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.