Uncovering the Origin of Divergence in the CsM(CrO$_4$)$_2$ (M = La, Pr, Nd, Sm, Eu; Am) Family through Examination of the Chemical Bonding in a Molecular Cluster and by Band Structure Analysis

TL;DR

This study investigates the electronic structure and bonding differences in a family of f-block chromates, revealing that americium exhibits unique covalent bonding characteristics driven by orbital degeneracy, which explains its structural divergence.

Contribution

The paper combines molecular cluster and band structure analysis to uncover the origin of divergence in americium's bonding compared to lanthanides in CsM(CrO$_4$)$_2$ compounds.

Findings

AmIII compounds show greater covalent f-orbital contributions.

Am-O covalency is driven by orbital degeneracy, not overlap.

Am compounds exhibit polymorphism with lower symmetry structures.

Abstract

A series of f-block chromates, CsM(CrO$_4$)$_2$ (M = La, Pr, Nd, Sm, Eu; Am), were prepared revealing notable differences between the AmIII 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 LnIII compounds. In order to probe the origin of these differences, the electronic structure of $\alpha$-CsSm(CrO$_4$)$_2$, $\alpha$-CsEu(CrO$_4$)$_2$ and $\alpha$-CsAm(CrO$_4$)$_2$ 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 was found to be more than twice as large in the AmIII chromate than in the SmIII and EuIII 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 $\alpha$-CsAm(CrO$_4$)$_2$ is driven by the degeneracy of the 5f and 2p orbitals, and not by orbital overlap.