Uncovering the Origin of Divergence in the CsM(CrO₄)₂ (M = La, Pr, Nd, Sm, Eu; Am) Family through Examination of the Chemical Bonding in a Molecular Cluster and by Band Structure Analysis

Abstract

A series of f-block chromates, CsM(CrO) (M = La, Pr, Nd, Sm, Eu; Am), were prepared revealing notable differences between the Am derivatives and their lanthanide analogs. While all compounds form similar layered structures, the americium compound exhibits polymorphism and adopts both a structure isomorphous with the early lanthanides as well as one that possesses lower symmetry. Both polymorphs are dark red and possess band gaps that are smaller than the Ln compounds. In order to probe the origin of these differences, the electronic structure of α-CsSm(CrO), α-CsEu(CrO), and α-CsAm(CrO) were studied using both a molecular cluster approach featuring hybrid density functional theory and QTAIM analysis and by the periodic LDA+GA and LDA+DMFT methods. Notably, the covalent contributions to bonding by the f orbitals were found to be more than twice as large in the Am chromate than in the Sm and Eu compounds, and even larger in magnitude than the Am-5f spin-orbit splitting in this system. Our analysis indicates also that the Am-O covalency in α-CsAm(CrO) is driven by the degeneracy of the 5f and 2p orbitals, and not by orbital overlap.

Origins of the odd optical observables in plutonium and americium tungstates

“…On the other hand, we also demonstrate that the unusual color of plutonium tungstate does not stem directly from the 5f electrons, as in both systems the valence band has mostly O-2p character and the conduction band has mostly W-5d character. We also find that a similar pattern is realized in the 4f and 5f chromates α-CsM(CrO 4 ) 2 (M = Sm, Eu, Am) recently synthesized in ref. 14.…”

“…72 and employing the standard fully-localized limit form for the double-counting functional 73. Consistently with previous work,14 in our calculations we assume that the screened values of the Hund's coupling constant and the local Coulomb interaction strength are J = 0.7 eV and U = 6.0 eV, respectively. The LDA + DMFT calculations are performed at room temperature ( T = 290 K) utilizing the continuous time quantum Monte Carlo (CTQMC) impurity solver,72,74 while the LDA + GA calculations are performed at T = 0 K.…”

“…Furthermore, the array of research efforts directed toward understanding the general preference of soft-donor ligands for 5f over 4f elements has led to a description of a covalent bond with minimal orbital overlap, but little build-up of electron density between atoms in most cases 5–13. However, even this is currently undergoing revision in light of the high kinetic energy of electrons in 5f elements 14. We have added to this discourse on the unusual nature of f-block bonding with the characterization of trivalent lanthanide and actinide borates where the actinides structurally deviating from their lanthanide congeners and from each other 15.…”

“…There have been few previously described low-valent actinides complexed with d 0 metal-containing oxoanions despite their potential importance to the nuclear fuel cycle 31–37. We have recently reported several plutonium molybdates and a family of chromates that have unusual electronic characteristics and display structural deviation from the lanthanides 14,38,39…”

Synthesis and Utility of Neptunium(III) Hydrocarbyl Complex

“…As af inal comparison, the ionic radii of six-coordinate Np III and Ce III are both 1.01 , [31] hence we can compare the distances in 1 to the Ce III analogue, [28] which sits on apseudo threefold axis of symmetry.H owever,t he CeÀC(methine) bond distances are 2.617 3,which are D = 0.025(3) (NpÀ C7), 0.036(3) (NpÀC16), and 0.043(3) (NpÀC25) longer than the Np À C(methine) distances in 1.T he closest lanthanide with similar bond distances to 1 is the Nd analogue which has an ionic radius of 0.953 .The analogous Nd complex has NdÀC(methine) distances of 2.588(3) [28] ,w ith the average NpÀC(methine) distance in 1 being 2.582(4) .T his comparison demonstrates the enhanced actinide-ligand bonding that is well-established in 4f versus 5f complexes. [32] To demonstrate that 1 can be used as astarting material to obtain other trivalent neptunium complexes,t he reaction of 1 with three equivalents of HO 2 C(2,6-Mes 2 -C 6 H 3 ) [33] was attempted (Scheme 1). Ap ale-yellow solution was afforded.…”