Speaker
Dr
Stefan Minasian
(Lawrence Berkeley National Laboratory)
Description
The exact nature of chemical bonds between actinides and light atoms such as carbon, nitrogen, oxygen, and fluorine is of widespread interest because these interactions control the physics and chemistry of many technologically important processes. Among approaches explored previously, ligand K-edge X-ray absorption spectroscopy (XAS) has emerged as an effective method for quantitatively probing electronic structure and orbital mixing. The presence of covalent mixing is observed as a pre-edge feature in the ligand K-edge XAS, which only has transition intensity if the final state metal orbital contains a component of ligand p orbital character. For actinide–chloride bonds, the Cl K-edge pre-edge intensity is related to percentage covalency by using a copper tetrachloride intensity standard. Recent advances have shown that quantitative pre-edge peak intensities can be measured at the K-edges for light atoms through a combination of XAS with a scanning transmission X-ray microscope (STXM), non-resonant inelastic X-ray scattering (NIXS), and hybrid density functional theory calculations (DFT). However, corresponding intensity standards have not been developed.
Herein, a new effort is discussed that employs these techniques to define experimental intensity standards and garner a quantitative understanding of bonding interactions with d- and f-block coordination compounds. Oxygen K-edge XAS measurements and DFT studies began with the lanthanide dioxides and sesquioxides, LnO2 and Ln2O3 (Ln = Ce, Pr, Tb), which were chosen because 4f covalency in the Ln–O bonds is already well-established from L3-edge X-ray spectroscopies. Features in the O K-edge XAS increase in intensity in analogy to the L3-edge measurements, suggesting that both techniques provide equivalent information regarding covalency. Results from an ongoing collaboration with theorists tie these experimental trends to changes in orbital mixing with the σ and π-bonding 4f orbitals. Following a similar approach, ytterbocene, (C5H5)2Yb, was chosen to standardize intensities at the carbon K-edge. Preliminary efforts to use these benchmarks in quantitative determinations of covalency for a variety of actinide molecules and materials will be discussed.
Primary author
Dr
Stefan Minasian
(Lawrence Berkeley National Laboratory)
Co-authors
Dr
Corwin Booth
(Lawrence Berkeley National Laboratory)
Dr
David Clark
(Los Alamos National Laboratory)
Dr
David Shuh
(LBNL)
Dr
Dennis Nordlund
(SLAC National Accelerator Facility)
Dimosthenis Sokaras
(SLAC National Accelerator Facility)
Dr
Enrique Batista
(Los Alamos National Laboratory)
Dr
Jason Keith
(Colgate University)
Dr
Richard Martin
(Los Alamos National Laboratory)
Dr
Stosh Kozimor
(Los Alamos National Laboratory)
Dr
Tolek Tyliszczak
(Lawrence Berkeley National Laboratory)
Tsu-Chein Weng
(SLAC National Accelerator Facility)
Dr
Wayne Lukens
(Lawrence Berkeley National Laboratory)
Dr
Xiao-Dong Wen
(Los Alamos National Laboratory)
