Shedding light on the enigmatic TcO2·xH2O structure with density functional theory and EXAFS spectroscopy

08 July 2022, Version 1
This content is a preprint and has not undergone peer review at the time of posting.


The β-emitting 99Tc isotope is a high-yield fission product in 235U and 239Pu nuclear reactors, raising special concern in nuclear waste management due to its long half-life and the high mobility of pertechnetate (TcO4−). In the conditions of deep nuclear waste repositories, retention of Tc is achieved via biotic and abiotic reduction of TcO4− to compounds like amorphous TcO2·xH2O precipitates. It is generally accepted that these precipitates have linear (Tc(μ-O)2(H2O)2)n chains, with trans H2O. 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, TcO2·xH2O 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 TcO2 polymorph.


Chain structures
Density functional calculations
EXAFS spectroscopy
Nuclear waste management

Supplementary materials

Supplementary Material -- Shedding light on the enigmatic TcO2·xH2O structure with density functional theory and EXAFS spectroscopy
Method details and validation.


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Comment number 1, Andreas Scheinost: Sep 11, 2023, 16:28

cif files of Re oxides, Tc oxides, and corresponding chain structures are available at