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

Abstract 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 −). Under the conditions of deep nuclear waste repositories, Tc is retained through 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.


Introduction
Technetium is the lightest element without a stable isotope and is almost completely artificial; only trace amounts are found in nature, formed from spontaneous fission in uranium minerals. 99m Tc (metastable) is of great importance in nuclear medicine as a γ-ray emitter, [1] while the β-emitting 99 Tc (ground state) has no practical use and raises special concern in nuclear waste management due to its long half-life (ca. 2.1 × 10 5 years) and relatively high formation yield (� 6 %) in 235 U and 239 Pu nuclear reactors.
In water and in absence of other complexing agents, Tc prevails in oxidation states VII and IV. [2] Under nonreducing conditions, Tc VII forms the soluble TcO 4 À anion, which is highly mobile in the environment due to its weak interaction with adsorbent surfaces. Under the reducing conditions of natural, anoxic sediments as well as in nuclear waste repositories, however, numerous studies suggest that Tc precipitates as TcO 2 · xH 2 O, either as colloids or associated with Fe II -bearing mineral phases, which also act as catalysts for the Tc reduction. [2,3] Understanding the Tc redox processes in these conditions is essential for evaluating the safety of nuclear disposal sites; however, to date, even the structure of the TcO 2 · xH 2 O precipitates remains enigmatic and needs to be resolved.
Lukens et al. [4] proposed that TcO 2 · xH 2 O is formed by linear chains of equally spaced edge-sharing TcO 6 octahedra with terminal H 2 O ligands in the trans positions. In the same year, Vichot et al. [5] arrived at similar conclusions, but argued that the structure would alternate shorter and longer TcÀ Tc distances along the chain, based on data available for TcO 2 crystals. [6] However, these chain structures were based exclusively on EXAFS spectroscopy, which only provides radial interatomic distances and estimates of coordination numbers, and analogies to known crystal structures of other systems, like CoCl 2 · 2 H 2 O and RuO 2 · 2 H 2 O. In addition, in a recent work, Yalçintaş et al. [3a] showed that both the chain model with equally spaced Tc atoms (similar to the model by Lukens et al. [4] ) and the one with alternating TcÀ Tc distances (similar to crystalline TcO 2 ) could describe equally well their own EXAFS measurements of a fresh TcO 2 · xH 2 O precipitate, thereby demonstrating that a conclusive characterization of TcO 2 · xH 2 O based on EXAFS alone is impossible, even though EXAFS is, to the best of our knowledge, the only experimental technique applicable in this case due to the noncrystalline nature of TcO 2 · xH 2 O.
In this work, in order to unambiguously assign the TcO 2 · xH 2 O structure, we use density-functional theory (DFT) to obtain energetically viable structural models. The results from the DFT modeling are then used in the interpretation of EXAFS spectra of fresh and aged samples, showing that TcO 2 · xH 2 O precipitates are polymeric structures consisting of zigzag chains of TcO 6 octahedra with H 2 O groups in a cis configuration. In addition, we show that these polymeric chains eventually combine laterally, forming a tridimensional structure that might evolve to a new, low-energy TcO 2 crystal phase.

Results and Discussion
For the DFT modeling, our strategy was to derive infinite TcO 2 · 2H 2 O chains from TcO 2 crystal structures constructed from the crystallographic coordinates of the three known ReO 2 polymorphs, namely α-ReO 2 (P2 1 /c), [7] β-ReO 2 (Pbcn), [8] and γ-ReO 2 (P4 2 /mnm). [9] Although only one TcO 2 phase has been characterized experimentally (almost identical to α-ReO 2 ), [10] the use of ReO 2 as initial reference is justifiable because Tc and Re have very similar crystal chemistry [11] and, thus, TcO 2 counterparts of all ReO 2 polymorphs should be expected to form. The corresponding α-, β-, and γ-TcO 2 structures were obtained by replacing the Re atoms with Tc and performing full geometry optimizations, as described in the Experimental Section. Indeed, the optimized TcO 2 structures turned out to be comparable to the ReO 2 phases (Table S1 in the Supporting Information).
The calculated TcO 2 crystal structures are represented in Figure 1a-c. All three polymorphs consist of edge-sharing TcO 6 octahedra forming unidimensional chains that are laterally interconnected via corner-sharing O atoms. The chain configuration is characteristic of each TcO 2 polymorph: in α-TcO 2 , the chains follow a straight path, with consecutive TcÀ Tc pairs alternating between shorter and longer distances; the chains in γ-TcO 2 are also straight, but the Tc atoms are separated by identical distances; in β-TcO 2 , consecutive Tc atoms are also separated by identical distances, but the chains follow a distinctive zigzag path. From each TcO 2 crystal structure, we extracted a single chain and converted the terminal O atoms into H 2 O groups to balance the total electric charge, resulting in the TcO 2 · 2H 2 O chains shown in Figure S2a-c. We refer to these chains as α-, β-, and γ-TcO 2 · 2H 2 O, to indicate that the chains were derived from the α-, β-, and γ-TcO 2 crystal structures, respectively. We also considered hydroxide chains ( Figure S2df) where all O atoms were converted into OH groups; however, these chains turned out to be energetically disfavored, as shown in Table S3.
All structures were fully optimized (lattice vectors and atomic coordinates) using AMS/BAND [12] with the PBE [13] density functional, scalar relativistic effects, [14] and numerical atomic orbitals (NAOs) augmented with a triple-zeta polarized (TZP) set of Slater-type basis functions. For the chains, D3 dispersion corrections [15] were also included. Further computational details are provided in the Experimental Section. Figure 1d-f shows the optimized structures and relative energies calculated for the TcO 2 · 2H 2 O chains. The β-TcO 2 · 2H 2 O structure (zigzag) is clearly the energetically most favored one; the energy difference to the next structure, α-TcO 2 · 2H 2 O, is already significantly high (29.4 kJ mol À 1 per formula unit). Note that, whereas βand γ-TcO 2 · 2H 2 O retained their general structure, α-TcO 2 · 2H 2 O rearranged into an oxyhydroxide chain, which can be represented as Tc(μ-O)(μ-OH)(OH)(H 2 O); nonetheless, for convenience, we will continue referring to this structure as α-TcO 2 · 2H 2 O.
The TcÀ O bond lengths as well as TcÀ Tc distances calculated for the optimized TcO 2 · 2H 2 O chains are shown in Table 1 in comparison with the corresponding parameters obtained for the TcO 2 crystal structures and from EXAFS spectra. The EXAFS parameters were determined by shell fitting (for details see the Supporting Information) from experimental spectra of a fresh and an aged sample. The spectrum of the fresh sample is the one published by Yalçintaş et al., [3a] measured within a month after sample preparation; the shell fitting was redone in this work. The spectrum of the aged sample, on the other hand, is presented in this work for the first time; in this case, the TcO 2 · xH 2 O precipitate was stored in air, at room temperature, for about four years prior to the EXAFS measurement. The experimental EXAFS spectra of both samples were obtained at the Rossendorf Beamline at ESRF, [16] under identical conditions, most importantly, by using a closed-cycle He cryostat to maintain a temperature of 15 K and anoxic conditions during the measurement. Aged [b] α- The fresh sample was measured within a month of preparation; [3a] the aged sample was stored for four years under room conditions prior to measurement. The geometry of α-TcO 2 · H 2 O converged to a mixed protonation state, as shown in Figure 1d). For the TcO 2 crystals, peak A is associated with the first TcÀ O coordination shell, B with the nearest intrachain TcÀ Tc neighbors, C with the nearest interchain TcÀ Tc neighbors, and D with the second intrachain TcÀ Tc neighbors.
with the distances in the calculated β-TcO 2 · 2H 2 O chain, as shown in Table 1. The Tc-Tc distances (which characterize the chain structure) differ by � 0.02 Å, whereas the TcÀ (μ-O) bonds differ by � 0.03 Å; the differences are slightly larger for the TcÀ OH 2 bonds (0.16 Å for the fresh sample and 0.09 Å for the aged sample), but still in good agreement in both cases. The discrepancy between TcÀ OH 2 bonds in the fresh and aged samples is reflected in the sharper splitting of the Fourier transform magnitude (FTM) peaks corresponding to the nearest TcÀ O and TcÀ Tc distances (peaks A and B in Figure 2). The most significant difference between the fresh and aged samples, however, is the absence of signals related to the longer Tc-Tc distances in the former, either because of a high static disorder of the chains or because the chains are too short to show such backscattering pairs consistently (note that thermal disorder can be excluded since both the fresh and the aged samples were measured at 15 K). The most conclusive proof of the formation of zigzag chains in the TcO 2 · xH 2 O precipitates comes from the EXAFS analysis of the aged sample. As shown in Figure 3, the χ(k) spectrum of the aged precipitate contains high-frequency signals between 5 and 7 Å À 1 that are absent in the spectrum of the fresh sample. These signals correspond to second and third intrachain TcÀ Tc distances of 4.63 and 7.03 Å shown in Table 1, which are only possible in the β-TcO 2 · 2H 2 O chain (calculated as 4.71 and 7.16 Å). Interestingly, TcÀ Tc distances of 3.80, 5.06, and 6.04 Å, corresponding to the interchain distances observed in the β-TcO 2 crystal, could also be fitted, indicating that the precipitate develops a tridimensional organization with aging ( Figure 4). Nonetheless, in the sample analyzed here, this tridimensional structure is likely in its initial stage, otherwise the EXAFS spectrum would bear a stronger resemblance to the spectrum of the β-TcO 2 crystal in Figure 2, especially with respect to peak C.
The absence of interpretable EXAFS signals related to longer TcÀ Tc distances in the fresh sample and the presence of the signals associated with long, interconnected parallel chains in the aged sample is indicative of the aging process in TcO 2 · xH 2 O precipitates. In our interpretation, the short β-TcO 2 · 2H 2 O chains combine with each other to form longer zigzag chains via condensation reactions, thereby releasing H 2 O. As the chains become longer, condensation reactions leading to cross-linked chains start to take place. The shortening of the second Tc-O path in the experimental EXAFS (denoted TcÀ OH 2 in Table 1) from 2.39 Å in the fresh to 2.14 Å in the aged sample is an additional indication of this condensation process. This scenario is consistent with the thermodynamic evidence compiled by Grambow [18] that TcO 2 · xH 2 O would release H 2 O over time and that the process takes place very slowly; as discussed above, after four years the aged sample still shows early signs of crosslinking between chains.

Conclusion
In conclusion, by combining DFT calculations with EXAFS measurements, we have demonstrated that, contrary to the currently anticipated linear structure, TcO 2 · xH 2 O precipitates are polymeric structures formed of zigzag chains of edge-  sharing TcO 6 octahedra with terminal H 2 O ligands at the cis positions (Figure 1e). Differences in the EXAFS of a fresh sample and a sample aged for four years indicate that the length of the polymeric chains increases slowly over time and might lead to crystallization of a yet uncharacterized TcO 2 phase analogous to β-ReO 2 (Figure 1b), which our calculations (Table S2) show to be energetically equivalent to the known P2 1 /c [10] (α-TcO 2 ) phase.

TcO 2 · 2H 2 O infinite chains:
The initial TcO 2 · 2H 2 O chains were constructed by extracting a single chain of edge-sharing TcO 6 octahedra from each TcO 2 crystal phase and saturating the O atoms with H according to two approaches, as shown in Figure S2: (aÀ c) terminal O atoms were converted into H 2 O ligands and bridging O atoms were left unprotonated, resulting in (Tc(μ-O) 2 (OH 2 )) n chains; (dÀ f) terminal and bridging O atoms were converted into OH groups, resulting in (Tc(OH) 2 (μ-OH) 2 ) n chains. Finally, the atomic coordinates and lattice parameter of each structure were fully optimized using the method described above for the crystal structures, supplemented with Grimme's D3 corrections [15] to improve the description of dispersion interactions with OH and H 2 O groups. Like for the crystal structures, these calculations were conducted with the AMS/BAND [12] program, using regular k-space grids with quality set to "Good". Note that these systems are periodic only along the chain direction.
Single-point calculations: Energies and electronic properties were calculated for the optimized structures with the same methods used in the preceding geometry optimizations, except for a denser k-space grid (quality set to "VeryGood"). For comparison, singlepoint calculations were also carried out within the FHI-aims program, [19] using the PBE [13] and HSE06 [20] density functionals with a "tier 1" set of atom-centered NAO basis functions; relativistic effects were described with the "atomic ZORA" approach; [19] for the TcO 2 · 2H 2 O infinite chains, the Tkatchenko-Scheffler (TS) method [21] was used for the dispersion interactions.

TcO 2 · xH 2 O samples and EXAFS measurements
Fresh sample: The preparation and EXAFS spectrum of the fresh sample were reported by Yalçintaş et al. [3a] The TcO 2 · xH 2 O precipitate was prepared by acidifying a pertechnetate solution with concentrated HCl, then adding Zn to generate nascent hydrogen. After the reaction was completed, NaOH (20 M) was added to obtain a black precipitate, which aged for one week. The sample was stored under Ar at liquid nitrogen temperature. The EXAFS was measured within 30 days of preparation at the Rossendorf Beamline (BM20 at ESRF, Grenoble, France) in fluorescence mode at the Tc Kedge (21044 eV). The sample was kept at 15 K during the measurement. Further details can be found in the original publication. [3a] Aged sample: The sample was prepared by hydrolysis of K 2 [TcCl 6 ], which was previously synthesized by an established procedure. [22] Solid K 2 [TcCl 6 ] (390 mg, 1 mmol) was dissolved in 0.5 mL H 2 O, which resulted in the precipitation of a dark brown solid. An aqueous solution of KOH (2 mL, 0.1 M) was added, and the suspension was stirred for 5 h at room temperature. The formed black brown solid was filtered off and washed with water (5 × 1 mL). The absence of Cl À in the final washing solution was checked by the addition of Ag(NO 3 ). The thus formed TcO 2 · xH 2 O was dried in air at room temperature. Yield: practically quantitative. The sample was prepared and stored at room temperature in air for four years prior to the EXAFS measurement, which was conducted identically to the fresh sample.

EXAFS analyses
EXAFS shell fitting: Tc K-edge EXAFS shell fittings were conducted in R-space with WinXAS [23] using the β-TcO 2 · 2H 2 O chain structure optimized with DFT ( Figure 2e) for the oxygen coordination to Tc and for intrachain TcÀ Tc distances. Interchain TcÀ Tc distances from the DFT-derived β-TcO 2 crystal structure were also included. The spectrum of the fresh precipitate was well fit by four oxygen atoms at 2.01 Å and two oxygen atoms at 2.39 Å, and 8 additional fourlegged multiple scattering paths arising from the quasi squareplanar configuration of the four nearest oxygen atoms (Table S4). Furthermore, we obtained two Tc atoms at 2.55 Å. These radial distances are in excellent agreement with the local structure of β-TcO 2 · 2H 2 O, except for the two longer TcÀ O distances, which represent the two water molecules in the coordination sphere and are about 0.2 Å longer than predicted by DFT. Note that a similarly long distance has been determined by Lukens et al., [4] also by EXAFS shell fitting. The spectrum of the fresh precipitate does not reveal any backscattering peaks at longer TcÀ Tc distances that could be fitted. For the aged sample, in addition to the local structure obtained for the fresh sample, TcÀ Tc distances corresponding to the 2nd and 3rd intrachain shells and for the three interchain shells (Figure 4) could also be fitted (Table S4 and Figure 3), in good agreement with the parameters in the β-TcO 2 · 2H 2 O chain and β-TcO 2 crystal. The amplitude reduction factor S 0 2 was fixed to 0.8 for all fits.
EXAFS simulation: EXAFS spectra were simulated for the DFToptimized structures with the program FEFF9.6.4 [17] using the selfconsistent field mode with a global Debye-Waller factor of 0.003 Å, amplitude reduction factor of 0.9, and ΔE 0 = 0.