Actinide 5f Occupations: The Case of PuO2
Ashima Bajaj1, Harry Ramanantoanina2, Bianca Schacherl2
1Department of Chemistry University at Buffalo, State University of New York Buffalo, Buffalo, New York 14260-3000, United States.
Inorganic Chemistry
|June 10, 2025
Summary
This study clarifies the difference between formal and effective 5f electron counts in actinide chemistry using PuO2. Experimental and computational data reveal significant covalent bonding, indicating an effective 5f occupation closer to Pu4+.
Area of Science:
- Actinide chemistry
- Solid-state chemistry
- Computational chemistry
Background:
- Formal 5f electron count (n_open) is integer-valued in actinides.
- Effective 5f occupation (n_f) accounts for electron donation/back-donation, yielding non-integer values.
- Distinguishing n_f from n_open is crucial for understanding actinide bonding.
Purpose of the Study:
- To investigate the distinction between n_f and n_open in PuO2.
- To quantify electron donation and its impact on 5f occupation in PuO2.
- To provide experimental evidence for covalent bonding in PuO2.
Main Methods:
- Relativistic electronic structure calculations.
- Pu M5-edge high-energy-resolution X-ray absorption and emission spectroscopy.
- Valence band resonant inelastic X-ray scattering (RIXS).
Main Results:
- Total electron donation to Pu in PuO2 ranges from 2.4 to 3.1 electrons.
- 5f electron donation is sensitive to calculation approximations, estimated at 1.6 or 0.8 electrons.
- RIXS confirms Pu 5f electron density in the valence band, indicating covalent bonding.
- Experimental Pu M5/M3-edge spectra suggest n_f in PuO2 is closer to Pu4+(aq).
Conclusions:
- PuO2 exhibits significant covalent character due to 5f electron delocalization.
- The effective 5f occupation in PuO2 is crucial for accurate chemical descriptions.
- Experimental and computational methods combined provide a comprehensive understanding of bonding in PuO2.
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