Picomolar Traces of Americium(III) Introduce Drastic Changes in the Structural Chemistry of Terbium(III): A Break in the “Gadolinium Break”

The crystallization of terbium 5,5′-azobis[1H-tetrazol-1-ide] (ZT) in the presence of trace amounts (ca. 50 Bq, ca. 1.6 pmol) of americium results in 1) the accumulation of the americium tracer in the crystalline solid and 2) a material that adopts a different crystal structure to that formed in the absence of americium. Americium-doped [Tb(Am)(H2O)7ZT]2 ZT⋅10 H2O is isostructural to light lanthanide (Ce–Gd) 5,5′-azobis[1H-tetrazol-1-ide] compounds, rather than to the heavy lanthanide (Tb–Lu) 5,5′-azobis[1H-tetrazol-1-ide] (e.g., [Tb(H2O)8]2ZT3⋅6 H2O) derivatives. Traces of Am seem to force the Tb compound into a structure normally preferred by the lighter lanthanides, despite a 108-fold Tb excess. The americium-doped material was studied by single-crystal X-ray diffraction, vibrational spectroscopy, radiochemical neutron activation analysis, and scanning electron microcopy. In addition, the inclusion properties of terbium 5,5′-azobis[1H-tetrazol-1-ide] towards americium were quantified, and a model for the crystallization process is proposed.