Chemical Trends in Solid Alkali Pertechnetates

Abstract

Insight into the solid-state chemistry of pure technetium-99 (Tc) oxides is required in the development of a robust immobilization and disposal system for nuclear waste stemming from the radiopharmaceutical industry, from the production of nuclear weapons, and from spent nuclear fuel. However, because of its radiotoxicity and the subsequent requirement of special facilities and handling procedures for research, only a few studies have been completed, many of which are over 20 years old. In this study, we report the synthesis of pure alkali pertechnetates (sodium, potassium, rubidium, and cesium) and analysis of these compounds by Raman spectroscopy, X-ray absorption spectroscopy (XANES and EXAFS), solid-state nuclear magnetic resonance (static and magic angle spinning), and neutron diffraction. The structures and spectral signatures of these compounds will aid in refining the understanding of Tc incorporation into and release from nuclear waste glasses. NaTcO shows aspects of the relatively higher electronegativity of the Na atom, resulting in large distortions of the pertechnetate tetrahedron and deshielding of the Tc nucleus relative to the aqueous TcO. At the other extreme, the large Cs and Rb atoms interact only weakly with the pertechnetate, have closer to perfect tetrahedral symmetry at the Tc atom, and have very similar vibrational spectra, even though the crystal structure of CsTcO is orthorhombic while that of RbTcO is tetragonal. Further trends are observed in the cell volume and quadrupolar coupling constant.

Redetermination of the crystal structure of NaTcO4at 100 and 296 K based on single-crystal X-ray data

“…A similar phenomenon has previously been observed in the structure of anilinium pertechnetate (Maruk et al, 2010). The greatest distortion of the TcO 4 À anion from an ideal tetrahedral configuration reported by Weaver et al 2017is confirmed by our analysis of the O-Tc-O angles in the NaTcO 4 structure, but the difference is not as high as in the model from the neutron diffraction experiment (Weaver et al, 2017). The maximum deviation values are 3.12 at 100 K and 3.08 at 296 K for the sodium salt and are larger in comparison with the potassium and rubidium salts, because the sodium cation has the smallest ionic radius compared to K + and Rb + and hence has the highest polarizing ability.…”

“…The structure of anhydrous NaTcO 4 , determined here on the basis of X-ray diffraction data of a single crystal recorded both at room and low temperature, belongs to the CaWO 4 structural type (space group type I4 1 /a). The obtained bond lengths and angles are similar to those obtained from previous X-ray powder (Keller & Kanellakopulos, 1963;Schwochau, 1962) and neutron powder diffraction studies (Weaver et al, 2017) Lattice parameters determined here with the precision of 0.0002-0.0005 Å at 296 K (Table 1) are close to those of a = 5.342 (3) Å , c = 11.874 (6) Å given by Weaver et al (2017). The lattice parameters at 100 K are a = 5.2945 (2) Å , c = 11.7470 (5) Å (single crystal measurement).…”

“…Another aspect for obtaining more precise structure data on pertechnates is to clarify if pseudo-Jahn-Teller distortions of d 0 -tetraoxide anions really take place when compared with previous determinations (German et al, 1987a;Spitsyn et al, 1987;Tarasov et al, 1983Tarasov et al, , 1991. In this context we have reinvestigated the crystal structure of NaTcO 4 that is known from powder diffraction data only, namely by inspection of its X-ray powder diffraction pattern (Schwochau, 1962;Keller & Kanellakopulos, 1963) and Rietveld refinement of neutron powder diffraction data (Weaver et al, 2017).…”

“…Radiochim. Acta, 1, 107-108] and neutron powder diffraction data using the Rietveld method [Weaver et al (2017). Inorg.…”

Challenges and Solutions for Handling and Characterizing Alkali-Tc-Oxide Salts

“…Within the last few decades there has been a significant increase in research efforts to learn how best to store and dispose of Tc for these purposes [8][9][10][11][12][13][14][15][16]. Coupled with these efforts has been a push for advanced analyses of Tc bearing materials using state-of-the-art equipment [12,[17][18][19][20][21][22][23]. X-ray absorption fine structure (XAFS) has been utilized to investigate the near-field structures of Tc in both crystals and soft matter.…”