SPring-8, the large synchrotron radiation facility

Speaker: John S. Tse, Ph.D. D.Sc.

Language: English

Affiliation: Department of Physics and Engineering Physics University of Saskatchewan

Title: New Insights to Old Proposals

Abstract:
In this presentation, three currently accepted proposals relating to, (i) the nature of chemical bonding for bimetallic alloys with significant differences in the element’s electronegativities, (ii) the relationship between the 57Fe Mossbauer chemical shift with the oxidation state and (iii) valence transition of Eu in the oxide are examined with recent experiments and theoretical calculations. The Zintl concept seems to be well suited to describe the bonding and structure of binary alloys formed from elements with very different electronegativities, such as the K-Ag alloys and metal polyhydride systems. However, the simple model fails to explain the sequence of structural transformations of the related Na-Au alloys. The valence electron distribution obtained from maximum entropy analysis of the diffraction patterns differs from the expected simple ionic picture. Hybridized Na 3s-3p orbitals are found to bond with the Au 6d. Thus, a comprehensive understanding of the structure and bonding requires an extension of the model. The 57Fe chemical shifts of minerals measured at high pressure are often used to assign the oxidate state from a correlation established at ambient pressure. Instead, it is found that the chemical shift range for pure Fe to high pressure almost covers the full range of the common Fe oxidation states. Calculations confirm the experimental observation and show that the oxidation state is not a well-defined quantity of mineral oxides under extreme pressure due to the back donation of electrons from the surrounding oxygen atoms. The isostructural transition of compressed EuO has been attributed to the change of valence state to Eu(III). It is shown, using global-hybrid density functional theory calculations, the features reported in the Eu L-edge XANES can be explained by the change in the electron population density of the empty crystal field split t2g – eg Eu 6d manifolds. The suggestion is confirmed by inelastic x-ray scattering of Eu N4,5, multiplet excitations.