Synthesis and Characterization of 5- and 6- Coordinated Alkali Pertechnetates
Synthesis and Characterization of 5- and 6- Coordinated Alkali Pertechnetates
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5-和6-配位碱高锝酸盐的合成和表征
DOI：
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发表时间：
2017
期刊：
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通讯作者：
J. McCloy
中科院分区：
文献类型：
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作者：
Jamie L. Weaver;C. Soderquist;P. Gassman;E. Walter;W. Lukens;J. McCloy
氧化物玻璃中锝-99（^99Tc）的局部化学性质对于理解核废料玻璃中Tc的掺入和长期释放非常重要，无论是遗留国防废物还是燃料后处理废物。Tc优选在玻璃中形成Tc（VII）、Tc（IV）或Tc（0），这取决于熔体的还原水平。氧化物玻璃中的Tc（VII）通常被认为是被碱金属包围的孤立的高锝酸盐TcO_4^−阴离子，但当Tc浓度较高时，偶尔也会以碱金属高锝酸盐如KTcO_4和NaTcO_4的形式析出。在这些情况下，Tc（VII）通过氧4-配位。对在氧化条件下和制备核废料玻璃的温度下形成的碱金属-锝-氧化物化学的重新研究表明，已经报道了更高配位的碱金属Tc（VII）氧化物物种，包括那些具有TcO_5^−和TcO_6^−阴离子的物种。本文综述了碱金属Tc（VII）及其它碱金属Tc氧化物的化学性质，沿着了有关的合成条件。此外，我们还尝试制备了K、Na和Li的5-和6-配位高锝酸盐化合物，即TcO_5^-和TcO_6^-。结果发现，高配位物种对水非常敏感，并且容易分解成各自的高锝酸盐。很难获得纯的化合物，但经常发现高锝酸盐和其他相的混合物，如X射线吸收光谱（XAS）、中子衍射（ND）和拉曼光谱所证明的。低温电子顺磁共振（EPR）测量表明，在Na_3TcO_5和Na_5TcO_6化合物中可能存在Tc（IV）和Tc（VI）。据推测，较小的抗衡阳离子将产生更稳定的高次氯酸盐。为了证实合成方法，制备了LiReO_4和Li_5ReO_6，它们的拉曼光谱与文献中的相符。随后，合成了Tc型LiTcO_4和Li_5TcO_6，并通过ND、拉曼光谱、XANES和EXAFS对其进行了表征。Li_5TcO_6是一种边缘稳定的化合物，似乎具有与已知的Li_5ReO_6相同的结构。碱锝酸盐化合物的稳定性的实验工作的影响和可能的作用，Tc的挥发进行了讨论。
The local chemistry of technetium-99 (^99Tc) in oxide glasses is important for understanding the incorporation and long-term release of Tc from nuclear waste glasses, both those for legacy defense wastes and fuel reprocessing wastes. Tc preferably forms Tc(VII), Tc(IV), or Tc(0) in glass, depending on the level of reduction of the melt. Tc(VII) in oxide glasses is normally assumed to be isolated pertechnetate TcO_4^− anions surrounded by alkali, but can occasionally precipitate as alkali pertechnetate salts such as KTcO_4 and NaTcO_4 when Tc concentration is high. In these cases, Tc(VII) is 4-coordinated by oxygen. A reinvestigation of the chemistry of alkali-technetium-oxides formed under oxidizing conditions and at temperatures used to prepare nuclear waste glasses showed that higher coordinated alkali Tc(VII) oxide species had been reported, including those with the TcO_5^− and TcO_6^− anions. The chemistry of alkali Tc(VII) and other alkali-Tc-oxides is reviewed, along with relevant synthesis conditions. Additionally, we report attempts to make 5- and 6-coordinate pertechnetate compounds of K, Na, and Li, i.e. TcO_5^− and TcO_6^−. It was found that higher coordinated species are very sensitive to water, and easily decompose into their respective pertechnetates. It was difficult to obtain pure compounds, but mixtures of the pertechnetate and other phase(s) were frequently found, as evidenced by x-ray absorption spectroscopy (XAS), neutron diffraction (ND), and Raman spectroscopy. Low temperature electron paramagnetic resonance (EPR) measurements showed the possibility of Tc(IV) and Tc(VI) in Na_3TcO_5 and Na_5TcO_6 compounds. It was hypothesized that the smaller counter cation would result in more stable pertechnetates. To confirm the synthesis method, LiReO_4 and Li_5ReO_6 were prepared, and their Raman spectra match those in the literature. Subsequently, the Tc versions LiTcO_4 and Li_5TcO_6 were synthesized and characterized by ND, Raman spectroscopy, XANES, and EXAFS. The Li_5TcO_6 was a marginally stable compound that appears to have the same structure as that known for Li_5ReO_6. Implications of the experimental work on stability of alkali technetate compounds and possible role in the volatilization of Tc are discussed.