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

A series of f-block chromates, CsM(CrO ₄) ₂ (M = La, Pr, Nd, Sm, Eu; Am), were prepared revealing notable differences between the Am ^III 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 ^III compounds. In order to probe the origin of these differences, the electronic structure of α-CsSm(CrO ₄) ₂, α-CsEu(CrO ₄) ₂, and α-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 were found to be more than twice as large in the Am ^III chromate than in the Sm ^III and Eu ^III 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 ₄) ₂ is driven by the degeneracy of the 5f and 2p orbitals, and not by orbital overlap.