Insight into the Structural and Emissive Behavior of a Three-Dimensional Americium(III) Formate Coordination Polymer

We report the structural, vibrational, and optical properties of americium formate (Am(CHO₂)₃) crystals synthesized via the in situ hydrolysis of dimethylformamide (DMF). The coordination polymer features Am³⁺ ions linked by formate ligands into a three-dimensional network that is isomorphous to several lanthanide analogs, (e. g., Eu³⁺, Nd³⁺, Tb³⁺). Structure determination revealed a nine-coordinate Am³⁺ metal center that features a unique local C_3v symmetry. The metal–ligand bonding interactions were investigated by vibrational spectroscopy, natural localized molecular orbital calculations, and the quantum theory of atoms in molecules. The results paint a predominantly ionic bond picture and suggest the metal-oxygen bonds increase in strength from Nd−O<Eu−O<Am−O. The optical properties were probed using diffuse reflectance and photoluminescence spectroscopies. Notably, the rarely reported ⁵D_1′→⁷F_0′ emission band is observed and dominates the emission spectrum. This behavior is unusual and is attributed to the C_3v coordination environment of the metal center.