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Shedding Light on the Enigmatic TcO2·xH2O Structure with Density Functional Theory and EXAFS Spectroscopy

Von Wiley-VCH zur Verfügung gestellt

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.

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