AVS 69 Session AC+MI+TH-TuA: Chemistry and Physics of the Actinides/Rare Earths

The computational modeling of rare-earthy and heavy-element complexes requires an accurate treatment of spin-orbit and electron correlation effects to fully understand the physical underpinnings of their chemical properties. The fully correlated frequency-independent Dirac-Coulomb-Breit Hamiltonian in a multireference framework provides the most accurate description of electron-electron interaction before going to a genuine relativistic quantum electrodynamics theory of many-electron systems. Our recent studies using many-body multireference methods suggest that there is a significant correlation effect of inner-valence electrons in rare-earth and heavy-element complexes. Ignoring this effect could lead to inaccurate descriptions of molecular properties, such as covalency, bonding, and spectroscopic response. In this talk, we will examine how the correlation effect of inner-valence electrons is manifested in molecular properties with a focus on the covalency in rare-earth and heavy-element complexes.

Actinides and lanthanides are f-electron elements, and thus their chemical behavior is very similar if they have the same valence. Recently, the demand for lanthanides, especially specific lanthanides such as Nd, Tb, and Dy, is increasing with the realization of a carbon-neutral society and the shift to EVs for vehicle, and there is a growing need to find new resources and recycle them from urban mines. On the other hand, in the case of geological disposal of radioactive waste generated from nuclear power generation, separation of minor actinides such as Am, which are long-lived α-nuclides, is very important from the viewpoint of recycling in a geological repository in a small country like Japan, and the establishment of separation and transmutation technology is desired. This means that the need for separation of specific f elements from solutions such as high-level liquid waste containing a wide variety of lanthanides and actinides will, needless to say, increase more and more.

Here, we have been new synthesized phenanthroline amide derivatives (phenanthroline amide: PTA) with N and O as donor elements for these purposes. These ligands achieve separation of a special f-element by recognize slight size differences or utilize differences in their interactions with the donor elements.

A terpyridine derivative was synthesized with the lanthanides and several trivalent actinides targeted at probing differences between americium and curium. In addition to investigating high quality structural data on both the americium and curium concluding in a slight increase in covalent character in the americium system, the cerium and berkelium analogs proved to be even more interesting, exhibiting enhanced covalent character in the system. This terpyridyl system continues to provide novel behavior throughout the f-elements exemplifying the inverse trans influence throughout the series with the interaction between the terpyridyl and the nitrate molecule in the same plane.

Plutonium is both electropositive and highly reactive, such that an oxide film of varying thickness is always present on metal samples.It is of interest from a safety point of view (reduced handling/processing) to investigate methods that either prevent or slow down the rate of the corrosion reaction of the metal.This can be achieved by alloying with a suitable quantity of gallium [1] or via the formation of a surface film of plutonium oxycarbide, PuO_xC_y (where x+y ≤ 1) [2].Plutonium oxycarbide films have the NaCl structure and were initially called plutonium monoxide, PuO.Plutonium oxycarbide films are formed from the bulk to surface diffusion of the ubiquitous carbon impurity in the metal [3].However, to date, there little understanding the electronic properties of plutonium oxycarbide films.The aim of this work is to investigate the electronic structure of plutonium oxycarbide surface films using X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and inverse photoemission spectroscopy (IPES).XPS measurements have been acquired at elevated temperature following the Pu 4f, O 1s and C 1s / Pu 5p_1/2 spectral regions on both alloyed and pure plutonium samples following sputter cleaning and oxidation at room temperature.UPS and IPES were acquired at room temperature following the formation of the surface plutonium oxycarbide film. All three spectroscopic measurements display a two peak structure consistent with a partially localized and itinerant 5f electronic structure for plutonium oxycarbide. Finally, the Auger parameter (Pu N₇O₅V – Pu 4f_7/2) for plutonium oxycarbide films of varying carbon and oxygen stoichiometries are compared to the recently reported values for plutonium metal and the homoleptic plutonium oxides [1].

[1] XPS characterization of a PuGa-7 at. % alloy, P. Roussel, S. C. Hernandez, J. J. Joyce, K. S. Graham, T. Venhaus, J. Vac. Sci. Technol. A 41, 023204 (2023).

[2] Retardation of plutonium oxidation by a PuO surface film, D. T. Larson, D. L. Cash, J. Vac. Sci. Technol. 9, 800 (1972).

[3] Initial studies of plutonium corrosion, P. Roussel, D. S. Shaw, D. A. Geeson, J. Nucl. Sci. Technol.39:sup3, 78 (2002).

Nanostructures (particularly with sizes below 10 nm) are inherently challenging to characterize on the atomic scale, due to broadening which occurs in diffraction-based characterization methods, and the high concentration of surface defects and energy-minimization effects. Characterization challenges compound when investigating nanoscale actinide oxides, such as uranium oxide, due to radioactive sample constraints and rich electronic structure which can potentially stabilize a wide range of crystallographic arrangements. To address these challenges, x-ray spectroscopic and scattering based methods are used to probe challenging nanoscale systems in detail over multiple length scales. In particular, synthetic developments are paired with robust characterization, in order to improve knowledge of nanoparticle growth and transformation, as well as develop ways to make advanced x-ray characterization analysis more informed and accessible, in tandem.

Janoschek, et al. described the ground state of plutonium (Pu) as being, “governed by valence fluctuations, that is, a quantum mechanical superposition of localized and itinerant electronic configurations.” [Science Advances, 1, 6 (2015)]. However, a casual internet search of the electronic structure of Pu gives [Rn]7s²5f⁶ which is wholly insufficient to describe the complex nature of its multiconfigurational ground state as described by Janoschek, et al. A better description might be given by

[Rn]7s^{2+ γ _s,Pu} 5f^{6+ γ _f,Pu} 6d^{0+ γ _d,Pu} 7p^{0+ γ _p,Pu}

where γ_s,Pu, γ_f,Pu, γ_d,Pu, γ_p,Pu represent potentially non-integer deviations from the integer electron occupancies of their respective orbitals. By judiciously selecting bounding conditions for how the eight valence electrons can distribute themselves within the given orbitals, a limited number of occupancy configurations can be identified with an identifiable number of cumulative allowable electron permutations. For example, the bounding conditions

-2≤ γ_s,Pu ≤0,

-2≤ γ_f,Pu ≤2,

0≤ γ_d,Pu ≤2,

γ_{p ,Pu} =0, and

γ_s,Pu+γ_f,Pu+γ_d,Pu+γ_{p ,Pu}=0

result in nine possible occupancy configurations

7s²5f⁶6d⁰7p⁰, 7s²5f⁵6d¹7p⁰, 7s²5f⁴6d²7p⁰, 7s¹5f⁷6d⁰7p⁰, 7s¹5f⁶6d¹7p⁰, 7s¹5f⁵6d²7p⁰, 7s⁰5f⁸6d⁰7p⁰, 7s⁰5f⁷6d¹7p⁰ , 7s⁰5f⁶6d²7p⁰

with 487,630 cumulative possible electron permutations. With the specific number of permutations identified, principles of statistical mechanics can be applied to multiconfigurational ground states.

Similar generalized representations of ground state electronic structures will be described for each element in the periodic table. Classical Boltzmann’s entropies for the respective electronic structures will be calculated by considering the possible electron permutations as statistical mechanics microstates. These entropies will then be compared to known standard molar entropies.

LA-UR-23-24803