Plan
Comptes Rendus
no. 6-7

Account / Revue
Salts of a sandwich-type bis 9-tungstoarsenate(III) tris(oxorhenate(V)) polyoxoanion as precursors to tungsten–rhenium bronzes
Alexei Besserguenev1  ; Michael T. Pope1
1 Department of Chemistry, Box 571227, Georgetown University, Washington, DC 20057-1227, USA
Comptes Rendus. Chimie, Volume 8 (2005) no. 6-7, pp. 933-955.

Résumés

Potassium and ammonium salts of the tungstoarsenate anion [(ReVO)3(AsW9O33)2]9– (1) have been isolated and characterized by elemental analysis, infrared and 183W NMR spectroscopy. The five-line NMR spectrum indicates a sandwich structure of C2v symmetry analogous to that found for [As2W21O69(H2O)]6–. Thermal treatment of both salts of 1 at temperatures to 900 °C leads to materials in which ~95% of the rhenium is retained, in contrast to the Keggin-derived K4PW11ReO40 which loses rhenium at ~500 °C. Based upon powder diffraction analysis, following quantitative loss of As2O3 at 420–430 °C, the following phases were identified in samples heated to 600 and 725 °C. From the potassium salt, a monoclinic K2W3O10-type bronze, and from the ammonium salt a mixture of hexagonal and tetragonal WO3 bronzes. Reflections corresponding to ReO2, ReO3, solid solutions of ReO3 in WO3, and Re2O7 were absent. The incorporation of rhenium into tungsten bronze phases suggests that technetium might behave similarly, opening a possible route to storage of 99Tc waste.

Les sels d’un sandwich-type bis 9-tungstoarsenate(III) tris(oxorhenate(V)) polyoxoanion en tant que précurseurs des bronzes tungstène–rhénium. Les sels de potassium et ammonium de l’anion tungstoarsenate(III) [(ReVO)3(AsW9O33)2]9– (1) ont été isolés et caractérisés par analyse élémentaire, spectroscopie infrarouge et 183W RMN. Le spectre RMN cinq lignes indique une structure sandwich de symétrie C2v analogue à la structure connue de [As2W21O69(H2O)]6–. Le traitement thermique des sels de 1 aux températures jusqu’à 900 °C conduit à des matériaux qui conservent 95% de leur contenu en rhénium, contrairement à K4PW11ReO40, de structure Keggin, qui perd son rhénium à ~500 °C. Sur la base de l’analyse par diffraction sur poudre, les phases suivantes ont été identifiées dans les échantillons chauffés à 600 et 725 °C après perte quantitative de As2O3 à 420–430 °C. On a obtenu, à partir du sel de potassium, un bronze monoclinique de type K2W3O10 et, à partir du sel d’ammonium, un mélange de bronzes WO3 hexagonaux et tétragonaux. Les réflexions correspondant à ReO3, à ReO2, aux solutions solides de ReO3/WO3 et à Re2O7 étaient absentes. L’incorporation du rhénium dans les phases de bronzes de tungstène suggère que le technétium se comporte de la même façon, qui ouvrant une possible voie au stockage des déchets de 99Tc.

Métadonnées
Reçu le :
Accepté le :
Publié le :
DOI : 10.1016/j.crci.2004.10.005
Keywords: Polyoxometalates, Tungsten, Rhenium, Tungsten bronzes
Mots clés : Polyoxometallates, Tungstène, Rhénium, Bronzes de tungstène
Alexei Besserguenev 1 ; Michael T. Pope 1

1 Department of Chemistry, Box 571227, Georgetown University, Washington, DC 20057-1227, USA 
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     title = {Salts of a sandwich-type bis {9-tungstoarsenate(III)} {tris(oxorhenate(V))} polyoxoanion as precursors to tungsten{\textendash}rhenium bronzes},
     journal = {Comptes Rendus. Chimie},
     pages = {933--955},
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Alexei Besserguenev; Michael T. Pope. Salts of a sandwich-type bis 9-tungstoarsenate(III) tris(oxorhenate(V)) polyoxoanion as precursors to tungsten–rhenium bronzes. Comptes Rendus. Chimie, Volume 8 (2005) no. 6-7, pp. 933-955. doi : 10.1016/j.crci.2004.10.005. https://comptes-rendus.academie-sciences.fr/chimie/articles/10.1016/j.crci.2004.10.005/

Version originale du texte intégral

1 Introduction

The use of polyoxometalates [1–4] as potential sequestering agents and/or as sources of storage matrices (‘waste forms’) for radioactive waste has attracted attention for several years. One particularly troubling isotope is technetium-99 with a half-life of ~105 years that is readily transformed to the volatile heptaoxide during the vitrification processes currently envisioned for production of waste forms, or is converted to soluble pertechnetates that have high mobility in ground waters. We have recently been investigating the potentialities of polyoxometalates for the technetium problem, using in the first instance rhenium as a non-radioactive surrogate [5]. In related studies we have shown that tungsten bronzes incorporating lanthanide and actinide ions are efficiently produced by thermal decomposition at relatively low temperatures of the corresponding polyoxotungstate complexes [6]. The inertness of tungsten bronzes make them possible attractive targets for waste forms.

With the exception of hexametalate-supported triscarbonylrheniun(I) complexes prepared under nonaqueous [7] or aqueous [5] conditions, previously-characterized rhenium-containing polyoxometalates incorporate rhenium, usually in oxidation state V, in place of one or two tungsten(VI) atoms in Keggin [8], Wells-Dawson [9] or decatungstate [10] structures. Examples of analogous technetium(V) Keggin derivatives have also been described [11].

We report here the synthesis and characterization of a new type of tungstorhenate derivative with relatively high rhenium content (Re/W = 1:6). Thermal decomposition of potassium and ammonium salts of the new complex does not result in loss of rhenium at temperatures up to 900 °C, and leads to new tungsten–rhenium bronzes.

2 Experimental

2.1 Syntheses

Potassium pentachloro(oxo)rhenate(V) was prepared by reduction of 55% perrhenic acid (Alfa Aesar) with hydriodic acid according to the method of Ivanov-Emin et al. [12]. Isolated yield, 60%; IR, 940 cm–1 (Re=O). Potassium and ammonium salts of [PW11O39]7– were isolated from stoichiometric mixtures of PO43– and WO42– adjusted to pH 5.2 and their identities confirmed by 31P-NMR and IR. Sodium 9-tungstoarsenate(III), Na9AsW9O33·nH2O (AsW9) was prepared according to the literature [13]. IR, 935(vs), 898(vs), 783(vs), 727(vs), 515(w), 472(w), 443(w), 411(w) cm–1.

The potassium salt of [(ReO)3(AsW9O33)2]9– (K-1) was prepared as follows. Potassium pentachloro(oxo)rhenate(V) (K2ReOCl5, 0.67 g, 1.5 mmol) was added to a solution of potassium thiocyanate (KSCN, 1.95 g, 20 mmol) in 20 ml of deoxygenated water. After the rhenium complex had dissolved completely, sodium perchlorate (NaClO4, 3.04 g) was added and the mixture was cooled to 5 °C in an ice bath. A white precipitate of potassium perchlorate was filtered off. Sodium 9-tungstoarsenate(III) (2.74 g, 1 mmol) was added to the solution of the rhenium(V) complex. The color of the reaction mixture color changed from green to red-brown and the mixture was stirred for 30 min at room temperature. Any undissolved AsW9 was filtered off. Saturated potassium chloride solution (20 ml) was added to the reaction mixture and it was cooled to 5 °C in an ice bath. A dark brown precipitate (1.15 g, 40%) was filtered off and suction dried. Anal. Found (Calc. for K9[(ReO)3(AsW9O33)2]·23 H2O): K 6.1 (6.0), Na 0.3 (0), Re 9.76 (9.49), As 2.57 (2.54), W 56.1 (56.2). An ammonium salt (NH4-1) was prepared similarly. The product (yield 0.85 g, 30%) was precipitated by addition of 20 ml of 5 M (NH4)Cl.

The water-soluble potassium salt of [PW11ReO40]4– (‘PW11Re’), previously only known as a tetra-n-butylammonium salt [8], was prepared by dissolving K7PW11O39·15 H2O, (6.44 g, 2 mmol) in 60 ml of a 0.06 M-acetate buffer (pH 4) that had been deaerated with a stream of nitrogen for 30 min. The solution was heated to 50 °C and the K2ReOCl5 (0.92 g, 2 mmol) was added. The reaction mixture was heated at 50 °C under a nitrogen atmosphere for 1.5 h. Saturated potassium chloride solution (80 ml) was added and the resulting purple precipitate (1.45 g) was filtered off. 31P-NMR showed an impurity (probably unreacted PW11) at –10.9 ppm, as well as the major product at –15.1 ppm. The product was dissolved in 40 ml of deaerated water; the suspension was heated to ~50 °C until the solid dissolved completely and saturated potassium chloride solution was added and the mixture cooled to 5 °C in an ice bath. The precipitate (1.29 g, 21%) was collected and suction dried. The procedure was repeated two times until only one peak (–15.1 ppm) was observed in the 31P-NMR spectrum. Tungsten-183 NMR: δ/ppm (relative intensity): –99 (2), –115 (2), –116 (1), –207 (2), –274 (2), –290 (2).

2.2 Instrumental

Infrared spectra were measured on a Nicolet 7000 spectrometer from 4000 to 400 cm–1 with resolution of 2 cm–1. All samples were prepared as KBr pellets. Thermogravimetric analysis (TGA) curves were obtained on a TGA 2050 analyzer (TA Instruments) from room temperature to 950 °C under a nitrogen atmosphere. Alumina pans were used for all experiments. Differential scanning calorimetry (DSC) was recorded on a DSC 2910 calorimeter (TA Instruments) from room temperature to 725 °C under a nitrogen atmosphere. Platinum pans were used for all measurements. Cyclic voltammetry was performed on a BAS 100A electrochemical analyzer using a glassy carbon working electrode and platinum counter electrode using a sweep rate of 500 mV s–1. Potentials are reported with respect to a Ag/AgCl reference electrode.

NMR spectra were recorded on Bruker AM 300 (31P, 183W) or Varian Mercury 300 (31P) spectrometers. Typical parameters for phosphorus-31 NMR spectra were: offset frequency, 121.497 MHz; bandwidth, 3650 Hz and repetition rate, 0.6 Hz. Chemical shifts were measured relative to 85% phosphoric acid. Typical parameters for 183W NMR spectra were: offset frequency, 12.504 MHz; bandwidth, 5 KHz and repetition rate, 0.4 Hz. Chemical shifts were measured relative to 2 M Na2WO4. For collection of 183W-NMR spectra potassium salts of PReVW11 and 1 were exchanged to lithium salts using lithium perchlorate (LiClO4). For the exchange, 1–2 g of the polyoxometalate sample, a stoichiometric amount of LiClO4 and 2 ml of D2O were mixed together and stirred for 30 min. Insoluble KClO4 was filtered off and the filtrate was used to collect the NMR spectrum.

Powder diffraction patterns were collected on a RIGAKU R-Axis Rapid XRD automated powder diffractometer equipped with an Image Plate detector. For a typical sample preparation, fine powder was mixed with vaseline and the mixture was smeared onto the surface of a single crystal silicon sample holder. The data were collected in reflection geometry and were integrated using the RIGAKU R-Axis Rapid XRD software package, producing diffraction intensity vs. 2θ diffraction angle dependencies. These were processed using the Materials Data Jade 5.0 XRD software package. Final processing using Jade included background subtraction and peak search. Phase identification was performed by comparing peaks found with powder diffraction patterns of pure compounds stored in the database provided with the Jade software package.

3 Results and discussion

3.1 Synthesis and characterization of 1

Since several examples of sandwich complexes of the type [M3(AsW9O33)2]12– (e.g. with M = Mn(H2O), Co(H2O), Ni(H2O), VO2+, WO4+) are known [14,15], reaction of {ReO3+} and AsW9 in a 3:2 molar ratio proceeds straightforwardly. It is necessary, however, to prevent rapid hydrolysis of [ReOCl5]2– in aqueous solution by complexation with excess thiocyanate, and to exchange sodium for potassium in the resulting solution to allow for ready subsequent dissolution of AsW9. Although crystals suitable for X-ray structural determination have not yet been obtained, there is little doubt about the composition and structure of 1. The molecular formula is supported by elemental analysis of the potassium salt and the infrared spectrum in the metal-oxygen stretching region is virtually identical to that of Na9AsW9O33 indicating the presence of AsW9 building blocks. The 183W NMR spectrum of 1 shows five lines of relative intensity 1:2:2:2:2 at –79, –95, –102, –134, and –145 ppm, respectively, instead of the anticipated two lines for a complex of D3h symmetry. This result implies that the three sandwiched ReO3+ groups are arranged with one Re–O vector directed towards the interior of the anion and the other two directed outwards, i.e. in similar fashion to the corresponding WO4+ groups in [As2W21O69(H2O)]6– (≡[H2O(WO)3(AsW9O33)2]6–) to give a structure of C2v symmetry, see Fig. 1. The presence of an ‘external’ water ligand in 1 cannot be confirmed in the absence of a crystal structure, although it seems probable.

Fig. 1

Two polyhedral representations of the structure of [As2W21O69(H2O)]6– [15] showing the reduction in symmetry from D3h to C2v caused by the orientation of the equatorial WO4+ groups. Small gray spheres, W; open circles, terminal oxygens; gray circle, water ligand. An analogous structure with equatorial ReO3+ groups is proposed for 1.

In contrast to the Keggin-derived rhenium derivatives which display a rich electrochemical behavior corresponding to Re(V)–Re(VI) and Re(VI)–Re(VII) couples [8], cyclic voltammograms of 1 show only an irreversible anodic feature at +1.1 V vs. Ag/AgCl. We presume that the proximity of the rhenium centers in 1 allows facile electron transfer and effective disproportionation of any Re(VI), e.g. via

Re(VI) + Re(V) → Re(IV) + Re(VII) {ReO4–}

Attempts to oxidize 1 by addition of bromine water led to immediate decolorization.

3.2 Thermal decomposition of 1

We have previously shown that ammonium salts of polyoxotungstates containing lanthanide and actinide heteroatoms may be thermally decomposed to yield cubic tungsten bronzes that might be suitable as waste forms for lanthanide and actinide radioisotopes [6]. We, therefore, decided to explore the thermal behavior of rhenium-containing polytungstates as models for the corresponding technetium species. Several years ago Sleight and Gillson [16] had investigated solid solutions of WO3 and ReO3, which are isoelectronic with the cubic bronzes MxWO3.

Comparative thermogravimetric scans of K4[PW11ReO40] and K3[PW12O40] revealed that the former salt lost its rhenium content, presumably as Re2O7, at temperatures above 500 °C. In contrast both K-1 and NH4-1 yielded mixed rhenium–tungsten bronzes. Fig. 2 shows the TGA and DSC results for these two salts. Following loss of crystal water below 200 °C for both salts and volatilization of NH3 by 400 °C. The exothermic DSC peaks at 417 °C (K) and 433 °C (NH4) correspond to weight losses of 3.56% and 3.88%, respectively, attributed to the decomposition of the anion via loss of As2O3 (ca. 3.33% and 3.46%). No other significant weight loss was observed up to 950 °C. Between 600 and 950 °C the sample lost 0.31% (NH4) 1.1% (K) of its initial weight, indicating that most of the rhenium was still left in the sample. Elemental analysis of the ammonium salt heated to 950 °C in the TGA apparatus indicated that ~95% of the rhenium was left in the sample after heating. Anal. (Calc. for 1.5(Re2O5):18(WO3)): Re 10.89 (11.51); W 67.7 (68.2).

Fig. 2

TGA and DSC curves of (NH4)9[(ReO)3(AsW9O33)2]·nH2O (A) and K9[(ReO)3(AsW9O33)2]·nH2O.

Phase identification in the samples of potassium and ammonium salts of 1 was carried out using powder diffraction. Analysis of powder diffraction patterns of the potassium salt samples decomposed at 600 and 725 °C (Fig. 3) indicates the presence of a monoclinic compound similar to potassium tritungstate, K2W3O10 (Fig. 4) [17]. Most of the observed diffraction peaks correspond to the powder diffraction pattern of K2W3O10. Observed differences in intensities of two low-angle peaks result from obstruction by the sample holder and absorption of diffracted rays by cavities on the real (not smooth) surface of the sample. Some peaks, which were not found in the diffraction pattern of K2W3O10 could be still indexed on the basis of the K2W3O10 monoclinic unit cell (see Section 5, Tables S1 and S2). Four reflections which could not be indexed on the basis of the K2W3O10 monoclinic cell do not correspond to any of the known rhenium oxides: ReO2 [18], ReO3 [19], Re2O7 [20] or to the ReO3–WO3 solid solutions [16]. The unit cell constants of the monoclinic cell were refined based on the assigned Miller indexes and are given in Table 1 (see Tables S7–S15 in Section 5). The refinement indicated no significant differences between unit cell parameters of samples heated to 600 and 725 °C.

Fig. 3

Powder diffractograms of samples of K9[(ReO)3(AsW9O33)2]·n H2O, heated to 600 °C (A), 725 °C (B), and 900 °C (C). (*) Unidentified reflections. (Si) reflections from silicon sample holder.

Fig. 4

Monoclinic K2W3O10 structure. Crystallographic b-axis perpendicular to the plane of paper. Potassium cations (gray spheres) occupy positions in the large channels formed by tungsten oxide framework.

Table S1

Phase identification for a sample of K9[(ReO)3(AsW9O33)2]·n H2O heated to 600 °C in TGA apparatus under N2 atmosphere

2θ(obs)d(obs) (Å)I(obs) (%)Phase IDd(card) (Å)I(card) (%)hkl2θ(card)Δ(2θ)
11.7397.53238.8K2W3O107.540070.0  00211.727–0.012
12.0997.308923.4K2W3O107.320070.0–10212.081–0.018
16.1895.47061.8K2W3O105.450010.0–20116.250  0.062
16.9685.22096.1K2W3O105.220020.0–10316.971  0.003
17.4795.069617.0K2W3O105.060040.017.512  0.034
19.7984.48063.9K2W3O104.480020.0  20119.801  0.003
20.5794.31242.7
21.4284.14335.2
21.8584.062836.3K2W3O104.060070.0  10321.873  0.015
22.2683.988914.8K2W3O103.990040.0–10422.262–0.006
22.9883.865699.3K2W3O103.860080.0  01023.022  0.034
23.6383.76089.7K2W3O103.760020.0  00423.643  0.005
24.5783.619025.8K2W3O103.610060.0–11124.640  0.063
26.0183.421944.1K2W3O103.410060.0–11226.110  0.093
27.2273.27266.0K2W3O103.270040.0  10427.249  0.022
28.1873.163395.4K2W3O103.1600100.0–21128.217  0.030
28.9073.0861100.0K2W3O103.0800100.0–30428.966  0.059
29.4173.033873.1K2W3O103.030080.0  00529.454  0.037
30.0472.971611.2K2W3O102.970020.030.063  0.016
31.8972.803338.8K2W3O102.800070.0  30231.936  0.039
33.1372.701220.4K2W3O102.700060.0  01433.152  0.015
33.8072.649225.2K2W3O102.630060.0–31234.061  0.254
35.087a2.55552.0K2W3O102.548––31335.199  0.112
36.0562.48891.0
36.6762.448329.6K2W3O102.440060.0–30636.805  0.129
37.216a2.414023.0K2W3O102.409––31437.289  0.073
37.6562.386821.3K2W3O102.400060.0–21537.441–0.215
38.7862.31984.6K2W3O102.320020.020538.783–0.003
39.716a2.26761.1K2W3O102.264–  31239.783  0.067
40.376a2.23208.9K2W3O102.240––31540.218–0.158
41.1762.190512.0K2W3O102.186060.0  21441.265  0.089
42.8452.10892.5K2W3O102.120010.0–50442.611–0.234
43.8652.06225.5K2W3O102.050040.0  50044.141  0.275
44.666a2.02712.6K2W3O102.022––41544.775  0.109
45.5251.99086.5K2W3O101.986020.0  10745.642  0.117
46.275a1.96033.0K2W3O101.9609–  11646.260–0.015
46.9351.934323.9K2W3O101.928020.0–50647.097  0.162
47.7251.90418.2K2W3O101.907020.0–41647.647–0.078
48.295a1.88296.3K2W3O101.8851–  00848.235–0.060
50.0451.821117.9K2W3O101.830020.0–40849.785–0.259

a Reflections were indexed manually by calculating d-spacings and 2θ-angles for monoclinic cell of K2W3O10.

Table S2

Phase identification for a sample of K9[(ReO)3(AsW9O33)2]·n H2O heated to 725 °C in DSC apparatus under N2 atmosphere

2θ(obs)d(obs) (Å)I(obs) (%)Phase IDd(card) (Å)I(card) (%)hkl2θ(card)Δ(2θ)
11.8097.48781.6K2W3O107.540070.0  00211.727–0.082
12.1997.24935.2K2W3O107.320070.0–10212.081–0.118
14.0196.31203.9
16.3495.41721.9K2W3O105.450010.0–20116.250–0.099
17.0095.20862.3K2W3O105.220020.0–10316.971–0.037
17.5085.06127.5K2W3O105.060040.017.512  0.004
19.8294.47383.0K2W3O104.480020.0  20119.801–0.028
21.5384.122518.1
21.9484.046326.4K2W3O104.060070.0  10321.873–0.075
22.3583.973012.7K2W3O103.990040.0–10422.262–0.096
23.0783.850855.1K2W3O103.860080.0  01023.022–0.056
23.7573.74218.3K2W3O103.760020.0  00423.643–0.114
24.6683.606017.9K2W3O103.610060.0–11124.640–0.027
26.1183.409130.1K2W3O103.410060.0–11226.110–0.007
27.3173.26207.4K2W3O103.270040.0  10427.249–0.068
28.2773.1534100.0K2W3O103.1600100.0–21128.217–0.060
29.0073.075776.3K2W3O103.0800100.0–30428.966–0.041
29.5573.019756.1K2W3O103.030080.0  00529.454–0.103
30.1772.959113.5K2W3O102.970020.030.063–0.113
30.566a2.92232.2K2W3O102.914–  21130.657  0.091
32.0172.793132.1K2W3O102.800070.0  30231.936–0.081
33.2472.692517.7K2W3O102.700060.0  01433.152–0.094
33.9472.638619.6K2W3O102.630060.0–31234.061  0.115
34.926a2.56681.7K2W3O102.569–  40034.889–0.037
35.276a2.54221.8K2W3O102.543––40435.260–0.016
36.196a2.47961.3K2W3O102.480–  30336.189–0.007
36.8062.439933.5K2W3O102.440060.0–30636.805–0.001
37.3462.405922.3K2W3O102.400060.0–21537.441  0.095
37.7962.378218.5K2W3O102.370060.0–40537.933  0.137
38.9062.31294.8K2W3O102.320020.0  20538.783–0.124
39.736a2.26651.8K2W3O102.267––20739.718–0.018
40.756a2.21217.6K2W3O102.221, 2.203–  4   30   02, 440.575, 40.923–0.177,   0.167
41.3162.183413.9K2W3O102.186060.021441.265–0.051
42.5562.12262.5K2W3O102.120010.0–50442.611  0.055
43.015a2.10103.4K2W3O102.106–  01642.902–0.113
44.0352.05477.1K2W3O102.050040.0  50044.141  0.105
44.786a2.02204.6K2W3O102.022––41544.775–0.011
45.3551.99793.9K2W3O101.996040.0–20845.401  0.045
45.7051.98346.9K2W3O101.986020.0  10745.642–0.063
46.475a1.95235.8K2W3O101.951––11746.500  0.025
47.1551.925853.9K2W3O101.928020.0–50647.097–0.058
47.8551.899210.5K2W3O101.907020.0–41647.647–0.208
48.4651.87677.4K2W3O101.870020.0  22248.650  0.186
49.5651.83764.7K2W3O101.830020.0–40849.785  0.220

a Reflections were indexed manually by calculating d-spacings and 2θ-angles for monoclinic cell of K2W3O10.

Table 1

Phase composition of ammonium and potassium salts of [(ReO)3(AsW9O33)2]9– following thermal degradation

Decomposition temperature (°C)K-salt(NH4)-salt
600Potassium bronze (K2W3O10-type): a = 10.91(2) Å, b = 3.881(4) Å, c = 15.97(2) Å, β = 109.0(1)°Mixture of two phases: hexagonal bronze: a = 7.324(7) Å, c = 7.519(7) Å; tetragonal bronze: a = 5.243(2) Å, c = 3.879(1) Å
725Potassium bronze (K2W3O10-type): a = 10.86(2) Å, b = 3.866(5) Å, c = 15.96(2) Å, β = 109.0(1)°Mixture of two phases: hexagonal bronze: a = 7.2890(5) Å, c = 7.5644(6) Å; tetragonal bronze: a = 5.2457(9) Å, c = 3.868(1) Å
900Potassium bronze (WO3· ½ H2O pyrochlore-type): a = 10.172(3) ÅMixture of two phases: hexagonal bronze: a = 7.316(4) Å, c = 7.545(3) Å; orthorhombic bronze: a = 7.372(5) Å, b = 7.499(5) Å, c = 3.860(3) Å
Table S7

Unit cell refinement results of a sample of K9[(ReO)3(AsW9O33)2]·n H2O heated to 600 °C in TGA apparatus under N2 atmosphere

hkl2θ(cal)2θ(obs)Δ(2θ)d(cal) (Å)d(obs) (Å)Δ(d) (Å)
  00211.71211.739–0.0277.54967.5323  0.0173
–10212.05712.099–0.0427.33447.3089  0.0255
–20116.23616.1890.0485.45475.4706–0.0159
–10316.88916.968–0.0795.24525.2209  0.0243
–20217.36817.479–0.1115.10185.0696  0.0321
  20119.89719.7980.0994.45864.4806–0.0220
  10322.00521.8580.1474.03604.0628–0.0268
–10422.27322.2680.0053.98803.9889–0.0009
  01022.89622.988–0.0923.88093.8656  0.0153
  00423.54923.638–0.0893.77483.7608  0.0140
–11124.52024.578–0.0573.62743.6190  0.0083
–11225.95326.018–0.0643.43033.4219  0.0083
–21128.19728.1870.0103.16223.1633–0.0011
–30428.85528.907–0.0523.09163.0861  0.0055
  00529.55629.4170.1393.01983.0338–0.0139
  30231.88131.897–0.0162.80472.8033  0.0014
  01433.07833.137–0.0592.70592.7012  0.0047
–31233.92233.8070.1162.64052.6492–0.0088
–30636.73036.6760.0542.44482.4483–0.0035
–21537.24737.2160.0312.41202.4140–0.0019
–40537.75837.6560.1022.38052.3868–0.0062
  21441.08641.176–0.0902.19512.1905  0.0046
  50043.83443.865–0.0312.06362.0622  0.0014
–20845.44945.525–0.0771.99401.9908  0.0032
–50646.95646.9350.0211.93351.9343–0.0008
–41647.59547.725–0.1301.90901.9041  0.0049
  02248.39248.2950.0971.87941.8829–0.0036
Table S8

Unit cell refinement results for a sample of K9[(ReO)3(AsW9O33)2]·n H2O heated to 725 °C in DSC apparatus under N2 atmosphere

hkl2θ(cal)2θ(obs)Δ(2θ)d(cal) (Å)d(obs) (Å)Δ(d)
  00211.72111.809–0.0897.54417.4878  0.0563
–10212.07212.199–0.1277.32507.2493  0.0757
–20116.31016.349–0.0405.43025.4172  0.0131
–10316.90017.009–0.1095.24205.2086  0.0334
–20217.42517.508–0.0835.08515.0612  0.0239
  20119.98519.829  0.1574.43914.4738–0.0348
  10322.05121.948  0.1034.02774.0463–0.0187
–10422.28322.358–0.0753.98633.9730  0.0132
  01022.98323.078–0.0943.86643.8508  0.0156
  00423.56623.757–0.1913.77213.7421  0.0299
–11124.61324.668–0.0553.61403.6060  0.0080
–11226.03826.118–0.0793.41933.4091  0.0102
–21128.31228.277  0.0353.14963.1534–0.0038
–30428.92729.007–0.0803.08413.0757  0.0084
  00529.57829.557  0.0213.01773.0197–0.0021
  30232.01732.017  0.0002.79312.7931  0.0000
  01433.15333.247–0.0942.70002.6925  0.0074
–31234.06133.947  0.1142.63002.6386–0.0086
–30636.77936.806–0.0282.44172.4399  0.0018
–21537.32337.346–0.0232.40732.4059  0.0014
–40537.86137.796  0.0652.37432.3782–0.0039
  21441.21541.316–0.1012.18852.1834  0.0051
–50442.68342.556  0.1272.11662.1226–0.0060
  50044.05844.035  0.0232.05362.0547–0.0010
–20845.46945.355  0.1141.99311.9979–0.0048
  10745.70445.705–0.0011.98351.9834  0.0001
–50647.09347.155–0.0621.92811.9258  0.0024
–41647.72047.855–0.1351.90431.8992  0.0051
  02248.57648.465  0.1121.87271.8767–0.0040
–40849.74949.565  0.1841.83131.8376–0.0064
Table S9

Unit cell refinement results for a sample of K9[(ReO)3(AsW9O33)2]·n H2O heated to 900 °C in TGA apparatus under N2 atmosphere

h k l2θ(cal)2θ(obs)Δ(2θ)d(cal) (Å)d(obs) (Å)Δ(d) (Å)
1 1 115.07315.079–0.0065.87305.8707  0.0022
3 1 129.09128.937  0.1543.06713.0830–0.0160
2 2 230.41530.307  0.1082.93652.9467–0.0102
4 0 035.26335.246  0.0172.54312.5442–0.0012
3 3 138.54638.486  0.0602.33372.3372–0.0035
5 1 146.34146.255  0.0861.95771.9611–0.0034
4 4 050.72750.635  0.0921.79821.8013–0.0031
5 3 153.22953.185  0.0451.71941.7208–0.0013
5 3 359.54459.513  0.0311.55131.5520–0.0007
6 2 260.30460.332–0.0281.53351.5329  0.0006
4 4 463.28663.174  0.1131.46821.4706–0.0024
7 1 165.47365.373  0.1001.42441.4263–0.0019
7 3 171.13371.052  0.0801.32431.3256–0.0013
8 0 074.57174.762–0.1901.27151.2688  0.0028
7 5 181.95982.082–0.1231.17461.1731  0.0014
6 6 282.62282.581  0.0411.16681.1673–0.0005
8 4 085.26585.350–0.0851.13731.1364  0.0009
Table S10

Unit cell refinement results of hexagonal phase found in a sample of (NH4)9[(ReO)3(AsW9O33)2]·n H2O heated to 600 °C in TGA apparatus under N2 atmosphere

h k l2θ(cal)2θ(obs)Δ(2θ)d(cal) (Å)d(obs) (Å)Δ(d) (Å)
1 0 013.95114.009–0.0586.34286.3165  0.0263
0 0 223.64823.938–0.2903.75933.7143  0.0449
2 0 028.11428.077  0.0363.17143.1754–0.0040
1 1 234.15334.107  0.0462.62312.6266–0.0035
2 0 237.05636.896  0.1602.42402.4342–0.0101
2 1 244.80145.085–0.2842.02132.0092  0.0121
2 2 049.75649.685  0.0711.83101.8335–0.0024
3 1 051.93551.855  0.0801.75921.7617–0.0025
3 1 153.44753.264  0.1831.71291.7184–0.0055
1 1 454.85655.094–0.2381.67221.6656  0.0067
4 0 058.12557.994  0.1321.58571.5890–0.0033
4 0 263.63563.473  0.1611.46101.4644–0.0033
4 1 168.92569.013–0.0881.36121.3597  0.0015
2 2 471.93071.882  0.0481.31161.3123–0.0008
4 1 272.74672.772–0.0271.29891.2985  0.0004
0 0 675.86175.792  0.0691.25311.2541–0.0010
3 3 078.25278.491–0.2391.22071.2175  0.0031
5 0 279.71079.691  0.0191.20201.2022–0.0002
2 0 682.74582.651  0.0941.16541.1665–0.0011
3 3 283.12983.166–0.0371.16101.1606  0.0004
Table S11

Unit cell refinement results of tetragonal phase found in a sample of (NH4)9[(ReO)3(AsW9O33)2]·n H2O heated to 600 °C in TGA apparatus under N2 atmosphere

h k l2θ(cal)2θ(obs)Δ(2θ)d(cal) (Å)d(obs) (Å)Δ(d) (Å)
0 0 122.90822.938–0.0303.87893.8739  0.0050
1 1 023.98223.938  0.0443.70763.7143–0.0067
1 0 128.60228.637–0.0353.11843.1146  0.0038
1 1 133.40433.397  0.0082.68022.6808–0.0006
2 0 034.17234.107  0.0662.62172.6266–0.0049
2 0 141.54141.486  0.0552.17212.1749–0.0028
1 2 145.14545.085  0.0592.00672.0092–0.0025
0 0 246.80246.835–0.0331.93941.9382  0.0013
2 2 049.10348.975  0.1281.85381.8584–0.0045
1 0 250.10750.095  0.0121.81901.8194–0.0004
1 1 253.25953.264–0.0051.71851.7184  0.0002
2 2 154.84255.094–0.2521.67261.6656  0.0071
3 0 157.81457.994–0.1801.59351.5890  0.0045
2 0 259.21159.224–0.0121.55921.5589  0.0003
1 3 160.69260.673  0.0181.52461.5251–0.0004
1 2 262.04962.083–0.0341.49451.4938  0.0007
2 3 168.89869.013–0.1141.36171.3597  0.0020
2 2 270.17070.053  0.1181.34011.3421–0.0020
4 0 071.97671.882  0.0941.31081.3123–0.0015
3 0 272.77972.772  0.0071.29841.2985–0.0001
1 0 375.70075.792–0.0921.25541.2541  0.0013
4 0 176.67176.786–0.1151.24191.2403  0.0016
1 1 378.23878.491–0.2531.22091.2175  0.0033
3 3 181.70881.801–0.0931.17761.1765  0.0011
4 2 082.14082.161–0.0211.17251.1722  0.0003
2 3 282.91182.651  0.2601.16351.1665–0.0030
2 0 383.25183.166  0.0851.15961.1606–0.0010
2 1 385.73685.701  0.0351.13221.1326–0.0004
Table S12

Unit cell refinement results of hexagonal phase found in a sample of (NH4)9[(ReO)3(AsW9O33)2]·n H2O heated to 725 °C in DSC apparatus under N2 atmosphere

h k l2θ(cal)2θ(obs)Δ(2θ)d(cal) (Å)d(obs) (Å)Δ(d) (Å)
1 0 014.01814.039–0.0216.31256.3031  0.0094
0 0 223.50223.528–0.0263.78223.7781  0.0041
1 1 127.13726.947  0.1903.28333.3060–0.0227
1 1 234.13634.147–0.0102.62442.6236  0.0008
2 0 237.06836.926  0.1412.42332.4323–0.0089
2 1 244.88045.115–0.2352.01792.0080  0.0100
0 0 448.07347.965  0.1081.89111.8951–0.0040
3 0 249.53249.655–0.1231.83881.8345  0.0043
2 2 050.01150.085–0.0741.82231.8198  0.0025
1 0 450.32750.314  0.0131.81161.8120–0.0004
3 1 052.20351.944  0.2591.75081.7589–0.0081
3 1 153.69253.504  0.1891.70571.7113–0.0056
1 1 454.63154.874–0.2431.67861.6717  0.0069
2 0 456.69756.634  0.0641.62221.6239–0.0017
3 1 258.00157.944  0.0571.58881.5902–0.0014
4 0 058.43158.164  0.2681.57811.5848–0.0066
4 0 263.86063.963–0.1031.45641.4543  0.0021
3 0 466.41266.402  0.0091.40651.4067–0.0002
4 1 169.27569.132  0.1431.35521.3577–0.0024
2 2 471.89171.942–0.0511.31221.3114  0.0008
4 1 273.04272.832  0.2101.29431.2975–0.0032
3 1 473.67773.662  0.0151.28471.2850–0.0002
2 1 574.15374.092  0.0611.27771.2786–0.0009
5 0 075.19775.242–0.0461.26251.2618  0.0007
0 0 675.32175.342–0.0211.26071.2604  0.0003
3 2 375.66875.612  0.0571.25581.2566–0.0008
1 0 677.07876.931  0.1471.23631.2383–0.0020
3 3 078.70078.641  0.0591.21481.2156–0.0008
4 0 478.94878.911  0.0371.21171.2121–0.0005
4 1 379.16579.321–0.1561.20891.2069  0.0020
3 3 179.90979.881  0.0281.19951.1998–0.0004
4 2 181.63881.686–0.0481.17841.1778  0.0006
2 0 682.28282.361–0.0781.17081.1699  0.0009
2 2 582.86782.931–0.0641.16401.1633  0.0007
3 1 584.58384.739–0.1561.14471.1430  0.0017
5 1 085.59485.701–0.1071.13381.1326  0.0011
5 1 186.78586.745  0.0401.12121.1216–0.0004
Table S13

Unit cell refinement results of tetragonal phase found in a sample of (NH4)9[(ReO)3(AsW9O33)2]·n H2O heated to 725 °C in DSC apparatus under N2 atmosphere

h k l2θ(cal)2θ(obs)Δ(2θ)d(cal) (Å)d(obs) (Å)Δ(d) (Å)
0 0 122.97322.938  0.0353.86813.8739–0.0059
1 1 023.97123.978–0.0073.70933.7082  0.0011
1 1 133.44233.407  0.0362.67722.6800–0.0028
2 0 034.15734.147  0.0102.62292.6236–0.0007
2 0 141.56641.516  0.0502.17092.1734–0.0025
1 2 145.16545.115  0.0492.00592.0080–0.0021
0 0 246.94146.855  0.0861.93401.9374–0.0034
2 2 049.07949.035  0.0441.85461.8562–0.0016
1 0 250.23650.314–0.0781.81461.8120  0.0026
1 1 253.38053.294  0.0861.71491.7175–0.0026
2 2 154.85154.874–0.0231.67231.6717  0.0006
1 3 055.33655.304  0.0321.65881.6597–0.0009
3 0 157.82057.944–0.1241.59331.5902  0.0031
2 0 259.31959.094  0.2251.55661.5620–0.0054
1 3 160.69560.693  0.0021.52461.5246  0.0000
1 2 262.15262.003  0.1481.49231.4955–0.0032
2 3 063.93563.963–0.0291.45491.4543  0.0006
2 3 168.89568.922–0.0281.36181.3613  0.0005
2 2 270.25970.322–0.0631.33861.3376  0.0010
4 0 071.93971.942–0.0031.31141.3114  0.0000
3 0 272.86472.832  0.0321.29711.2975–0.0005
1 3 275.43375.452–0.0191.25911.2589  0.0003
1 0 375.93375.921  0.0121.25211.2522–0.0002
4 0 176.66176.672–0.0111.24201.2418  0.0001
3 3 077.06976.931  0.1381.23641.2383–0.0019
1 1 378.46778.521–0.0551.21791.2172  0.0007
4 1 179.18879.321–0.1331.20861.2069  0.0017
3 3 181.69481.686  0.0081.17771.1778–0.0001
4 2 082.09582.061  0.0341.17301.1734–0.0004
2 3 282.98482.931  0.0531.16271.1633–0.0006
2 1 385.95385.970–0.0171.12991.1298  0.0002
Table S14

Unit cell refinement results of hexagonal phase found in a sample of (NH4)9[(ReO)3(AsW9O33)2]·n H2O heated to 900 °C in TGA apparatus under N2 atmosphere

t2θ(cal)2θ(obs)Δ(2θ)d(cal) (Å)d(obs) (Å)Δ(d) (Å)
1 0 013.96614.019–0.0536.33596.3122  0.0237
0 0 223.56023.568–0.0073.77303.7718  0.0011
1 1 024.31224.208  0.1043.65803.6735–0.0155
1 0 227.49127.527–0.0363.24173.2376  0.0041
2 0 028.14528.227–0.0833.16803.1589  0.0091
1 1 234.11034.167–0.0562.62632.6221  0.0042
2 0 237.02337.196–0.1742.42612.4152  0.0109
3 0 042.78142.945–0.1652.11202.1043  0.0077
2 1 244.78844.885–0.0972.02192.0177  0.0042
3 0 249.41349.480–0.0671.84291.8406  0.0023
2 2 049.81449.805  0.0091.82901.8293–0.0003
1 0 450.43250.355  0.0771.80801.8106–0.0026
2 2 151.35951.235  0.1241.77761.7816–0.0040
3 1 051.99652.134–0.1381.75731.7529  0.0043
2 1 352.73452.654  0.0791.73441.7368–0.0024
3 1 153.49653.554–0.0581.71151.7098  0.0017
1 1 454.69954.555  0.1441.67671.6807–0.0041
2 2 255.81255.774  0.0371.64581.6468–0.0010
2 0 456.74856.664  0.0841.62091.6231–0.0022
3 1 257.83557.864–0.0291.59301.5922  0.0007
4 0 058.19558.184  0.0111.58401.5842–0.0003
2 1 462.63662.483  0.1531.48191.4852–0.0033
4 0 263.66163.713–0.0521.46051.4594  0.0011
3 2 064.00163.993  0.0081.45361.4537–0.0002
3 0 466.38966.433–0.0431.40691.4061  0.0008
1 1 567.02667.123–0.0971.39511.3933  0.0018
4 1 067.71467.662  0.0521.38261.3835–0.0009
3 2 269.20769.413–0.2061.35641.3529  0.0035
4 1 272.79072.692  0.0981.29821.2997–0.0015
3 1 473.60473.482  0.1221.28581.2877–0.0018
2 1 574.21273.932  0.2801.27681.2809–0.0041
0 0 675.53775.682–0.1451.25771.2556  0.0020
4 1 378.94979.151–0.2021.21161.2090  0.0026
3 3 179.57079.631–0.0611.20371.2030  0.0008
4 2 080.07880.111–0.0331.19741.1970  0.0004
1 1 680.73180.591  0.1391.18931.1910–0.0017
4 2 181.28881.083  0.2051.18261.1851–0.0025
2 0 682.44382.481–0.0381.16891.1685  0.0004
2 2 582.86183.021–0.1601.16411.1622  0.0018
3 2 483.97883.911  0.0671.15141.1522–0.0007
3 1 584.56584.430  0.1351.14491.1464–0.0015
4 2 284.89784.930–0.0341.14131.1409  0.0004
5 1 186.40086.350  0.0501.12521.1258–0.0005
2 1 687.54587.560–0.0151.11341.1133  0.0002
Table S15

Unit cell refinement results of orthorhombic phase found in a sample of (NH4)9[(ReO)3(AsW9O33)2]·n H2O heated to 900 °C in TGA apparatus under N2 atmosphere

h k l2θ(cal)2θ(obs)Δ(2θ)d(cal) (Å)d(obs) (Å)Δ(d) (Å)
0 0 123.02423.088–0.0633.85963.8492  0.0104
0 2 023.71023.747–0.0373.74953.7437  0.0057
2 0 024.12424.208–0.0843.68603.6735  0.0125
0 1 125.94226.027–0.0853.43173.4207  0.0110
1 2 026.65126.687–0.0363.34203.3376  0.0045
1 1 128.67028.687–0.0183.11113.1093  0.0019
0 2 133.28733.267  0.0202.68942.6909–0.0016
2 0 133.59233.637–0.0452.66572.6622  0.0034
2 2 034.08034.167–0.0862.62862.6221  0.0064
1 2 135.50235.586–0.0842.52652.5207  0.0058
2 2 141.53241.586–0.0542.17262.1699  0.0027
0 3 143.07942.945  0.1332.09812.1043–0.0062
3 2 044.02144.135–0.1142.05532.0502  0.0050
1 3 144.88044.885–0.0052.01792.0177  0.0002
3 1 145.35445.465–0.1111.99791.9933  0.0046
0 0 247.05147.035  0.0161.92981.9304–0.0006
0 4 048.52048.585–0.0651.87471.8724  0.0024
4 0 049.41049.480–0.0701.84301.8406  0.0025
1 1 250.32650.355–0.0281.81161.8106  0.0010
2 0 253.55853.554  0.0031.70961.7098–0.0001
0 4 154.35954.334  0.0251.68631.6870–0.0007
2 4 054.89954.964–0.0651.67101.6692  0.0018
4 2 055.51255.434  0.0781.65401.6561–0.0021
1 4 155.88455.774  0.1101.64391.6468–0.0030
3 3 157.73057.864–0.1341.59561.5922  0.0034
2 2 259.36359.283  0.0791.55561.5575–0.0019
2 4 160.30660.364–0.0571.53351.5321  0.0013
4 2 160.88460.743  0.1411.52031.5235–0.0032
3 4 062.23562.014  0.2211.49051.4953–0.0048
4 3 062.56662.483  0.0831.48341.4852–0.0018

The powder diffraction pattern of the potassium salt heated to 900 °C differs significantly from the patterns collected for samples decomposed at lower temperatures. Since the TGA curve does not show any stepwise weight loss between 725 and 950 °C, the observed change of phase composition suggests an existence of a phase transition in this temperature interval. A search on the powder diffraction database shows that the observed powder diffraction pattern corresponds to those of hydrated tungsten oxide WO3·0.5H2O and a number of substituted tungsten bronzes, KCr0.33W1.67O6 with pyrochlore-type structures (Fig. 5; Section 5, Table S3) [21].

Fig. 5

Pyrochlore-type structure viewed perpendicular to the (110) plane, showing locations of potassium cations (gray spheres).

Table S3

Phase identification for a sample of K9[(ReO)3(AsW9O33)2]·n H2O heated to 900 °C in TGA apparatus under nitrogen atmosphere

2θ(obs)d(obs) (Å)I(obs) (%)Phase IDd(card) (Å)I(card) (%)h k l2θ(card)Δ(2θ)
14.0896.28103.4
15.0795.8707100.0KCr0.33W1.67O65.8800100.01 1 115.055–0.024
21.4984.13008.1
23.8583.72664.9
27.6873.21932.1
28.3173.14905.6Si3.135491 1 128.4423  0.1253
28.9373.083024.4KCr0.33W1.67O63.0700100.03 1 129.062  0.125
30.3072.946713.2KCr0.33W1.67O62.940085.02 2 230.378  0.070
34.3462.60881.6
35.2462.54422.2KCr0.33W1.67O62.546015.04 0 035.221–0.025
37.3062.40832.5
38.4862.33721.6KCr0.33W1.67O62.337015.03 3 138.489  0.003
46.2551.96111.8KCr0.33W1.67O61.960035.05 1 146.283  0.027
47.2351.92278.9Si1.92012 2 047.3027  0.068
48.0351.892526.3
50.6351.80133.9KCr0.33W1.67O61.800050.04 4 050.673  0.038
53.1851.72083.1KCr0.33W1.67O61.722030.05 3 153.143–0.041
56.3141.63231.8Si1.637453 1 156.1222–0.192
57.0841.61211.5
59.5131.55202.3KCr0.33W1.67O61.555015.05 3 359.387–0.127
60.3321.53293.8KCr0.33W1.67O61.537040.06 2 260.153–0.179
63.1741.47060.7KCr0.33W1.67O61.472010.04 4 463.106–0.067
63.5231.46331.4
65.3731.42632.9KCr0.33W1.67O61.429020.07 1 165.236–0.137
71.0521.32562.0KCr0.33W1.67O61.329025.07 3 170.844–0.208
71.3721.32051.0
74.7621.26883.4
81.6111.17870.9KCr0.33W1.67O61.17805.07 5 181.671  0.060
82.5811.16732.1KCr0.33W1.67O61.170020.06 6 282.350–0.231
84.9311.14090.6KCr0.33W1.67O61.140015.08 4 085.015  0.084

A rough estimate of the rhenium contribution to the intensity of diffraction, based on the formula of the decomposed product, shows that rhenium should contribute about {3(Re)/[18(W) + 8 × 69(O)/74 + 19 × 9(K)/74]} × 100% ≈ 11% of total diffraction intensity. The intensities of the unidentified peaks increase with increase of temperature, for example, the reflection with d = 4.13 Å increases from 5.9% at 600 °C to 18% at 725 and 900 °C, which probably indicates decomposition of the monoclinic phase and formation of a cubic phase. At 600 °C, according to this estimate, a considerable part of the rhenium should be in the monoclinic phase, but it is unclear how much rhenium, if any is contained in the monoclinic, or cubic phase at the higher temperature.

A powder diffraction study of decomposition products resulting from heating the ammonium salt of 1 to 725 °C, shows two different crystallographic phases corresponding to hexagonal [22,23] and tetragonal [24] bronzes (Fig. 6, Tables 1, S4 and S5). As for the potassium salt a few low-intensity reflections could not be identified as known rhenium oxides and bronzes [18,20–22,25], or indexed in either tetragonal [24] or hexagonal [22,23] cell settings. A rough estimate of the rhenium contribution to the intensity of diffraction on the basis of decomposition product’s formula: Re1.9W12O41 suggests that rhenium should contribute about: {2(Re)/[12(W) + 8 × 41(O)/74]} × 100% ≈ 12% of observed diffraction intensity, which is about three times greater than the most intense non-identified peak: 3.9% (Tables S4 and S5). This estimate suggests that considerable part of rhenium is incorporated into the observed tetragonal and hexagonal phases.

Fig. 6

Powder diffractograms of (NH4)9[(ReO)3(AsW9O33)2]·n H2O heated to 600 °C (A), 725 °C (B), and 900 °C. (*) Unidentified reflections. (Δ) Reflections from the hexagonal phase.

Table S4

Phase identification for a sample of (NH4)9[(ReO)3(AsW9O33)2]·n H2O heated to 600 °C in TGA apparatus under N2 atmosphere

2θ(obs)d(obs) (Å)I(obs) (%)Phase IDd(card) (Å)I(card) (%)h k l2θ(card)Δ(2θ)
14.0096.31655.2hexagonala6.326453.01 0 013.987–0.022
21.5184.12632.5
22.9383.873984.5tetragonalb3.860070.00 0 123.022  0.084
23.9383.7143100.0tetragonal3.7100100.01 1 023.966  0.028
26.2783.38872.4
26.6773.33883.4
28.0773.175442.6hexagonal3.1642100.02 0 028.179  0.102
28.6373.114624.3tetragonal3.110030.01 0 128.680  0.043
33.3972.680864.1tetragonal2.673060.01 1 133.497  0.100
34.1072.626638.7hexagonal2.624514.01 1 234.135  0.028
35.5872.52071.3
36.8962.434219.4hexagonal2.424442.02 0 237.051  0.155
41.4862.174919.6tetragonal2.170025.02 0 141.583  0.097
45.0852.00929.0tetragonal2.005012.02 1 145.186  0.100
46.8351.93827.9tetragonal1.92908.00 0 247.071  0.236
47.9641.89510.9hexagonal1.88599.00 0 448.213  0.249
48.9751.858412.2tetragonal1.854016.02 2 049.098  0.123
49.6851.83358.4hexagonal1.826933.02 2 049.875  0.190
50.0951.819419.3tetragonal1.81133.01 0 250.344  0.249
51.8551.76171.2hexagonal1.75536.03 1 052.058  0.203
53.2641.718412.2tetragonal1.713012.01 1 253.445  0.180
55.0941.665617.7tetragonal1.672014.02 2 154.864–0.230
56.4741.62815.4hexagonal1.620615.02 0 456.760  0.286
57.9941.58903.8hexagonal1.59158.03 1 257.894–0.100
59.2241.55893.4tetragonal1.55508.02 0 259.387  0.163
60.6731.52519.5tetragonal1.524020.03 1 160.720  0.047
62.0831.49388.5tetragonal1.491014.02 1 262.212  0.128

a Hexagonal diffraction pattern is compared with the diffraction pattern of hexagonal KAl0.33W2.67O9 (a = 7.307 Å, c = 7.546 Å).

b Tetragonal diffraction pattern is compared with a diffraction pattern of tetragonal Ca0.035WO3 (a = 5.246 Å, c = 3.858 Å).

Table S5

Phase identification for a sample of (NH4)9[(ReO)3(AsW9O33)2]·n H2O heated to 725 °C in DSC apparatus under N2 atmosphere

2θ(obs)d(obs) (Å)I(obs) (%)Phase IDd(card) (Å)I(card) (%)h k l2θ(card)Δ(2θ)
14.0396.30314.3hexagonala6.326453.01 0 013.987–0.052
19.3684.57911.5
21.4984.13003.9
22.9383.873960.9tetragonalb3.860070.00 0 123.022  0.084
23.5283.778116.9hexagonal3.773033.00 0 223.560  0.032
23.9783.7082100.0tetragonal3.7100100.01 1 023.966–0.012
26.8183.32172.7
28.1073.172133.2hexagonal3.1642100.02 0 028.179  0.072
28.6173.116720.7tetragonal3.110030.01 0 128.680  0.063
33.4072.680055.1tetragonal2.673060.01 1 133.497  0.090
34.1472.623631.7tetragonal2.624045.02 0 034.141–0.005
35.5262.52481.5
35.6462.51661.3
36.9262.432316.5hexagonal2.424442.02 0 237.051  0.125
41.5162.173418.6tetragonal2.170025.02 0 141.583  0.067
45.1152.00809.0tetragonal2.005012.02 1 145.186  0.070
46.8551.93746.5tetragonal1.92908.00 0 247.071  0.216
47.9651.89511.5hexagonal1.88599.00 0 448.213  0.248
49.0351.856212.0tetragonal1.854016.02 2 049.098  0.063
49.6551.83455.1hexagonal1.84136.03 0 249.460–0.195
50.0851.819813.7hexagonal1.826933.02 2 049.875–0.210
50.3141.81209.7tetragonal1.811020.01 0 250.344  0.030
51.9441.75891.1hexagonal1.75536.03 1 052.058  0.114
53.2941.71759.3tetragonal1.713012.01 1 253.445  0.150
53.5041.71135.9hexagonal1.70991.03 1 153.550  0.046
54.8741.671713.3tetragonal1.672014.02 2 154.864–0.011
55.3041.659715.4tetragonal1.659020.03 1 055.330  0.026
56.6341.62393.3hexagonal1.620615.02 0 456.760  0.126
57.9441.59023.0hexagonal1.59158.03 1 257.894–0.050
58.1641.58482.8hexagonal1.582315.04 0 058.264  0.100
59.0941.56203.5tetragonal1.55508.02 0 259.387  0.293
60.6931.524612.1tetragonal1.524020.03 1 160.720  0.027

a Hexagonal diffraction pattern is compared with the diffraction pattern of hexagonal KAl0.33W2.67O9 (a = 7.307 Å, c = 7.546 Å).

b Tetragonal diffraction pattern is compared with a diffraction pattern of tetragonal Ca0.035WO3 (a = 5.246 Å, c = 3.858 Å).

The hexagonal bronze structure is formed of WO6 octahedra connected by vertices (Fig. 7). Cations reside inside the large channels. The hexagonal structural framework is stable even if channels are empty, so that a hexagonal tungsten oxide (WO3) structure exists [22,23]. The tetragonal bronze structure is similar to the structure of perovskite [24,25]. The perovskite structure consists of a framework of WO6 octahedra connected by vertices (Fig. 8). This forms a cubic-type 3D structure, with interconnected pores partially occupied by cations. Since the tungsten octahedra are distorted, there is a deviation from ideal cubic symmetry. The a-axis of the smallest tetragonal cell is equal to the face diagonal of the cubic cell (a = 3.8 × √2 ≈ 5.22 Å). Some tetragonal bronzes have been indexed on the basis of a larger tetragonal cell with a ~ 7.4 Å, which approximately equals 2a for the cubic cell (Fig. 8). For both tetragonal cell types c (~3.8 Å) is close to the parameter for the cubic cell. Structures with orthorhombic or monoclinic symmetry with unit cells close to the larger tetragonal cell have also been reported [26,27]. Decomposition products found at 600 and 725 °C could be indexed on the basis of both tetragonal cells with a cell parameter of either ~5.22 Å (‘Ca0.035WO3’) or ~7.4 Å (‘H0.23WO3’), which in fact represent the same perovskite-type structure.

Fig. 7

Structure of hexagonal tungsten bronze (oxide) showing the channels parallel to the crystallographic c-axis.

Fig. 8

Structure of tetragonal tungsten bronze (oxide). A, a perspective along the c-axis. B, relationship between different lattice cell types. Perovskite-type cubic cell (a ~ 3.7 Å) is shown by dashed line; smallest tetragonal cell (a ~ 5.2 Å) is shown by continuous line; larger tetragonal cell (a ~ 7.4 Å) is shown by another type of dashed line. Crystallographic c-axis is perpendicular to the plane of paper.

Sleight and Gillson [16] studied formation of WO3–ReO3 solid solutions. They found cubic (a = 3.7574 Å), perovskite-type phases, similar to the tetragonal (orthorhombic) phase observed for the decomposition of ammonium salt of 1.

As for the potassium salt, a change in powder diffraction pattern was observed for a sample decomposed at 900 °C. Search of a match in the powder diffraction database shows a mixture of hexagonal (also observed at lower temperatures) and orthorhombic phases.

The lattice parameters of the orthorhombic compound are very similar to those of the tetragonal compound, suggesting a deformation of the tetragonal lattice. Orthorhombic deformation of a tetragonal lattice is easily demonstrated by splitting of the (1 1 0) reflection of the tetragonal phase (3.71, 100%, Tables S4 and S5) into the (0 2 0), (2 0 0) reflections of the orthorhombic phase (3.74, 38%; 3.67, 74%, Table S6).

Table S6

Phase identification for a sample of (NH4)9[(ReO)3(AsW9O33)2]·n H2O heated to 900 °C in TGA apparatus under N2 atmosphere

2θ(obs)d(obs) (Å)I(obs) (%)Phase IDd(card) (Å)I(card) (%)h k l2θ(card)Δ(2θ)
14.0196.31223.2hexagonala6.326453.01 0 013.987–0.032
19.3294.58841.6
21.5084.12821.1
23.0883.849237.9orthorhombicb3.8500100.00 0 123.082–0.005
23.5683.7718100.0hexagonal3.773033.00 0 223.560–0.008
23.7473.743737.9orthorhombic3.750065.00 2 023.707–0.040
24.2083.673574.4orthorhombic3.690095.02 0 024.098–0.110
26.0273.42070.5orthorhombic3.43002.00 1 125.955–0.072
26.6873.33765.4orthorhombic3.350010.01 2 026.586–0.101
27.5273.23762.0hexagonal3.240820.01 0 227.500–0.027
28.2273.158918.6hexagonal3.1642100.02 0 028.179–0.048
28.6873.109326.6orthorhombic3.100025.01 1 128.775  0.088
33.2672.690966.2orthorhombic2.686035.00 2 133.330  0.063
33.6372.662235.4orthorhombic2.662035.02 0 133.639  0.003
34.1672.622138.5hexagonal2.624514.01 1 234.135–0.032
35.5862.52077.5orthorhombic2.52506.01 2 135.524–0.063
36.1262.48431.5
37.1962.415223.8hexagonal2.424442.02 0 237.051–0.145
41.5862.169921.7orthorhombic2.173016.02 2 141.523–0.063
42.9452.10431.4hexagonal2.10955.03 0 042.834–0.111
44.1352.05022.9orthorhombic2.05602.03 2 044.005–0.130
44.3552.04061.4
44.8852.01774.7orthorhombic2.018010.01 3 144.879–0.007
45.4651.99336.8orthorhombic1.99806.03 1 145.353–0.112
45.8151.97892.5
47.0351.93048.7orthorhombic1.923020.00 0 247.227  0.191
48.5851.87246.2orthorhombic1.878010.00 4 048.430–0.155
48.8141.86411.7orthorhombic1.863–0 1 248.847  0.033
49.4801.84064.7hexagonal1.84136.03 0 249.460–0.020
49.8051.829310.0hexagonal1.826933.02 2 049.875  0.070
50.3551.810624.0hexagonal1.80792.01 0 450.436  0.081
52.1341.75291.3hexagonal1.75536.03 1 052.058–0.076
53.5541.70985.6hexagonal1.70991.03 1 153.550–0.004
53.7941.70275.6orthorhombic1.706010.02 0 253.682–0.113
54.3341.68707.3orthorhombic1.688016.00 4 154.301–0.033
54.5551.68079.8hexagonal1.67652.01 1 454.704  0.150
54.9641.66924.9orthorhombic1.665016.04 0 155.114  0.150
55.4341.65619.4orthorhombic1.657016.04 2 055.403–0.031
55.7741.646812.4orthorhombic1.64506.01 4 155.842  0.068
56.6641.62314.6hexagonal1.620615.02 0 456.760  0.096
56.9541.61553.3
57.8641.59224.1hexagonal1.59158.03 1 257.894  0.030
58.1841.58422.1hexagonal1.582315.04 0 058.264  0.080

a Hexagonal diffraction pattern is compared with the diffraction pattern of hexagonal KAl0.33W2.67O9 (a = 7.307 Å, c = 7.546 Å).

b Orthorhombic diffraction pattern is compared with the diffraction pattern of orthorhombic WO3 (a = 7.384 Å, b = 7.512 Å, c = 3.846 Å).

Thus thermal decomposition of the ammonium salt of [(ReO)3(AsW9O33)2]9– gives mostly a mixture of hexagonal and tetragonal (perovskite) type bronzes. The intensities of the diffraction peaks that belong to the hexagonal phase increase with increasing temperature, which suggests higher stability of the hexagonal phase at high temperatures. A structural phase transition is observed for the tetragonal phase between 725 and 900 °C, which results in an orthorhombic deformation of the observed unit cell.

4 Conclusions

Thermolysis of solid samples of ammonium and potassium salts of the new sandwich polyoxometalate anion [(ReO)3(AsW9O33)2]9– at 600–900 °C yield tungsten–rhenium bronzes based on K2W3O10 and hexagonal and tetragonal oxides. About 95% of the rhenium content of the original polyoxoanion is retained at 900 °C. This behavior suggests the possibility of generating analogous thermally stable technetium oxide bronzes for use as radioactive waste forms.

Acknowledgments

We thank the DOE for support of this work through the Office of Energy research and Environmental Management Science Program, Grant DE-FG07-96ER14695.

Supporting information

Tables S1–S15 listing phase identification and unit cell refinement from powder diffractograms of thermally-treated samples of 1-K, and 1-NH4.

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