Shedding Light on the Enigmatic TcO(2) ⋅ xH(2)O Structure with Density Functional Theory and EXAFS Spectroscopy

The β‐emitting (99)Tc isotope is a high‐yield fission product in (235)U and (239)Pu nuclear reactors, raising special concern in nuclear waste management due to its long half‐life and the high mobility of pertechnetate (TcO(4) (−)). Under the conditions of deep nuclear waste repositories, Tc is retained through biotic and abiotic reduction of TcO(4) (−) to compounds like amorphous TcO(2) ⋅ xH(2)O precipitates. It is generally accepted that these precipitates have linear (Tc(μ‐O)(2)(H(2)O)(2))( n ) chains, with trans H(2)O. Although corresponding Tc−Tc and Tc−O distances have been obtained from extended X‐ray absorption fine structure (EXAFS) spectroscopy, this structure is largely based on analogy with other compounds. Here, we combine density‐functional theory with EXAFS measurements of fresh and aged samples to show that, instead, TcO(2) ⋅ xH(2)O forms zigzag chains that undergo a slow aging process whereby they combine to form longer chains and, later, a tridimensional structure that might lead to a new TcO(2) polymorph.