Katharina Muller

Harald Foerstendorf

Robin Steudtner

Satoru Tsushima

Michael U Kumke

Greǵory Lefevre

Jorg Rothe

Harris Mason

Zoltań Szabo

Ping Yang

Christian K R Adam

Reḿi Andre

Brennenstuhl

Ion Chiorescu

Herman M Cho

Gaelle Creff

Fredeŕic Coppin

Kathy Dardenne

Christophe Den Auwer

†,∥∥ Bjorn Drobot

Sascha Eidner

⊥⊥ Nancy J Hess

Peter Kaden

Alena Kremleva

Jerome Kretzschmar

Sven Kruger

James A Platts

⊥, ¶ ¶ Petra J Panak

Robert Polly

∇∇ Brian A Powell

Thomas Rabung

Roland Redon

⧫⧫ E Reiller

Notker Rosch

† † Andrérossberg

Andreas C Scheinost

Bernd Schimmelpfennig

Georg Schreckenbach

Andrej Skerencak-Frech

∥∥∥ Vladimir Sladkov

⊥⊥⊥ Pier Lorenzo Solari

⊥⊥ Zheming Wang

### Nancy M Washton

Xiaobin Zhang

Interdisciplinary Round-Robin Test on Molecular Spectroscopy of the U(VI) Acetate System

ABSTRACT: A comprehensive molecular analysis of a simple aqueous complexing systemU(VI) acetateselected to be independently investigated by various spectroscopic (vibrational, luminescence, X-ray absorption, and nuclear magnetic resonance spectroscopy) and quantum chemical methods was achieved by an international round-robin test (RRT). Twenty laboratories from six different countries with a focus on actinide or geochemical research participated and contributed to this scientific endeavor. The outcomes of this RRT were considered on two levels of complexity: first, within each technical discipline, conformities as well as discrepancies of the results and their sources were evaluated. The raw data from the different experimental approaches were found to be generally consistent. In particular, for complex setups such as accelerator-based X-ray absorption spectroscopy, the agreement between the raw data was high. By contrast, luminescence spectroscopic data turned out to be strongly related to the chosen acquisition parameters. Second, the potentials and limitations of coupling various spectroscopic and theoretical approaches for the comprehensive study of actinide molecular complexes were assessed. Previous spectroscopic data from the literature were revised and the benchmark data on the U(VI) acetate system provided an unambiguous molecular interpretation based on the correlation of spectroscopic and theoretical results. The multimethodologic approach and the conclusions drawn address not only important aspects of actinide spectroscopy but particularly general aspects of modern molecular analytical chemistry.

INTRODUCTION

Exploring the migration pathways of contaminants in the ecosphere is a major field in the environmental sciences. In the past few decades, particular attention was paid to the migration behavior of radioactive heavy metals because of their chemoand radiotoxicity in humans. [START_REF] Choppin | Actinide speciation in the environment[END_REF] The dispersion of these elements in soils and rocks is mainly governed by their chemical speciation in the pore water and accordingly by the chemical reactions occurring at mineral-liquid interfaces. [START_REF] Maher | Environmental speciation of actinides[END_REF][START_REF] Geckeis | Mineral-water interface reactions of actinides[END_REF] Among the radionuclides, the high abundance of uranium (U) at sites considered for remediation activities and its relevance to the long-term safety assessment of nuclear waste repositories have led to numerous studies on the hydrolysis reactions of the hexavalent, highly soluble uranyl cation UO 2 2+ and its complexing reactions with ubiquitous inorganic and organic ligands. [START_REF] Burns | Nuclear fuel in a reactor accident[END_REF][START_REF] Berto | Advances in the investigation of dioxouranium(VI) complexes of interest for natural fluids[END_REF][START_REF] Guillaumont | Update on the Chemical Thermodynamics of U, Np[END_REF][START_REF] Hummel | Chemical Thermodynamics of Compounds and Complexes of U, Np[END_REF] Additionally, waste forms from other parts of the nuclear fuel cycle are of concern such as remedies of the uranium mining, milling, and enrichment industries, or uranium from other industrial processes, for example geothermal energy generation or phosphate fertilizer production. [START_REF] Campbell | Biogeochemical aspects of uranium mineralization, mining, milling, and remediation[END_REF] Previously, the thermodynamic data of such complex species were primarily derived from macroscopic approaches, such as potentiometric and extraction experiments. [START_REF] Guillaumont | Update on the Chemical Thermodynamics of U, Np[END_REF] However, the assignment of species to distinct molecular complexes with empirical formulas was mainly accomplished without structural information. In fact, the existence of most of the postulated species and their contributions to the multifaceted aqueous chemistry of the UO 2 2+ ion still await unequivocal evidence from spectroscopic or theoretical approaches. In recent years, progress in the advancement not only of spectroscopic techniques but also of quantum chemical (QC) approaches has provided an increasing amount of spectral data related to the aqueous speciation of U(VI). However, the state of knowledge still appears partially contradictory and unclear. [START_REF] Drobot | Speciation studies of metals in trace concentrations: the mononuclear uranyl(VI) hydroxo complexes[END_REF][START_REF] Muller | Aqueous uranium(VI) hydrolysis species characterized by attenuated total reflection fouriertransform infrared spectroscopy[END_REF][START_REF] Moll | Uranium(VI) chemistry in strong alkaline solution: speciation and oxygen exchange mechanism[END_REF][START_REF] Drobot | Combining luminescence spectroscopy, parallel factor analysis and quantum chemistry to reveal metal speciation -a case study of uranyl(VI) hydrolysis[END_REF] The idea of a multimethodological round-robin test (RRT) on the aqueous speciation of an actinide system was born in the aftermath of the closing round table discussion at the first Workshop on Advanced Techniques in Actinide Spectroscopy (ATAS) held in November 2012 at Dresden, Germany. During this discussion, scientists working in experimental or theoretical disciplines of actinide research shared their perspectives on the contributions from the other discipline and their expectations associated therewith. It became obvious that the current knowledge is inconsistent because of the multitudinous data provided by various spectroscopic and theoretical approaches, the meaning and interpretability of which are often hard to convey to the highly specialized scientists in different disciplines. Hence, this interdisciplinary test on a relatively simple actinide complex system was intended to elucidate the collection of knowledge from data acquisition to the resulting interpretational approaches. Consistencies and discrepancies in the results and in the way of best practice obtained by each single spectroscopic or theoretical technique applied in various laboratories and their sources should be disclosed to elaborate the potentials and limitations of each single technique. Furthermore, the benefit of multimethodical approaches for the molecular study of a selected complex system was to be assessed (cf. Figure S1).

The selection of the chemical system to be investigated and of the analytical techniques to be applied by the participants was mainly based on experience within the actinide geochemistry community as well as the feasibility of performing experiments encompassing the broad array of techniques available in leading laboratories in the field of geochemical sciences. After extensive discussions between potential participants, the aqueous U(VI) acetate system appeared to be the most implementable system in terms of experimental effort, radiation safety, sample preparation, and applicability to sophisticated spectroscopic techniques, namely, vibrational, luminescence, X-ray absorption, and nuclear magnetic resonance (NMR) spectroscopy. For several reasons (e.g., organizational aspects, few number of participants, etc.), techniques, such as UV-vis absorption spectroscopy or mass spectrometry that may also provide high information value on species distribution, were not considered.

The U(VI) acetate system has been examined since the 1950s. Mainly thermodynamic data on the complexing species and recently structural information by the application of spectroscopy and quantum chemistry have been derived. Up to now, the presence of four species for this complexing system is discussed in the acidic pH range (pH ≤ 4): UO 2 (H 2 O) 5 2+ , UO 2 CH 3 COO + , UO 2 (CH 3 COO) 2 , and UO 2 (CH 3 COO) 3 -, denoted below as the (1,0), (1,1), (1,2), and (1,3) species, respectively (cf. Figure S2). [START_REF] Lucks | Aqueous uranium(VI) complexes with acetic and succinic acid: speciation and structure revisited[END_REF] A more recent study takes fewer species into account. [START_REF] Sladkov | Photochemical characterization of uranyl interaction with acetic acid[END_REF] Because of the formation of insoluble hydrolyzed U(VI) solid phases, for example, schoepite, the pH range considered in this RRT was restricted to a maximum value of 3.5. The details of the RRT organization are provided in the following section and in the Supporting Information (Sections S1-S4).

More than 40 scientists from 20 institutions joined the RRT. According to the technique applied, the results were initially collected by nominated scientists acting as cluster speakers for a single technique applied in several laboratories. During a special session of the second ATAS workshop held in November 2014, preliminary summary reports were presented and discussed extensively. This work represents the outcome of this RRT. First, a survey of the experimental and theoretical data obtained is given for each technique. Second, the results are combined into a discussion focusing on the consequences for the knowledge on the U(VI) acetate speciation. Finally, general conclusions and perspectives of the RRT are provided. We believe that the methodology as well as the outcome of this RRT appeals to a broad community of inorganic chemists, actinide chemists, analytical chemists, geochemists, and environmental engineers.

ORGANIZATION OF THE RRT

The RRT was initiated in late 2013 at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR). After the approval of the participating institutions, five clusters were formed according to the spectroscopic and theoretical techniques, namely, vibrational spectroscopy (VIB, five institutions), laser-induced luminescence spectroscopy (LUM, nine institutions), NMR spectroscopy (five institutions), X-ray absorption spectroscopy (XAS, three laboratories located at three different European synchrotron sources), and QC calculations (QCC, six institutions) (cf. Figure S1). An overview of the clusters and the respective participating institutions is given in Table S1. For each cluster, a representative speaker was nominated.

For each analytical method, participants received four samples, denoted U1-U4, and respective reference samples, denoted B1-B4 (see Table S2). Those samples were uniformly prepared at HZDR. The participants received detailed information on sample preparation as it is given in the Supporting Information (Section S3). Apart from the requirement of measuring the samples as received, no further restrictions or guidelines were given to the participants to avoid significant impact on the specific approaches of the participants. The performance of the measurements and analysis of the raw data (e.g., data transformations, background subtraction, smoothing, deconvolution, etc.), as well as data interpretation, were entirely at the responsibility of each single operator or operating team. The Supporting Information (Section S4) provides detailed information on each cluster, describes experimental procedures, and lists all relevant setting parameters of the participants for each analytical method and quantum chemistry.

The period for active participation was from January to August 2014. Each participant submitted the raw data obtained, a detailed description of settings, the data analysis procedures applied, and the interpretational approaches of the derived molecular structures to the respective cluster speaker who in turn provided a summary report of the cluster to the organizers of the RRT.

RESULTS AND DISCUSSION

3.1. Results from the Clusters. All clusters delivered substantial data sets for intercomparison to determine consistencies and discrepancies within the applied analytical methods. For the sake of brevity, only a summary of the most important aspects can be given here. But, a description and interpretation of the whole data and its discussion with up-todate literature as well as of different interpretative approaches of the participants are given in detail in the Supporting Information (Section S5). Furthermore, the original data from each participant are given in Section S6.

3.1.1. Vibrational Spectroscopy. All vibrational spectra of the samples U1-U4 obtained by IR and Raman spectroscopy are shown in Figure 1. The focus is set on the spectral region of the asymmetric (ν 3 (UO 2 )) and symmetric (ν 1 (UO 2 )) stretching modes for clarity (Figure 1A,B). All other features observed in the spectra of the different samples mainly reflect the modes of the acetate molecule, which do not show a change in frequency but in intensity (Figures S9 andS10). The spectra provided by the single contributors are given in different colors. A detailed description of the spectroscopic features and their interpretation is given in the Supporting Information (Section S5.1).

The vibrational spectroscopic data of this RRT provided by five independent participants are in excellent agreement for all samples (cf. Table S8). For IR spectroscopy, the interpretation of IR data by all participants is consistent, and four different species could be spectrally identified. Generally, all species found in the IR spectra should be intrinsically observed in the Raman spectra as well. However, Raman spectra of the U(VI) acetate complexes are more difficult to interpret because of the strongly interfering bands of the symmetric stretching mode ν 1 (UO 2 ) of the different species and a vibrational mode of the acetate ligand. Consequently, the results of this cluster demonstrate that IR and Raman spectroscopy are strongly complementary techniques for the aqueous speciation of actinyl complexes.

3.1.2. Luminescence Spectroscopy. A survey of the timeresolved emission spectra revealed significantly different sets of acquisition parameters, resulting in a large time gate that ranged from a few nanoseconds to 100 ms (Figure 2A). Beside the gate width, parameters such as the initial time delay and the number and size of the gate steps strongly varied between the data sets provided by the participants (cf. Table S5 in the Supporting Information). However, general analysis of the time-resolved spectra from each participant suggested multi-exponential decay behavior, indicating the presence of at least two distinct species showing characteristic peak positions and luminescence lifetimes regardless of the variation in the spectral parameter applied (cf. Figures S11-S19 in the Supporting Information).

For a detailed analysis of the spectral data, the reader is referred to the Supporting Information. In summary, the strongly varying acquisition parameters applied by the participants of the LUM cluster required additional global data analysis prior to an adequate data evaluation (Figure 2B,C).

After an internal referencing of selected spectra, the relative peak positions show only marginal deviations, demonstrating the applicability of luminescence spectroscopy to the investigation of U(VI) speciation and the identification of single species. However, among the results of all participants, the obtained luminescence lifetimes displayed a broad distribution range. These discrepancies likely arose from the use of significantly differing experimental setups and algorithms for the spectral analysis. These aspects were already addressed during the first interlaboratory RRT on aqueous solutions of U(VI) studied by LUM. [START_REF] Billard | Aqueous solutions of uranium(VI) as studied by time-resolved emission spectroscopy: A round-robin test[END_REF] 3.1.3. X-ray Absorption Spectroscopy. The discussion of the XAS results will be focused on samples U1 (pH 1) and U4 (pH 3.5), which were analyzed by all groups (cf. Figure S20). As demonstrated in a recent comprehensive study, samples U2 (pH 2) and U3 (pH 2.5) are known to contain species mixtures that are impossible to be unequivocally resolved by XAFS analysis based on four data points, that is, U(VI) acetate under four different pH conditions. [START_REF] Lucks | Aqueous uranium(VI) complexes with acetic and succinic acid: speciation and structure revisited[END_REF] A detailed discussion of the curvefitting results and their interpretation is given in the Supporting Information (Section S5.3). To sum up most important findings (Table 1), we can conclude that

• The data sets for samples U1 and U4 obtained at different beamlines are consistent. • U1: the presence of the (1,0) species was unambiguously confirmed by all three teams, and the structure reported in the literature was reproduced better than within typical extended X-ray absorption fine structure (EXAFS) error margins. • U2 and U3: complex mixtures of all the species considered cannot be differentiated using EXAFS through measurements of only four data points, but a clear spectral trend is discernible from the EXAFS measurements of participants XAS-2 and -3. • U4: the dominant presence of the (1,3) species was confirmed by all the cluster's participants. However, uncertainties in CN determination might cause different interpretations of the coordination schemes for the same data set.

The final point underpins the general importance of a priori speciation information that might be necessary for obtaining a conclusive picture from EXAFS curve fit analysis. Conversely, any EXAFS shell fitting analysis will be prone to be biased by such a priori knowledge.

3.1.4. NMR Spectroscopy. The NMR data are remarkably consistent and show little variation among the five independent groups. A detailed discussion of both 1 H and [START_REF] Lucks | Aqueous uranium(VI) complexes with acetic and succinic acid: speciation and structure revisited[END_REF] C spectra is given in the Supporting Information (Section S5.4). The use of an internal standard as well as the reliability of modern NMR spectrometers allows excellent and reproducible results. Based on the control samples, the impact of the pH on the acetate spectra was found to be negligible. Hence, the speciation of the acetate molecule does not significantly change in this pH range. Conversely, systematic changes are observed in the spectra of the U(VI) acetate samples with increasing pH. This effect is most dramatic in the [START_REF] Lucks | Aqueous uranium(VI) complexes with acetic and succinic acid: speciation and structure revisited[END_REF] C spectra (Figure 3), where the signal representing the carboxyl group broadens considerably with pH. This line broadening is attributed to the exchange between free and bound acetate groups, which is in the range of the relaxation time of the resonance signal. NMR spectroscopy can be used to examine high exchange rates of such molecular complexes in solution and has successfully been applied to binary and ternary U(VI) complexes. [START_REF] Aas | Equilibria and dynamics in binary and ternary uranyl oxalate and acetate/fluoride complexes[END_REF] The observation of only one averaged peak per position in the 1 H and [START_REF] Lucks | Aqueous uranium(VI) complexes with acetic and succinic acid: speciation and structure revisited[END_REF] C spectra of the U(VI) acetate complex system even at the higher field clearly indicates a fast exchange rate. The differences observed in the line widths of the signals measured at the same field (at 400 MHz) are mainly due to the variations in the homogeneity of the magnetic field, that is, due to the differences in the shimming procedure. At higher fields, the chemical shifts differ more (in Hz) between the exchanging sites (free acetate and complexes), and the exchange-averaged signals are broader compared to those in the 400 MHz spectra.

As neither the chemical shifts of the individual exchanging species nor their relative population are known, simulation of the spectra via a matrix formalism cannot be applied to extract kinetic information. Adequate results can be achieved only by running experiments at lower temperatures, but these are far Figure 2. Time regimes applied by all participants of the LUM cluster during the acquisition of the luminescence spectra (A). Emission spectra of sample U1 as measured (B) and after calibration (C) using the peak maximum at 490 nm as an internal standard. Only spectra of four participants could be used for calibration (for details, see Supporting Information 5.2).

beyond the scope of this RRT. Nevertheless, the observations of increased linewidths and chemical shifts in the current data set are consistent with earlier findings, suggesting a correlation of the linewidths with the concentration of the U(VI) acetate species. [START_REF] Aas | Equilibria and dynamics in binary and ternary uranyl oxalate and acetate/fluoride complexes[END_REF] 3.1.5. Theory/QCC. DFT is the most widely used method for calculations of uranyl(VI) complexes because U(VI) formally exhibits a 5f 0 electronic configuration, and thus a singleconfigurational approach is appropriate. The effect of the spinorbit coupling is only marginal for most of the properties considered here, and therefore, it was neglected. We tested various DFT functionals and a wave function method (MP2). The structural and spectroscopic properties of the two most relevant species, that is, the complexes (1,0) and (1,3), and the energy of ligand replacement, that is from (1,0) to (1,3), were calculated. The results are given in Tables S13 andS14 (Supporting Information). The detailed discussion in the Supporting Information (Section S5.5) focuses on the results obtained with the same basis set for various methods (Programs Gaussian, TURBOMOLE, and ParaGauss), as summarized in Tables S10 and2, together with ADF results in solution for comparison.

The results suggest that the calculated spectroscopic parameters depend considerably on the optimized structures. Generally, all methods satisfactorily reproduce the U-O ax distances, whereas the U-O eq distances are generally overestimated compared to experimental data. To fully evaluate the calculated results, they must be compared with the experimental results, which will be done in the next section.

3.2. Molecular Aspects of the U(VI) Speciation Derived from Each Cluster's Results. In this section, the results and interpretational approaches obtained from each cluster are analyzed with respect to the aqueous U(VI) acetate speciation. As a synopsis, Table 3 displays the consistency of the data obtained and the results of interpretational approaches. The predominant U(VI) species in samples U1 and U4 are expected to be the (1,0) and (1,3) species, respectively. At variance, for samples U2 and U3, the presence of several species is expected where the predominance of uranyl complexes with an increasing number of ligands increases with pH. Thus, to evaluate the data across all clusters, the (1,0) and (1,3) species are discussed first.

The unequivocal data provided by the spectroscopic clusters for sample U1 facilitate the assignment of the spectral features observed to the predominant aqueous (1,0) species. This conclusion is explicitly supported by the VIB and XAS clusters. All participants performing Raman and infrared spectroscopy concordantly assigned the spectral data showing the symmetric (ν 1 (UO 2 )) and asymmetric (ν 3 (UO 2 )) stretching vibrational modes to the fully hydrated uranyl ion. Furthermore, the structure of the aqueous uranyl complex, in which the UO 2 2+ moiety is coordinated by 5 water molecules in the equatorial plane, is unambiguously reproduced by EXAFS shell fitting by all three participating beamline groups. However, regarding the results of the U1 sample from the LUM cluster, the assignment of the predominant species as the (1,0) species is mainly based on the prevailing conditions.

The (1,3) species is unequivocally identified by all participants in the VIB and XAS clusters. Based on the highly consistent spectral data, the interpretational approaches used by the participants differ only slightly. Most participants interpreted the spectral data in terms of the predominance of the (1,3) species with a structure, in which all three acetate ligands are bidentately coordinated to the uranyl moiety. From the VIB data, only one set of Raman spectra was not assigned to the (1,3) species, while in one set of IR spectra, the presence of an even higher coordinated U(VI) acetate complex, the (1,4) species, was discussed but was subsequently considered to be unlikely because of steric considerations. Moreover, according to one set of XAS data, the (1,3) species was considered to exhibit a mixed type of coordination of the ligands, that is, two bidentately and one monodentately bound ligand.

In contrast, the assignment of spectral features to the (1,1) and (1,2) species is generally less clear. Only the vibrational spectra exhibit a spectral feature, namely the frequency of the ν(UO 2 ) modes, that directly correlates with the formation of each species and thus allows the unambiguous assignment of four distinct species. However, even the interpretation of vibrational data is hampered by overlapping signals, and the assignment of these species is not completely consistent in case of the Raman spectra. Note, however, that the Raman spectroscopy results of this RRT convincingly revise previous literature data. [START_REF] Quiles | Infrared and Raman spectroscopic study of uranyl complexes: hydroxide and acetate derivatives in aqueous solution[END_REF] For the XAS and LUM clusters, identifying these species is not possible for various reasons. For EXAFS spectroscopy, this is mainly due to the number of samples being too small, as shown previously. [START_REF] Lucks | Aqueous uranium(VI) complexes with acetic and succinic acid: speciation and structure revisited[END_REF] Moreover, it has been Note that for better comparison the same line broadening factor (lb) was applied to the raw data (see Tables S11 andS12). demonstrated that the spectrum of the (1,2) species intrinsically cannot be derived from EXAFS data. [START_REF] Lucks | Aqueous uranium(VI) complexes with acetic and succinic acid: speciation and structure revisited[END_REF] Indeed, the LUM data from all contributors demonstrate that each sample shows its characteristic spectral fingerprint; however, specific species can be extracted only when an extensive parametric analysis of a larger data set is available than the one generated within this RRT.

U-O ax U-O eq ν 1 ν 3 ν 3 -ν 1 U-O ax U-O eq U-C carb ν 1 ν 3 ν 3 -
NMR spectroscopy is a non-optical spectroscopic technique that exclusively provides signals reflecting changes of the energy differences in nuclear spin states in hydrogen and carbon atoms of the ligand, which differentiates this technique from the other spectroscopic methods used in this RRT. On the one hand, the slight shifts observed in the 1 H and [START_REF] Lucks | Aqueous uranium(VI) complexes with acetic and succinic acid: speciation and structure revisited[END_REF] C spectra nicely represent the ongoing complexation of the uranyl moiety by acetate molecules as the pH increases. On the other hand, an assignment of the signals observed to distinct species is not possible. As already stated in Section 3.1.4, a more detailed analysis yielding speciation information can be obtained only by conducting much more extensive experiments, which are beyond the scope of this RRT.

A principal concern of this RRT was the juxtaposition of the results obtained from the experimental and theoretical approaches. For the sake of simplicity, the calculations focused on the (1,0) and (1,3) species assuming the penta-aquo complex UO 2 (H 2 O) 5 2+ for the (1,0) species and the exclusively bidentate complex UO 2 (CH 3 COO) 3

-for the (1,3) species. In general, the accuracy of the calculated values of the interatomic distances and the vibrational frequencies agree well with experimental results but the quality of the results depends slightly on the method applied (Table 2). Compared to the structures in the gas phase (Table S10), the calculations including solvation effects provide slightly longer U-O ax distances and slightly shorter U-O eq distances (Tables S10 and2). Solvation effects slightly improve the agreement with EXAFS results for U-O eq and in some cases also for U-O ax distances. As an example, the solvation calculations of both the (1,0) and (1,3) species at the MP2 level gave bond distances that are all within 0.02 Å of the values from the EXAFS data (Table 2). At the DFT level, the deviations are slightly larger. Depending on the type of functional, the deviations from the experimental bond distances can be as large as 0.1 Å (Table S14). For all methods, a trend to overestimating the lengths of U-O bonds to the ligands has to be noted, by 0.06 Å for (1,0) and by 0.03 Å for (1,3) on average. The geometry differences between the species (1,0) and (1,3) are qualitatively reproduced by all methods. The increase of U-O ax by about 0.01 Å und the increase of U-O eq by about 0.05 Å due to complexation, as observed by EXAFS, are overestimated for U-O ax and underestimated for U-O eq by about 0.02 Å in all calculations (Table 2). In general, calculated differences of properties should be more accurate than absolute values due to favorable error cancelation. On the other hand, the observed deviations from experiment are not unexpected due to limitations of the solvation modeling in the calculations and due to some contributions from species with less than three acetate ligands in probe U4 in the experiment (see Figure S2).

The calculated harmonic vibrational frequencies of the ν 1 and ν 3 (UO 2 ) modes are consistent with the experimental values. Not surprisingly, the better the agreement between the experimental and theoretical bond distances, the better tends to be the agreement in vibrational frequencies (irrespective of gas phase or aqueous phase calculations). For instance, the calculations at the MP2 level, which coincide well with the experimental bond distances, provide an average deviation of only 17 cm -1 (max. 27 cm -1 ) between the theoretical and experimental ν 1 /ν 3 (UO 2 ) frequencies (Table 2). The results obtained with the M06L functional with ParaGauss are only slightly less accurate. In line with the overestimation of the increase of the U-O ax bond due to acetate complexation in the calculations, the red shift of the uranyl stretching vibrations as a result of acetate complexation, which amounts to about 30 cm -1 in experiment, is overestimated by all methods by at least 30 cm -1 (Table 2). The difference between ν 3 and ν 1 is well reproduced by MP2 (Table 2, overestimated for (1,0) = 2 cm -1 , for (1,3) = 13 cm -1 ), but underestimated by DFT by 40 cm -1 for (1,0) and 30 cm -1 for (1,3) on average (Table 2). These findings of systematic deviations support the assumption of differences between experiment and theory due to model aspects in the calculations and additional species in the experimental probes (see above).

Besides, it should be noted that there are three essential sources of errors in computations: the model choice, the electronic structure method, including the particular (relativistic) treatment of the core electrons, and the solvation treatment. Commonly some error cancelation may be expected. One may therefore always argue that good agreement with experiments may simply be a coincidence and computational results always warrant a critical evaluation.

This RRT demonstrates that the basic molecular parameters of small actinide complexes obtained using QCC are accurate with respect to the experimental findings. In particular, the interatomic distances are obviously predicted with higher accuracy than vibrational frequencies (for which rescaling is a rather common practice). Deviations of calculated parameters from the experimental values, averaged over all methods applied, amount to 1-2.5% for bond lengths and to 4-6% for the uranyl stretching vibrations. For well-defined systems and with a proper choice of the method, an even better agreement with experiment is achievable. Small systematic deviations, especially of differences of properties, point to limitations of the models used in the calculations and uncertainties of speciation in the experimental probes: thus, such differences are of special interest and are helpful for improving calculations as well as the interpretation of experiments.

CONCLUSIONS AND PERSPECTIVES

4.1. Evaluation of the RRT. The concept of this RRT based on the contributions of a broad array of spectroscopic techniques and QC approaches to the aqueous U(VI) speciation exemplified for the U(VI) acetate system. This project intended to collect results from as many laboratories with dedicated expertise in the molecular analysis of actinides as possible. Hence, for the sake of practicability, the scale of the RRT had to be restricted to a relatively small number of samples, which can hardly be considered as adequate for a thorough investigation of an aqueous speciation as complex as the U(VI) acetate system. In consequence, a conclusive evaluation of the RRT shall be restricted to some general, yet still-instructive aspects. Table 4 summarizes the major findings of the applied techniques as well as their strengths and limitations concerning the U-acetate complex system.

The generally high consistency of the data, obtained within each cluster, reflects the accuracy and suitability of the techniques applied for the exploration of the uranyl complex system. In particular, those techniques conducted on commercial instruments provided highly consistent data, as shown by the results of the VIB and NMR clusters. Despite having worked on different beamlines and individually adapted setups, even accelerator-driven XAS provided highly consistent raw data. Regarding the characteristic specifications of the single facilities, one of the merits of this RRT is explicitly demonstrating the reproducibility of the performance of the three participating synchrotron beamlines.

A distinct comparison of the luminescence spectral data was only feasible after performing a correction routine for wavelength calibration, as described in Sections 3.1.2 and S5.2. At first, the consistency of luminescence data appeared to be quite low. This inconsistency might raise questions about the comparability of the spectral analysis, which was performed based on only simple spectral fingerprinting published previously. Conversely, when the spectra were referenced internally to the spectral distances of the peaks, the data within the LUM cluster demonstrated high reproducibility. However, considering the data of the luminescence decay curves provided by the respective participants, a comparison of the spectral data is hardly possible, as described in Section S5.2. In fact, these aspects were already addressed in a previous RRT in 2003. [START_REF] Billard | Aqueous solutions of uranium(VI) as studied by time-resolved emission spectroscopy: A round-robin test[END_REF] Multimethodological approaches to the analysis of molecular complexes are expected to benefit from the acquisition of complementary information. Potentially, this advantage might be derived from IR and Raman spectroscopy because of the intrinsic selection rules of the vibrational modes observed in the respective spectra. In fact, four prevailing species and the interacting functional groups in the molecular complexes could be evidently derived from the vibrational spectra (see Sections 3.1.1 and S5.1). This information was nicely reflected by the results in the NMR cluster, which provided evidence for the increasing degree of complexation of the uranyl moiety with increasing pH (see Sections 3.1.4 and S5.4). As the results from the LUM and XAS cluster indicated the presence of only three species, the missing complementarity of these techniques with the former ones must be discussed in more detail. Due to the intrinsic properties of the EXAFS spectra, the (1,2) species cannot be differentiated from the other species which is largely due to symmetry reasons as already mentioned above (see Sections 3.1.3 and S5.3). Hence, such intrinsic limitations, which might not be obvious even to advanced spectroscopists, must be understood prior to performing extensive series of experiments. The intrinsic luminescence properties of the species might also be responsible for the lower number of species observed using LUM. One of the speciesmost likely the (1,1) specieswas assumed to be not luminescent. Although an ultimate verification is still lacking, this explanation would clearly elucidate why comprehensive complementary information was not obtained from the LUM cluster. However, it has to be emphasized that the number of samples was certainly insufficient for a thorough investigation of the speciation, particularly in the case of luminescent spectroscopy.

Consequently, to gain maximum benefit from a multimethodological approach exploring the U(VI) acetate speciation, the techniques must be carefully selected, which requires a detailed knowledge of their capabilities and restrictions. Even then, experienced users might suffer from unexplored details of intrinsic properties of the molecules under investigation. At this point, theoretical calculations, as they represent an independent and thus complementary approach to molecular properties, appear particularly valuable. As shown in this RRT, results from various QC codes are very consistent when the same stationary points of the species under study are considered. Bond distances are well reproduced with the MP2 and a number of DFT approaches, while vibrational frequencies show slightly larger deviations, but are still useful for the assignment of the spectral findings to distinct molecular structures. Nevertheless, the general trends of geometry and vibrational parameters of the U(VI) acetate system are calculated in agreement with the experiment. Small systematic deviations from the experimental results, which are independent of the detailed method applied, are of special interest, as they point to weaknesses of the models applied and/or the need for a more detailed understanding of the system considered in the experiments. DFT calculations predict chemical shifts of NMR signals quite precisely. In the future, the development of algorithms providing reliable calculations of luminescent spectra is desired.

In summary, this RRT was very successful in terms of the high number of participants throughout the community and their acceptance to share their expertise and experimental (unpublished) data. Furthermore, the cooperation and willingness of the participants to contribute to the discussions during the compilation process of the RRT illustrate the profound interest in improving the performance of the various techniques and in continuing the knowledge transfer among scientists from various analytical disciplines to obtain a deeper understanding of basic molecular complexes in actinide chemistry.

4.2. Scope and Perspectives. In the past decades, significant progress in laser technology, microfluidics, digital signal processing, and the development of novel light detectors has contributed to dramatic efficiency enhancements of spectroscopic methods in terms of significantly reduced accumulation times and increase in quality of spectra. Consequently, due to improved signal-to-noise ratios in the spectra, the extraction of further molecular information by numerical analysis has gained importance in spectroscopic investigations. Apart from classical analysis methods, such as curve decomposition or factor analysis, which have been applied intensively ever since, various statistical methodschemometrics in particularhave been developed for processing these data and extracting essential information. However, only the accurate application of such statistical methods prohibits from misinterpretation of the results, which might lead to erroneous understanding otherwise. [START_REF] Shashilov | Advanced Statistical and Numerical Methods for Spectroscopic Characterization of Protein Structural Evolution[END_REF] This particularly refers to wellestablished fitting procedures, which often appear to be poorly documented in scientific articles. Hence, the provision of comprehensive numerical information necessary for the reproduction of such an analysis is indispensable. Moreover, we felt confident through this RRT that it is necessary to establish a method utilizing chemometrics as a routine tool for characterization of molecular structures. In the future, further development of dimension reduction and feature extraction methods such as self-organizing maps is foreseen to contribute further to molecular spectroscopy.

A key concern of this RRT was the elicitation of the mutual promotion of QC calculations and spectroscopic approaches. The results presented provide a widely consistent molecular understanding of the (1,0) and (1,3) uranyl acetate species, even though theoretical predictions and experimental findings showed some discrepancies to a different extent with respect to the different spectroscopic techniques. Hence, the development or improvement of algorithms explicitly considering the intrinsic properties of specific spectroscopic techniques is expected to enhance the concordance of theoretical and experimental approaches and appears therefore necessary. If an intense communicative exchange of theoreticians and experimentalists generates a mutual understanding of their operational modes and mindsets, future work might proceed in extracting specific information from complex experimental data and vice versa in verifying theoretical investigations by experimental findings.

On a technical level of instrumentation, this RRT demonstrated the importance of the availability of experimental parameters. Only detailed sets of relevant instrumental parameters strengthen the sustainability of the data presented and might provide the basis for future technical improvements. In this context, it was shown that a commonly accepted and applied calibration routine for LUM is mandatory though hard to be implemented throughout worldwide researchers. Nevertheless, we suggest a suitable calibration approach in the Supporting Information (cf. Figure S21). This approach follows the procedure already described earlier using the emission line of a conventional Hg lamp. [START_REF] Martinsen | Accurate and precise wavelength calibration for wide bandwidth array spectrometers[END_REF] In the future, it is eligible to represent luminescence spectra on an energy scale (wavenumber units) instead of a wavelength scale. This would facilitate the comparability of spectral features observed under different experimental setups, thereby data interpretation by various researchers becoming more perspicuous and comprehensible.

■ ASSOCIATED CONTENT

* S Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b00164.

Further information on the general RRT organization; U(VI) acetate calculated speciation; experimental details on sample preparation; clusters and experimental parameters of the spectroscopic setups including VIB, LUM, XAS, and NMR spectroscopy and QCC programs, methods, and basis sets with detailed description and interpretation of the results from each cluster; presentation of all data provided by the participants; calibration routine for LUM; and extended list of references (PDF)

Figure 1 .

 1 Figure 1. Vibrational spectra of samples U1-U4. Different colors correspond to different participants. Only the spectral regions of the ν 3 (UO 2 ) mode from IR (A) and of the ν 1 (UO 2 ) mode from Raman spectroscopy (B) are shown. Values are given in cm -1 .

a

  The multiple scattering path U-O ax1 -O ax2 -U was included in all fits of sample U1, structural model: uranyl acetate dihydrate.[START_REF] Howatson | Crystal and molecular structure of uranyl acetate dihydrate[END_REF] b Fixed fit parameter. c Parameter correlated with the preceding shell. d Significant improvement by including asymmetry parameter (3rd cumulant). e 3 and 4 legs multiple scattering path O ax1 -U-O ax2 . f Multiple scattering path U-O ax1 -O ax2 -U and MS paths to C meth included, distances and Debye-Waller factors correlated with single scattering values. g Values without phase shift correction. h Values in italics given for participant XAS-2 represent literature data taken from ref 13.

Figure 3 .

 3 Figure 3. 13 C NMR spectroscopic data of the methyl (left) and carboxyl regions (right). Comparison of the chemical shifts (top) and linewidths (below) for samples U1-U4 obtained by different RRT participants.Note that for better comparison the same line broadening factor (lb) was applied to the raw data (see TablesS11 and S12).

Table 1 .

 1 EXAFS Shell-Fitting Parameters a,h

	k-range (Å -1 )	R-range g (Å)	N	R (Å)	σ 2 (Å 2 )	ΔE 0 (eV)	S 0 2	R-factor (%)
				Sample U1
	2.6-12.1	1.0-6.0	participant XAS-1
		O ax	2 b	1.76(1)	0.0017 b	12.85 b	1.03 b	0.8 (k 2 -weighting)
		O eq	4.8(1)	2.41(1)	0.0070 b
	3.1-16.7		participant XAS-2
		O ax	2 b	1.766(1)	0.0011(1)	4.6	0.9 b
			2 b	1.764(1)	0.0013(1)	3.9	0.9 b
		O eq	5.3(3)	2.407(3)	0.0064(5)
			5.1(3)	2.403(3)	0.0067(5)
	3.2-15.7	1.0-3.3	participant XAS-3
		O ax	2 b	1.763(2)	0.0015	3.8	0.96	1.0 (k 2 -weighting)
		O eq	4.8(2)	2.404(3)	0.0070
				Sample U4
	3.4-12.1	1.0-6.0	participant XAS-1 e
		O ax	2 b	1.77(1)	0.0013 b	12.9 b	1.01 b	1.5 (k 2 -weighting)
		O eq	5.4(3)	2.46(1)	0.0075 b
		C carb	2.7 c	2.89(1)	0.0034 b
	3.1-16.7		participant XAS-2 f
		O ax	2 b	1.780(1)	0.00117(7)	6.2(3)	0.9 b
			2 b	1.780(1)	0.0014(1)	4.9(3)	0.9 b
		O eq	5.8(3)	2.464(3)	0.0082(6)
			5.8(3)	2.472(3)	0.0071(5)
		C carb	3.0(3)	2.887(6)	0.0042 b
			3.3(3)	2.880(5)	0.0042 b
		C meth	3 c	4.360(9)	0.00645 b
			3.3 c	4.362(8)	0.00645 b
	3.1-15.7	1.1-4.3	participant XAS-3 f
		O ax	2 b	1.773(2)	0.0016	4.3	0.96	0.9 (k 2 -weighting)
		O eq	6 b	2.44(1)	0.0087 d
		C carb	3 b	2.88(2)	0.0045
		C meth	3 c	4.37(2)	0.0041

Table 2 .

 2 Average Aqueous Phase U-O and U-C carb Bond Distances (Å), OUO ν 1 and ν 3 Stretching Vibrational Frequencies (cm -1 ), and NMR Chemical Shifts (ppm) of the (1,0) and (1,3) Species as Well as Gibbs Energy of Formation of UO 2 (CH 3 COO)3 -ΔG (kcal mol -1 ) at Different Theoretical Levels a

	UO 2 (H 2 O) 5	2+		UO 2 (CH 3 COO) 3 -
	bond distances	vibr. frequencies	bond distances	vibr. frequencies	NMR chemical shift c	energy
	b

Table 3 .

 3 Qualitative Summary of the Clusters' Outcome with Respect to the U(VI) Acetate Species a consistency of data and interpretation between the participants; +/-: high consistency of data and ambiguous interpretation between the participants; -/-: low consistency of data and interpretation if available; N/A: not available. b Deviation in assignment & interpretation.Unequivocal identification not feasible. d Deviation in interpretation of structure. e Due to low sample number nearly no interpretation approaches; global equilibration procedure required. f Variation due to weakly bound H 2 O-ligands.

	U(VI) species

a +/+: high c

Table 4 .

 4 Summary of Strengths and Limitations of Applied Analytical Tools and Their Relevance to the U(VI) Acetate System

	for U-Ac complex system

This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.Downloaded via CEA SACLAY on December 15, 2020 at 12:09:57 (UTC).See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

DOI: 10.1021/acsomega.9b00164 ACS Omega 2019, 4, 8167-8177

■ ACKNOWLEDGMENTS

We would like to thank Annika Maffert, Christa Muller, and Heidrun Neubert for sample preparation and packaging and Annette Rumpel, Ines Prescher, and Christine Roch for their assistance with radiation safety and shipment procedures. For analytical support, we acknowledge Karsten Heim, Sophia Kostudis, and Michael Steppert. Furthermore, kindly acknowledge the proofreading and the fruitful discussions with Vinzenz Brendler, Clemens Walther, Astrid Barkleit, Atsushi Ikeda-Ohno, Michael Patzschke, and Jochen Autschbach. We appreciate the synchrotron light sources SOLEIL at Gif-sur-Yvette, France, ESRF at Grenoble, France, and KIT Synchrotron Light Source at Karlsruhe, Germany, for the provision of beamtime. B.D. and K.B were supported by the German Federal Ministry of Economic Affairs and Energy (BMWi) under contract numbers 02E11415G and 02E11011. P.Y. was supported for this work by the Heavy Element Chemistry program at LANL sponsored by the United States Department of Energy Office of Basic Energy Sciences. Los Alamos National Laboratory is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the United States Department of Energy under Contract DE-AC52-06NA25396. G.S. acknowledges funding from the Natural Sciences and Engineering Council of Canada (NSERC, Discovery Grant). B.A.P. Experimental was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DE-SC0010355. Work at LLNL was supported by the Subsurface Biogeochemical Research Program of the U.S. Department of Energy's Office of Biological and Environmental Research. Prepared by LLNL under contract DE-AC52-07NA27344. The work of I.C., A.K. and S.K. has been supported by the German Federal Ministry of Economic Affairs and Energy (BMWi) under contract nos. 02E11001 and 02E11415E. This material is based upon work supported by the U.S. Department of Energy Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences, Heavy Element Chemistry program, and was performed in the William R. Wiley Environmental Molecular Sciences Laboratory, a D.O.E. national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at the Pacific Northwest National Laboratory (PNNL). PNNL is a multiprogram national laboratory operated for D.O.E. by Battelle.

■ AUTHOR INFORMATION

Corresponding Author *E-mail: k.mueller@hzdr.de. ORCID Katharina Muller: 0000-0002-0038-1638 Harald Foerstendorf: 0000-0002-8334-9317 Satoru Tsushima: 0000-0002-4520-6147 Michael U. Kumke: 0000-0002-3395-9379 Greǵory Lefevre: 0000-0002-1054-906X Harris Mason: 0000-0002-1840-0550 Ping Yang: 0000-0003-4726-2860 Christian K. R. Adam: 0000-0003-1449-7065 Herman M. Cho: 0000-0001-7072-2306 Christophe Den Auwer: 0000-0003-2880-0280 Bjorn Drobot: 0000-0003-1245-0466 Peter Kaden: 0000-0002-9414-2936 Alena Kremleva: 0000-0003-3330-0862 James A. Platts: 0000-0002-1008-6595 Robert Polly: 0000-0002-7024-7987 Brian A. Powell: 0000-0003-0423-0180 Notker Rosch: 0000-0002-4769-4332 Andreas C. Scheinost: 0000-0002-6608-5428 Georg Schreckenbach: 0000-0002-4614-0901 Andrej Skerencak-Frech: 0000-0003-2177-4462 Vladimir Sladkov: 0000-0002-1053-0473

Notes

The authors declare no competing financial interest.


Jai Prakash

Adel Mesbah

Sébastien Lebègue

James A Ibers
email: ibers@chem.northwestern.edu

Synthesis, crystal structure, and electronic structure of Ba 2 GeTe 3 (Te 2 )

Keywords: Barium germanium telluride, solid-state synthesis; crystal structure, TeTe single bond, electronic structure

Black crystals of Ba2GeTe3(Te2) were obtained by the "U-assisted" reaction of U, Ba, Ge, and Te at 1173 K using the sealed-tube method. The crystal structure of Ba2GeTe3(Te2) was determined by a single crystal X-ray study at 100(2) K. It crystallizes in space group Pnma of the orthorhombic crystal system with four formula units in a cell with constants a = 6.7900(14) Å, b = 23.720(5) Å, and c = 6.9300(14) Å. The crystal structure of Ba2GeTe3(Te2) can be described as pseudo one-dimensional with zig-zag chains 1 ∞[GeTe3 2 ] where each Ge atom is bonded to four Te atoms in a distorted tetrahedron. These chains are separated by the Ba 2+ cations and homoatomic dimers of Te2 2 .

Around each Ba atom there are nine Te atoms in a distorted tricapped trigonal prism. Charge balance in Ba2GeTe3(Te2) is achieved with 2 × Ba 2+ , 1 × Ge 4+ , 3 × Te 2 , and 1 × Te2 2 . Density functional theory (DFT) calculations suggest Ba2GeTe3(Te2) is a metal with Ge-and Te-derived states contributing the most around the Fermi level.

Introduction

The compounds A/Q/T and Ak/Q/T, where A = alkali metal = Li-Cs ; Ak = alkaline-earth metal = Mg, Ca, Sr, Ba; Q = chalcogen = S, Se, and Te; and T = Si, Ge, and Sn, have been extensively studied because of their rich structural chemistry. These compounds show diverse physical properties, such as semiconducting, nonlinear optics (NLO) materials, thermoelectric properties, solar energy converters, and detector materials [START_REF] Wu | An investigation of new infrared nonlinear optical material: BaCdSnSe4, and three new related cetrosymmetric compounds: Ba2SnSe4, Mg2GeSe4, and Ba2Ge2S6[END_REF][START_REF] Wu | Ba2GeS4 and Mg2SnS4: synthesis, structures, optical properties and electronic structures[END_REF][START_REF] Assoud | New Quaternary Barium Copper/Silver Selenostannates: Different Coordination Spheres, Metal-Metal Interactions, and Physical Properties[END_REF][START_REF] Marking | New Quaternary Thiostannates and Thiogermanates. A2Hg3M2S8 (A = Cs, Rb; M = Sn, Ge) through Molten A2Sx. Reversible Glass Formation in Cs2Hg3M2S8[END_REF]. For example, Li2Ga2GeS6 shows a strong second harmonic generation (SHG) effect [START_REF] Kim | Characterization of New Infrared Nonlinear Optical Material with High Laser Damage Threshold, Li2Ga2GeS6[END_REF] and the Li2In2TQ6 (T = Si, Ge; Q = S, Se) compounds are the few examples of IR NLO materials [START_REF] Yin | Synthesis, Structure, and Properties of Li2In2MQ6 (M = Si, Ge; Q = S, Se): A New Series of IR Nonlinear Optical Materials[END_REF].

The structures of most of these compounds contain tetrahedral TQ4 units. The condensation of TQ4 units results in the formation of infinite chains, layers, and networks of [TxQy n ] units. Compounds such as Mg2SnS4 [START_REF] Wu | Ba2GeS4 and Mg2SnS4: synthesis, structures, optical properties and electronic structures[END_REF] and Ba3GeS5 [START_REF] Pan | Ba3GeS5 and Ba3InS4Cl: Interesting size effects originated from the tetrahedral anions[END_REF] contain discrete units of [TQ4 4 ] and are classified as zero-dimensional. Other examples of [TxQy n ] units include the [Ge4S10 4 ] clusters in Ba2Ge4S10 [START_REF] Ribes | Etude Structurale de Thiocomposés à Groupement Anionique de Type Tetrane Na4X4S10 (X = Ge, Si) et Ba2Ge4S10[END_REF], the infinite chains of [GeS3 2 ] in Ba2Ge2S6 [START_REF] Wu | An investigation of new infrared nonlinear optical material: BaCdSnSe4, and three new related cetrosymmetric compounds: Ba2SnSe4, Mg2GeSe4, and Ba2Ge2S6[END_REF], and the [SnS5 6 ] units in the BaSn2S5 structure [START_REF] Li | Synthesis and Characterization of Ba2SnTe5: A New Zintl Phase Containing Unique One-Dimensional Chains of (SnTe3) 2and Dimeric Units of (Te2) 2[END_REF]. Apart from TQ4 units, a few compounds also contain ethane-like [T2Q6 6 ] units with homonuclear TT bonds, an example being K6Ge2S6 [START_REF] Eisenmann | Über Thio-, Selenido-und Telluridogermanate(III): Zur Kenntnis von K6Ge2S6, K6Ge2Se6 und Na6Ge2Te6[END_REF].

The current ICSD database [START_REF]The Inorganic Crystal Structure Database (ICSD) Web[END_REF] shows very few compounds in the ternary Ak/Ge/Q system: Ba2Ge4S10 [START_REF] Ribes | Etude Structurale de Thiocomposés à Groupement Anionique de Type Tetrane Na4X4S10 (X = Ge, Si) et Ba2Ge4S10[END_REF], Ba2Ge2S6 [START_REF] Wu | An investigation of new infrared nonlinear optical material: BaCdSnSe4, and three new related cetrosymmetric compounds: Ba2SnSe4, Mg2GeSe4, and Ba2Ge2S6[END_REF], Ba3GeS5 [START_REF] Assoud | From Yellow to Black: New Semiconducting Ba Chalcogeno-Germanates[END_REF], Ak2GeQ4 (Ak = Mg, Ca, Sr, and Ba; Q = S and Se) [START_REF] Wu | An investigation of new infrared nonlinear optical material: BaCdSnSe4, and three new related cetrosymmetric compounds: Ba2SnSe4, Mg2GeSe4, and Ba2Ge2S6[END_REF][START_REF] Wu | Ba2GeS4 and Mg2SnS4: synthesis, structures, optical properties and electronic structures[END_REF][START_REF] Assoud | From Yellow to Black: New Semiconducting Ba Chalcogeno-Germanates[END_REF], Sr2Ge2Se5 [START_REF] Johrendt | Strontium Selenogermanate(III) and Barium Selenogermanate(II,IV): Synthesis, Crystal Structures, and Chemical Bonding[END_REF], Ba2Ge2Se5 [START_REF] Johrendt | Strontium Selenogermanate(III) and Barium Selenogermanate(II,IV): Synthesis, Crystal Structures, and Chemical Bonding[END_REF], and only one telluride Ba2Ge2Te5 [START_REF] Brinkmann | Ba2Ge2Te5, ein neues Kettentelluridogermanat(III)[END_REF]. Most of these compounds contain Ge 4+ . However, there are few compounds with Ge 3+ or mixed Ge 2+/4+ . For example, Sr2Ge2Se5 [START_REF] Johrendt | Strontium Selenogermanate(III) and Barium Selenogermanate(II,IV): Synthesis, Crystal Structures, and Chemical Bonding[END_REF] and Ba2Ge2Te5 [START_REF] Brinkmann | Ba2Ge2Te5, ein neues Kettentelluridogermanat(III)[END_REF] contain exclusively Ge 3+ whereas Ge 2+ and Ge 4+ coexist in a mixed-valence compound Ba2Ge2Se5 [START_REF] Johrendt | Strontium Selenogermanate(III) and Barium Selenogermanate(II,IV): Synthesis, Crystal Structures, and Chemical Bonding[END_REF]. Surprisingly, there appear to be no known Ak/Ge/Q compounds with structures that have any homoatomic QQ bonds. Here we present the first such example: the synthesis, crystal structure, and electronic structure of the new ternary polytelluride Ba2GeTe3(Te2).

Experimental

Synthesis

Caution! Depleted uranium is an α-emitting radioisotope and its manipulation is considered a health risk. Its use requires appropriate infrastructure and personnel trained in the handling of radioactive materials.

The following reactants were used as received: Ba (Alfa 99.5%), Ge (Aesar 99.99%), and Te (Aldrich 99.8%). U powder was obtained through the hydridization of depleted-U turnings (Mfg Sci. Corp.) followed by decomposition of the hydride under vacuum [START_REF] Haneveld | Some Ternary Uranium Chalcogenides[END_REF].

Synthesis of single crystals of Ba2GeTe3(Te2

). Black crystals of Ba2GeTe3(Te2) were serendipitously obtained by combining the elements Ba, Ge, U, and Te. The aim of the reaction was to synthesize a new quaternary compound of Ba, Ge, U, and Te. The reactants were loaded into carbon-coated fused-silica tubes in an Ar-filled glove box, evacuated to 10 4 Torr, and flame sealed. The reaction contained 0.0346 g (0.252 mmol) Ba, 0.01g (0.042 mmol) U, 0.0061g (0.084 mmol) Ge, and 0.0965g (0.756 mmol) Te. The sealed tube containing the reaction mixture was heated from 298 K to 1053 K in 36 h, held there for 15 h, further heated to 1173 K in 24 h, and held there for 192 h. The tube was then cooled to 993 K over 99 h and finally to 298 K in a further 122 h. Semi-quantitative elemental analysis of the products was carried out with the use of a Hitachi S-3400 SEM equipped for EDX analysis. The black irregular shaped crystals showed Ba:Ge:Te  2:1:5. The sizes of these crystals were in the range of ~ 30 m to 150 m.

The reaction product also contained crystals of UOTe and BaTe. The crystals of Ba2GeTe3(Te2) were stable in air for at least one week as judged from unit-cell determinations.

Structure Determination

Single-crystal X-ray diffraction data were collected at 100(2) K on an APEX2 X-ray diffractometer equipped with graphite-monochromatized MoKα radiation [START_REF]5-1 Data Collection and Processing Software[END_REF]. The crystal-to-detector distance was 60 mm; the exposure time was 15 sec/frame. Collection of intensity data, cell refinement, and data reduction were performed using APEX2 as a series of 0.3 scans in φ and ω [START_REF]5-1 Data Collection and Processing Software[END_REF]. Face-indexed absorption, incident beam, and decay corrections were performed by the program SADABS [START_REF] Sheldrick | SADABS[END_REF]. The crystal structure was solved using the SHELX14 suite of programs [START_REF] Sheldrick | A short history of SHELX[END_REF]. Atom positions were standardized using the program STRUCTURE TIDY [START_REF] Gelato | STRUCTURE TIDY -a computer program to standardize crystal structure data[END_REF]. Structure drawings were made using the program CRYSTALMAKER [20]. Further details are given in Tables 1 and2, and in the Supporting Information.

Electronic structure calculation

To conduct our calculations we have used the VASP (Vienna ab Initio Simulation Package) [START_REF] Kresse | From ultrasoft pseudopotentials to the projector augmented-wave method[END_REF] code which implements density functional theory [START_REF] Kohn | Self-Consistent Equations Including Exchange and Correlation Effects[END_REF][START_REF] Hohenberg | Inhomogeneous Electron Gas[END_REF] using the projector augmented wave basis set [START_REF] Blöchl | Projector augmented-wave method[END_REF]. For the exchange-correlation functional, we have chosen the one of Heyd, Scuseria, and Ernzerhof [START_REF] Heyd | Hybrid functionals based on a screened Coulomb potential[END_REF][START_REF] Heyd | Erratum: "Hybrid functionals based on a screened Coulumb potential[END_REF][START_REF] Paier | Screened hybrid density functionals applied to solids[END_REF]. The crystal structure was kept identical to the experimental one, while to ensure proper convergence of our calculations, a 12 × 4 × 12 mesh was used to integrate over the Brillouin zone. The default cut-off was used for the plane wave part of the wave functions.

Results and discussion

Synthesis and Structure

Black single crystals of Ba2GeTe3(Te2) were first obtained using the sealed tube method by heating the elements Ba, Ge, U, and Te at 1173 K. A yield of Ba2GeTe3(Te2) crystals was about 25% (based on Ge). The reaction of stoichiometric amount of Ba, Ge, and Te did not yield the target Ba2GeTe3(Te2) phase, but instead a mixture of the phases BaTe, GeTe, Ba2Ge2Te5, and unreacted Te powder was obtained. The reaction of Ba, Ge, and Te in the same ratio as loaded in the initial reaction that involved uranium also tried using the same heating profile. However, the reaction product showed the presence of Ba2Ge2Te5 as a major phase. Our further attempts to synthesize this compound by varying the heating conditions failed. It appears that this compound cannot be synthesized without the participation of U in the reaction. There are other known examples of compounds such as CsTi5Te8 [START_REF] Gray | Synthesis and structure of CsTi5Te8: Relation to the TlV5S8, TlCr3S5, and similar channel structures[END_REF] that can only be synthesized by U-assisted reactions.

The single-crystal X-ray diffraction study of Ba2GeTe3(Te2) shows that it crystallizes with four formula units in the space group Pnma of the orthorhombic system in a cell of dimensions a = 6.790(1), b = 23.720 [START_REF] Kim | Characterization of New Infrared Nonlinear Optical Material with High Laser Damage Threshold, Li2Ga2GeS6[END_REF], and c = 6.930(1) Å. The site symmetries of the atoms in the asymmetric unit are: Ba1 (1), Ge1 (.m.), Te1 (1), Te2 (1), and Te3 (.m.). The crystal structure of Ba2GeTe3(Te2) is one-dimensional (Fig. 1). ] composition (Fig. 2). The Ge1Te distances are in the range of 2.571(1)2.683(1) Å. These distances are in good agreement with the corresponding distances in some known related compounds (Table 3). The Ba atoms in Ba2GeTe3(Te2) structure are surrounded by nine Te atoms (4 × Te1, 4 × Te2, and 1 × Te3) in a distorted tricapped trigonal prism geometry (Fig. 3). This structure also features homoatomic TeTe bonding between two Te2 atoms forming a Te2 2 dimer with a distance of 2.7924(5) Å. This distance is comparable to the single bond TeTe distances found in Cs4GeTe6 (2.735(1) and 2.746(1) Å) [START_REF] Brinkmann | Cs4GeTe6, ein o-Pertelluridogermanat(IV)[END_REF], Cs2GeTe4 (2.780(1) Å) [START_REF] Sheldrick | Methanolothermale Synthese und Kristallstruktur von Diselenido-und Ditelluridogermanaten(IV) der Alkalimetalle K-Cs[END_REF], Tl2GeTe5 (2.905(1) Å) [START_REF] Toure | Sur le ternaire Tl-Ge-Te: Etude structurale de la phase Tl2GeTe5[END_REF], K2GeTe4 (2.736(1) Å) [START_REF] Eisenmann | K2GeTe4, ein pertelluridogermanat mit kettenstruktur[END_REF], α-ThTe3 (2.7631(8) Å) [START_REF] Prakash | Synthesis and Crystal Structure of α-ThTe3[END_REF], and BaThTe4 (2.766(1) Å) [START_REF] Prakash | Syntheses, Crystal Structures, Resistivity Stuides, and Electronic Properties of Three New Barium Actinide Tellurides: BaThTe4, BaUTe4, and BaUTe6[END_REF].

The charge balance in this ternary Ba2GeTe3(Te2) compound can be achieved with 2 × Ba 2+ , 1 × Ge 4+ , 3 × Te 2 , and 1 × Te2 2 . Hence, the chemical formula of this closed shell Zintl compound is best described as Ba2GeTe3(Te2).

Electronic structure

In Fig. 4 we present our computed total (upper plot) and partial (lower plots) density of states (DOS) for Ba2GeTe3(Te2), as obtained with the HSE functional. Although relatively small, the total density has a finite value at the Fermi level; thus Ba2GeTe5 is a metal. From our partial density of states, it can be seen that Ge-and Te-derived states contribute the most around the Fermi level, while the contribution from Ba is almost negligible.

Conclusions

Crystals of the new ternary polytelluride, Ba2GeTe3(Te2), have been obtained by the solid-state "U-assisted" reaction of U, Ba, Ge, and Te at 1173 K. The single crystal X-ray study at 100(2) K indicates that Ba2GeTe3(Te2) crystallizes in space group Pnma of the orthorhombic crystal system with four units in a cell with constants of a = 6.7900 [START_REF] Brinkmann | Ba2Ge2Te5, ein neues Kettentelluridogermanat(III)[END_REF] Å, b = 23.720(5) Å, and c = 6.9300(14) Å. Charge balance in Ba2GeTe3(Te2) is achieved with 2 × Ba 2+ , 1 × Ge 4+ , 3 × Te 2 , and 1 × Te2 2 : (TeTe = 2.7924(5) Å). The crystal structure of Ba2GeTe3(Te2) can be described as pseudo one-dimensional with zig-zag chains of 1 ∞[GeTe3 2 ], where each Ge atom, which has m symmetry, is coordinated to four Te atoms that form a distorted tetrahedron. Each Ba atom is surrounded by nine Te atoms in a distorted tricapped trigonal prism. The Te2 2 units and Ba 2+ cations separate the 1 ∞[GeTe3 2 ] chains. Ba2GeTe3(Te2) is predicted to be a metal from the DFT calculations with Ge and Te-derived states contributing the most around the Fermi level.

Supporting information

The crystallographic CIF file for Ba2GeTe3(Te2) [i.e., Ba2GeTe5] has been deposited as entry CCDC-1935898 (https://www.ccdc.cam.ac.uk). a Some distances have been rounded for comparison. All the compounds contain germanium in +4 oxidation state.

λ = 0.71073 Å, T = 100(2) K. b R(F) = Σ ||Fo||Fc|| / Σ|Fo| for Fo 2 > 2σ(Fo 2 ). c Rw(Fo 2 ) = {Σ [w(Fo 2  Fc 2 ) 2 ] / ΣwFo 4 } 1/2 . For Fo 2 < 0, w -1 = σ 2 (Fo 2 ); for Fo 2  0, w -1 = σ 2 (Fo 2 ) + 0.0074 Fo 2 .

Fig. 1 :

 1 Fig. 1: The unit cell view of Ba2GeTe3(Te2) structure along (a) [100] and (b) [001] directions. The Ba, Ge, and Te atoms are shown in blue, green, and orange, respectively.

Fig. 2 :

 2 Fig. 2: A view of infinite zig-zag chain of [GeTe3 2 ] in Ba2GeTe3(Te2) structure.

Fig. 3 :

 3 Fig. 3: Local coordination environment of Ba atoms in Ba2GeTe3(Te2) structure. The BaTe interactions are shown as black lines and TeTe bond is shown as a red solid line.

Fig. 4 :

 4 Fig. 4: Density of states (DOS) for Ba2GeTe3(Te2).

a

  Some entries have been rounded to three significant figures to facilitate comparisons.

Table 1 . Crystallographic Data and Structure Refinement Details for Ba2GeTe3(Te2) structure a .

 1

		Ba2GeTe5
	Space group	Pnma
	a (Å)	6.7900(14)
	b (Å)	23.720(5)
	c (Å)	6.9300(14)
	V (Å 3 )	1116.1(4)
	Z	4
	ρ (g cm 3 )	5.863
	μ (mm 1 )	22.39
	R(F) b	0.016
	Rw(Fo2) c	0.027
	a

Table 2 : Selected Metrical Data for Ba2GeTe3(Te2). a

 2

	Atom pair	Distances (Å)	Angle (deg)
	Ge1Te1	2.5713(5) × 2	Te1Ge1Te1 130.05(2)
	Ge1Te3	2.6316(7)	Te1Ge1Te3 102.44(1) × 2
		2.6827(7)	Te1Ge1Te3 103.85(1) × 2
			Te3Ge1Te3 114.78(2)
	Te2Te2	2.7924(5)
	Ba1Te1	3.6048(7)
		3.6202(7)
		3.6332(7)
		3.7110(7)

Table 3 :GeTe Distances in Some Related Compounds with 4-Coordinated Ge atoms a .

 3

	Compound	Structure	GeTe (Å)	Ref.
	Ba2GeTe5	one-dimensional	2.571(1)2.683(1) this work
	Cs4GeTe6	zero-dimensional	2.537(3)2.623(2) [31]
	Cs2GeTe4	one-dimensional	2.549(1)2.650(3) [32]
	Tl2GeTe3	zero-dimensional	2.580(3)2.643(3) [37]
	α -Tl2GeTe5 zero-dimensional	2.604(1)2.664(5) [33]
	β-Tl2GeTe5	zero-dimensional	2.616(1)2.639(1) [38]
	K2GeTe4	one-dimensional	2.508(2)2.640(2) [34]

Acknowledgments

9

Use was made of the IMSERC X-ray Facility at Northwestern University, supported by the International Institute of Nanotechnology (IIN).


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Previously, plasma nitriding of 90CrMoV8 steel had shown the improvement in hardness; however the corrosion propensity in wood juice was shown to be increased [25]. Apart from this specific wood juice electrolyte there is no other literature available so far on corrosion studies of this steel in a common electrolyte found in the atmosphere. As it is known that halides accelerate corrosion in Fe based alloys and also these are commonly found atmospheric corrodents, in the present study electrochemical investigation was conducted in NaCl electrolyte.

Experimental Details

Plasma nitriding process:

Steel samples in as-received condition were cut from a sheet to a dimension of 10 x 10 x 3 mm 3 . The composition of the steel is given as below in the Table 1.

Table 1: Composition of 90CrMoV8 steel Element C Si Mn Cr Mo V F e
Wt. (%) 0.5 1.0 0.5 8.0 1.5 0.5 balance After metallographic polishing and ultrasonic cleaning in acetone, samples were placed in the vacuum chamber of the nitriding reactor and evacuated to 0.5 Pa. Then nitriding was performed in a glow discharge plasma of H2 and N2 (80:20) gas mixture the temperature range of 450-500 o C for 1-8h.

Microstructural characterization:

For the structural characterization the samples after nitriding were exposed to X-ray diffraction (XRD) (XRD -INEL CPS 120 diffractometer -θ/2θ configuration) using Co-kα radiation source. Cross sections of the samples were observed in scanning electron microscope (SEM-model SEM-Jeol JSM-5900) to inspect the nitrided layer. The cross sections after metallographic polishing and etching with Villela's reagent were exposed to SEM.

Electrochemical characterization:

To assess the corrosion resistance property, steel samples before and after plasma nitriding were subjected to detailed corrosion tests by following potentiodynamic polarization. To carry out these tests Biologic SP300 potentiostat with EC Lab software was used. The three electrode corrosion test cell constituted was consisting of the nitrided steel sample as the working electrode with 0.636 cm 2 exposed to the electrolyte, a saturated calomel electrode (SCE) as reference electrode and a platinum counter electrode. All the tests were performed in 3.5% (wt.%) NaCl electrolyte with pH=5.

Results andDiscussion

X-ray diffraction and phase evolution

From the previous studies on similar steel, it was found that there was no significant improvement in micro hardness for the steel samples nitrided for shorter durations within the range of 1-5 h. However, a significant improvement of hardness was shown after nitriding for 6 and 8 h at all the temperatures (450-520 o C) [25]. It can be seen from the following Fig. 1 that, nitriding at lower temperature of 450 o C improved the hardness to ~ 1150 Hv however, the case depth formed was shallow which was only ~ 40 µm thick [25].

With the view to obtain a wider case depth and hence to prolong the tool life, in the present studies, nitriding was performed at the temperature of 500 o C by varying the exposure time for 6 h and 8 h. Fig. 2 shows the XRD patterns of as-received and nitrided steels at 500 o C for different treatment times (6 h and 8 h). In the nitrided samples, mainly the peaks of Fe and γ′-(Fe, Cr) 4 N and ε-(Fe, Cr) 2-3 N were revealed. Fe and other nitride peaks are much broadened, which is indicative of nitrogen incorporation in the crystal lattice. Fig. 3 represents SEM micrographs of the cross sections of the nitrided steel at 500 o C for 6 and 8 h. It is evident that a very thin and uniform compound layer is formed on top of the surface. After this a diffusion/hard layer was formed. This thin compound layer is known to be beneficial from the corrosion resistance point of view. The bright particles on the surface are formed by agglomeration of iron nitrides particles which depend on the nitrogen concentration [20].

  Fig. 1 M 5

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