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

A series of f-block chromates, CsM(CrO₄)₂ (M = La, Pr, Nd, Sm; Am), were prepared revealing notable differences between the Am^III derivative and its 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-CsAm(CrO₄)₂ was determined computationally using both a molecular cluster approach featuring hybrid density functional theory and QTAIM analysis and by the periodic LDA+GA method. Taken together, these complementary methods demonstrate that while there is Am-O covalency in α-CsAm(CrO₄)₂, it is driven by the degeneracy of the 5f and 2p orbitals and not by orbital overlap.