the most attention due to its first-order isostructural insulator-to-metal transition (IMT) occurring at 0.65 GPa at room temperature. SmSe and SmTe undergo a continuous, second-order IMT above respectively 3 and 6 GPa. We have characterized the pressure dependence of the Sm valence in SmS, SmSe, and SmTe by Sm-L3 x-ray absorption spectroscopy in the partial fluorescence yield (PFY-XAS) and Sm-2p3d resonant x-ray emission spectroscopy (RXES). We find that, the larger the radius of the chalcogenide ion (See

Fig.1-Fe2O3

 

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