Cation–cation interactions and cation exchange in a seriesof isostructural framework uranyl tungstates$

Enrica Balboni a, Peter C. Burns a,b,n

a Department of Civil & Environmental Engineering & Earth Sciences, University of Notre Dame, Notre Dame, IN 46556, USAb Department of Chemistry & Biochemistry, University of Notre Dame, Notre Dame, IN 46556, USA

a r t i c l e i n f o

Article history:Received 20 December 2013Received in revised form30 January 2014Accepted 1 February 2014Available online 11 February 2014

Keywords:Uranyl-tungstatesCation–cation interactionsCation exchangeHydrothermal synthesis

a b s t r a c t

The isotypical compounds (UO2)3(WO6)(H2O)5 (1), Ag(UO2)3(WO6)(OH)(H2O)3 (2), K(UO2)3(WO6)OH(H2O)4 (3), Rb(UO2)3(WO6)(OH)(H2O)3.5 (4), and Cs(UO2)3(WO6)OH(H2O)3 (5) were synthesized, char-acterized, and their structures determined. Each crystallizes in space group Cc. (1): a¼12.979 (3), b¼10.238 (2), c¼11.302 (2), β¼102.044 (2); (2): a¼13.148 (2), b¼9.520 (1), c¼11.083 (2), β¼101.568 (2);(3): a¼13.111 (8), b¼9.930 (6), c¼11.242 (7), β¼101.024 (7); (4): a¼12.940 (2), b¼10.231 (2), c¼11.259(2), β¼102.205 (2); (5): a¼12.983 (3), b¼10.191 (3), c¼11.263 (4), β¼101.661 (4). Compounds 1–5 are aframework of uranyl and tungsten polyhedra containing cation–cation interactions. The framework hasthree symmetrically distinct U(VI) cations, one tungsten, sixteen to eighteen oxygen atoms, and in 2–5,one monovalent cation. Each atom occupies a general position. Each U(VI) cation is present as a typical(UO2)2þ uranyl ion in an overall pentagonal bipyramidal coordination environment. Each pentagonalbipyramid shares two equatorial edges with two other pentagonal bipyramids, forming a trimer. Trimersare connected into chains by edge-sharing with WO6 octahedra. Chains are linked through cation–cationinteractions between two symmetrically independent uranyl ions. This yields a remarkably complexsystem of intersecting channels that extend along [0 0 1] and [�1 1 0]. The cation exchange properties of2 and 3 were characterized at room temperature and at 140 1C.

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1. Introduction

The chemistry of U(VI) is dominated by the linear dioxo uranylion [1]. The uranyl ion is usually coordinated by four to six ligandsarranged at the equatorial vertices of square, pentagonal, orhexagonal bipyramids that are capped by the O atoms of theuranyl ion [2]. In inorganic compounds these bipyramids are mostoften linked into structural units through the sharing of equatorialedges or vertices, either directly between uranyl polyhedra orthrough various oxyanions, most commonly resulting in infiniteanionic sheets with cations located in the interlayer region [3].

Several studies have challenged our understanding of the normallyunreactive uranyl ion [4–9]. Of interest here is the occurrence ofcation–cation interactions (CCIs) in U(VI) uranyl compounds [10–19].CCIs are of considerable interest in actinide chemistry, as they result inunusual solid-state topologies [20], possible super-exchange pathwaysrelated to magnetic ordering [21–23], and they are implicated indisproportionation reactions in aqueous systems [24–26]. CCIs werefirst noted in solutions containing Np(V) neptunyl and U(VI) uranylfive decades ago [27], and in the solid state three decades ago [28].In actinide chemistry, a CCI occurs where an O atom of an actinyl ioncoordinates another actinyl ion in the equatorial plane of its corre-sponding bipyramid.

CCIs are rare in compounds containing An(VI) cations, account-ing for �2% of U(VI) uranyl compounds and only one Np(VI)compound, a neptunyl borate synthesized at 220 1C [29]. TheLewis basicity of the “yl” O atoms of actinyl ions is higher for An(V) cations, consequently CCIs are much more common for An(V)[30–34]. CCIs occur in �50% of Np(V) compounds, and have beenfound in several U(V) compounds synthesized in non-aqueoussystems [12,24,35–39].

Several recent studies have reported CCIs in U(VI) compounds[10,13–16,40,41]. We are particularly interested in the role of U(VI)CCIs in creating compounds with novel structural topologies and

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☆CAS #: (UO2)3(WO6)(H2O)5 (1) CAS# 427164; Ag(UO2)3(WO6)(OH)(H2O)3 (2)CAS# 427165; K(UO2)3(WO6)OH(H2O)4 (3) CAS# 427170; Rb(UO2)3(WO6)(OH)(H2O)3.5 (4) CAS# 427166; Cs(UO2)3(WO6)OH(H2O)3 CAS# 427171; Na(UO2)WO6(OH)(H2O)4 (3NaRT) CAS# 427172; K(UO2)WO6(OH)(H2O)3 (3Tl_RT) CAS#427167; Tl0.5K0.5(UO2)WO6(OH)(H2O)3 (3Tl_140) CAS# 427173; Ag(UO2)WO6(OH)(H2O)3 (2Na_140) CAS# 4427168; Tl0.8Ag0.2(UO2)WO6(OH)(H2O)3 (2Tl_140) CAS#427169.

n Corresponding author at: University of Notre Dame, Department of Chemistry &Biochemistry, 301 Stinson-Hall Remick, Notre Dame, IN 46556, USA.Tel.: þ1 574 631 7852; fax: þ1 574 631 9236.

E-mail addresses: pburns@nd.edu, Peter.Burns.50@nd.edu (P.C. Burns).

Journal of Solid State Chemistry 213 (2014) 1–8

properties. Here we report the syntheses, characterization, andcation exchange properties of a series of five isotypical uranyltungstates with integral CCIs.

2. Experimental

2.1. Synthesis

Caution: Uranium is radioactive and toxic. The experimentsdescribed herein should only be conducted by qualified indivi-duals in appropriate facilities.

Crystals of (UO2)3(WO6)(H2O)5 (1), Ag(UO2)3(WO6)(OH)(H2O)3(2), K(UO2)3(WO6)OH(H2O)4 (3), Rb(UO2)3(WO6)(OH)(H2O)3.5 (4),and Cs(UO2)3(WO6)OH(H2O)3 (5) were synthesized hydrothermally in23 mL Teflon-lined stainless steel Parr reaction vessels. After cooling,crystals were recovered by filtration, washed with ultrapure water andallowed to dry in air. The syntheses of compounds 1–5were optimizedby changing U:W:X ratios (X¼Cs, Rb, K or Ag), pH, and soaktemperature. The formation of compounds 1–5 is highly pH depen-dent. Each forms prismatic crystals that are dark yellow (1, 3–5) ordark orange (2) (Supplementary information). The optimized synthesismethods that provided superior yields are given here. (1) 4mL ofaqueous 0.25 M (UO2(NO3)2)(H2O)6 solution and 0.011 g of powderedWO3were combined, giving a solutionwith a pH of 2.88. The reactantswere heated at 220 1C for six days. The products consisted of crystalsof 1, as well as crystals of α-[(UO2)(OH)2]. The yield of 1was �70% onthe basis of uranium. (2) 4 mL of a 0.25 M aqueous solution of(UO2(NO3)2)(H2O)6, 0.011 g of powdered WO3, 0.02 g of powderedAgNO3, and 0.01 g of powdered Ag2SO4 were combined, giving asolutionwith a pH of 3.23. Soaking at both 220 and 265 1C for six daysyielded compound 2 as well as fine grained yellow precipitate. Theyield of compound 2was �60% on the basis of uranium. (3) 4 mL of a0.25 M aqueous solution of (UO2(NO3)2)(H2O)6, 0.012 g of powderedWO3, 0.054 g of powdered K2CO3, and 0.016 g of powdered KCl werecombined, giving a solutionwith a pH of 3.32. Soaking at both 220 and265 1C for six days yielded 3 as well as fine-grained yellow precipitate.The yield of compound 3was �50% on the basis of uranium. (4) 4 mLof a 0.25 M aqueous solution of (UO2(NO3)2)(H2O)6, 0.011 g of pow-dered WO3, and 0.1 g of powdered RbHCO3 were combined, giving a

solutionwith a pH of 3.53. Soaking at both 220 and 265 1C for six daysyielded compound 4 as well as a fine grained yellow precipitate andcrystals of α-[(UO2)(OH)2]. The yield of compound 4was �50% on thebasis of uranium. (5) 4 mL of a 0.25 M aqueous solution of (UO2(NO3)2)(H2O)6, 0.011 g of powdered WO3, 0.02 g of powdered Cs2CO3, and0.01 g of powdered CsCl were mixed, giving a solution with a pH of3.66. Soaking at both 220 and 265 1C for six days yielded compound 5as well as a fine-grained yellow precipitate. The yield of compound 5was �50% on the basis of uranium.

2.2. Cation exchange experiments

The cation-exchange properties of compounds 2 and 3 wereinvestigated by placing several crystals of either 2 or 3 in 1.5 to2 mL of saturated solutions of TlNO3 or NaCl. Crystals of 3 weresubjected to exchange experiments at room temperature over 30days (designated 3Na_RT, 3Tl_RT), while both 2 and 3 were soakedin solutions at 140 1C for six hours (designated 2Na_140, 2Tl_140,3Tl_140). In all cases crystals remained intact with sharp edgesand were of sufficient quality for structure determination viasingle crystal X-ray diffraction.

2.3. Single crystal X-ray diffraction

Crystals or crystal fragments of compounds 1–5, as well ascrystals from the ion exchange experiments 3Na_RT, 3Tl_RT,2Na_140, 2Tl_140 and 3Tl_140 were selected for data collection.Single crystal X-ray diffraction data were collected using either aBruker three-circle diffractometer equipped with an APEX CCDdetector or a Bruker Quazar micro-source system with an APEXIIdetector. A sphere of data was collected for each compound usingMoKα X-radiation, frame widths in ω of 0.51, and counting timesper frame ranging from 10 to 30 s, depending on the crystal size.Unit cell parameters were refined by least square techniques usingthe Bruker APEXII software, which was also used for data integra-tion and corrections for Lorentz, background and polarizationeffects. Empirical absorption corrections were applied using theprogram SADABS [42]. Structures were solved and refined usingSHELXTL [43] on the basis of F2 for all unique data. Systematicabsences were consistent with space group Cc in all cases, which

Table 1Crystallographic data for compounds 1–5.

Structure formula 1 2 3 4 5

(UO2)3(WO6)–(H2O)5

Ag(UO2)3(WO6)–(OH)(H2O)3

K(UO2)3(WO6)–(OH)(H2O)3

Rb(UO2)3(WO6)–(OH)(H2O)3.5

Cs(UO2)3(WO6)–(OH)(H2O)3

Formula weight 1169.94 1268.84 1200.17 1246.44 1293.88Temperature (K) 100 100 100 100 100Wavelength 0.71073 0.71073 0.71073 0.71073 0.71073Crystal system Monoclinic Monoclinic Monoclinic Monoclinic MonoclinicSpace group Cc (9) Cc (9) Cc (9) Cc (9) Cc (9)a (Å) 12.979 (3) 13.148(2) 13.111(8) 12.940(2) 12.983 (3)b (Å) 10.238 (2) 9.520(1) 9.930(6) 10.231(2) 10.191(3)c (Å) 11.302 (2) 11.083(2) 11.242(7) 11.259(2) 11.262(3)β (1) 102.044 (2) 101.568(2) 101.024(7) 102.205(2) 101.661 (4)Volume (Å3) 1468.7(5) 1395.1(4) 1436.6(1) 1456.9(4) 1459.5(8)Z 4 4 4 4 4μ (mm�1) 40.87 45.55 42.06 44.51 43.58F(0 0 0) 1944 2100 1988 2076 2132Reflections collected 8477 7927 8104 5884 5883Reflections unique 3313 3090 3213 3097 3199S 1.035 0.945 1.000 0.995 1.05R(F) for FO242s(FO2) R1¼0.0242 R1¼0.0238 R1¼0.0285 R1¼0.0394 R1¼0.0458Rw(FO2) wR2¼0.0605 wR2¼0.0506 wR2¼0.0586 wR2¼0.0929 wR2¼0.1253Largest difference peak and hole

(Å)3.14, �2.11 1.37, �2.47 1.35, �1.71 3.03, �4.74 3.21, �6.17

Theta range for data collection 2.56 to 27.57 2.66 to 27.55 2.59 to 27.61 2.56 to 27.45 2.56 to 27.55

E. Balboni, P.C. Burns / Journal of Solid State Chemistry 213 (2014) 1–82

was verified by the successful solution and refinement of thecrystal structures. The U atoms were located from direct-methodssolutions. Lighter atoms were located in difference-Fourier mapscalculated following subsequent refinement cycles. The finalstructure models included anisotropic displacement parametersfor U and W in 1–5, Ag, K, Rb and Cs in 2, 3, 4 and 5, respectively,and all oxygen atoms in 3. As is typical for many actinidecompounds, the data did not support refinement of anisotropicdisplacement parameters for all O atoms in all cases, and in nocases were the H atoms located. Additional information pertinentto the data collections is given in Tables 1 and 2. Selectedinteratomic distances are reported in Table 3 for compounds 1–5.

The structures of crystals selected from the cation exchangeexperiments were solved and refined as described above. Where asingle unique interstitial cation site was found, its occupancy wasrefined using scattering factors corresponding to the two potentialconstituents (the cation in the original crystal and that in the ion-exchange solution), with the total site occupancy constrained to beunity. The atomic coordinates and displacement parameters ofeach cation were constrained to be identical. In the case of of3Tl_140, difference-Fourier maps revealed two distinct interstitialcation sites. These were assigned as Tl and K on the basis of theirFourier peak heights, and subsequently were refined as twoindependent sites with the constraint that their total occupancybe unity, as required to provide charge balance.

2.4. Bond valence analyses

The bond valence sums incident upon each atom in 1–5 werecalculated [2,44]. In each case the calculated sums are in goodagreement with the formal valences of U6þ and W6þ . The bondvalence sums for compounds 1–5 incident on the alkali metal sitesare 1.20 vu for K, 0.94 vu for Rb, 1.12 vu for Cs and 0.73 vu for Ag. Oatoms were assigned as O2� , OH� or H2O on the basis of theirbond valence sums.

2.5. Infrared spectra

Infrared spectra were obtained for single crystals of 1–5 using aSensIR technology IlluminatIR-FT-IR microspectrometer. Single

crystals of each compound were placed on a glass slide, and thespectra were collected using a diamond ATR objective over therange of 650–4000 cm�1 (Supporting information).

2.6. UV–vis absorbance spectra

Absorbance spectra were acquired for each compound using aCraig Technologies UV–vis_NIR microspectrophotometer. Eachspectrum was collected for a single crystal from 250 to 1500 nm(Supporting information).

2.7. Thermogravimetric analysis

Thermogravimetric analyses (TGA) were perfomed for com-pounds 1 and 2 using a Netzch TG209 F1 IRIS thermal analyzer.15 mg of sample was loaded into an Al2O3 crucible in each case,and was heated from 20 to 900 1C at a rate of 5 1C/min underflowing nitrogen gas (Supporting information).

2.8. Chemical analyses

Energy dispersive spectra were collected using a LEO EVO 50scanning electron microscope for compounds 1–5 and the crystalsfrom the cation exchange experiments (Supporting information).

2.9. X-ray powder diffraction

X-ray powder diffraction data were collected for residuals of 1and 2 after TGA analysis, and for fine-grained precipitates recov-ered from the products of 2, 4 and 5 (Supplementary information).Data were collected using a Bruker D8 Advance automateddiffractometer at room temperature over the angular range of5–701 (2θ, CuKα) with a step width of 0.011 and a fixed countingtime of 2 s/step.

Table 2Crystallographic data for compounds 3Na_RT, 3Tl_RT, 2Tl_140, 2Na_140 and 2Tl_140.

3Na_RT 3Tl_RT 3Tl_140 2Na_140 2Tl_140Na[(UO2)3(WO6)(OH)](H2O)4

K[(UO2)3(WO6) OH](H2O)3

Tl0.5K0.5[(UO2)3-(WO6)(OH)](H2O)3

Ag0.95[(UO2)3-(WO6)(OH)](H2O)3

(Tl0.8Ag0.2) [(UO2)3-(WO6)(OH)](H2O)3

Formula weight 1196.15 1211.93 1287.85 1259.57 1337.36Temperature (K) 100 100 100 100 100Wavelength 0.71073 0.71073 0.71073 0.71073 0.71073Crystal system Monoclinic Monoclinic Monoclinic Monoclinic MonoclinicSpace group Cc (9) Cc (9) Cc (9) Cc (9) Cc (9)a (Å) 12.907(7) 12.887(5) 12.890(4) 13.149(2) 12.929(1)b (Å) 10.220(5) 10.272(4) 10.180(3) 9.529(1) 10.121(1)c (Å) 11.345(6) 11.305(5) 11.242(4) 11.086(1) 11.291(1)β (1) 101.496(6) 101.914(6) 102.187(3) 101.634(1) 101.788(1)Volume (Å3) 1466(13) 1464.3(1) 1442.0(8) 1360.7(3) 1446.6(3)Z 4 4 4 4 4μ (mm�1) 41.01 41.46 47.89 45.45 50.61F(0 0 0) 1994 2024 2132 20096 2260Reflections collected 8381 4994 8001 7739 8160Reflections unique 2894 2365 2579 3005 2911S 0.933 0.997 1.184 1.003 1.129R(F) for FO242s(FO2) 0.0405 0.051 0.0585 0.0213 0.0438Rw(FO2) 0.0801 0.1071 0.1315 0.0414 0.0912Largest difference peak and

hole (Å)1.76, �2.52 3.43, �2.73 5.82, �3.627 2.9, �1.92 8.05, �6.31

Theta range for datacollection

2.56–22.56 2.56–22.40 2.57–22.43 2.57–27.51 2.58–27.55

E. Balboni, P.C. Burns / Journal of Solid State Chemistry 213 (2014) 1–8 3

3. Results

3.1. Synthesis

Crystals of compounds 1–5 were obtained under hydrothermalconditions at 220 and 265 1C in good yield. The initial synthesis of1 provided a low yield, prompting exploration of conditions byvarying the U:W ratio from 1:1 to 40:1 and the pH from 1.5 to 7.The U:W ratio and pH values that yielded the highest and purestyields of 1 were used as the starting conditions for compounds2–5. Attempts to synthesize 3–5 using the nitrate forms of thecations of interest (KNO3, RbNO3 and CsNO3) were unsuccessful. X-ray powder diffraction data for 2, 4 and 5 indicate the presence ofuranyl oxyhydrates and tungsten oxides (Supplementaryinformation).

3.2. Cation coordination

Compounds 1–5 are an isotypical series of frameworks of uranyland tungsten polyhedra. The framework contains three symmetri-cally distict U(VI) cations, one W(VI) cation, sixteen to eighteen Oatoms, and in 2–5, one monovalent cation, with each atom occupyinga general position. Each U(VI) cation is present as a typical (UO2)2þ

uranyl ion with U�O bond lengths in the range of 1.713(18) to 1.868(17) Å and O–U–O bond angles ranging from 175.21 to 178.31. Eachuranyl ion is coordinated by five ligands arranged in the equatorialplane of pentagonal bypiramids, with Ueq distances ranging from2.175(15) to 2.532(17) Å. These values are consistent with averages ofU(VI) polyhedra in a variety of structures [3].

The single independent W(VI) cation is present in a distortedoctahedral arrangement of oxygen atoms. The W–O distances of 1,

Table 3Selected interatomic distances (Å) in the structures of 1–5.

Compound 1

U1–O9 1.781(8) U2–O6 1.761(8) U3–O3 1.793(8) W1–O13 1.766(8)U1–O2 1.792(9) U2–O15 1.792(8) U3–O5 1.849(8) W1–O10 1.833(8)U1–O7 2.180(8) U2–O7 2.244(8) U3–O7 2.191(8) W1–O4 1.884(7)U1–O4 2.415(7) U2–O11 2.318(7) U3–O11 2.267(7) W1–O1 1.910(7)U1–O5 2.418(8) U2–O13 2.353(8) U3–O1 2.295(8) W1–O11 2.012(7)U1–O1 2.432(7) U2–O4 2.429(7) U3–O1 2.416(7) W1–O8 2.327(8)U1–O14 2.519(10) U2–O12 2.520(8) U3–O12 2.525(8)Compound 2 Compound 4U1–O2 1.786(8) W1–O13 1.761(8) U1–O2 1.754(16) W1–O10 1.843(14)U1–O9 1.793(9) W1–O10 1.844(8) U1–O9 1.785(16) W1–O4 1.853(15)U1–O7 2.195(8) W1–O4 1.889(8) U1–O7 2.175(15) W1–O1 1.946(14)U1–O4 2.339(8) W1–O1 1.890(8) U1–O5 2.400(15) W1–O11 2.001(15)U1–O1 2.363(8) W1–O11 1.976(8) U1–O1 2.403(15) W1–O13 2.001(15)U1–O5 2.404(8) W1–O8 2.312(8) U1–O4 2.421(15) W1–O8 2.282(17)U1–O14 2.474(9) U1–O14 2.532(17)

Ag1–O12 2.412(8) Rb1–O17 2.830(3)U2–O6 1.783(8) Ag1–O9 2.453(9) U2–O6 1.792(16) Rb1–O14 2.897(1)U2–O15 1.800(8) Ag1–O15 2.471(8) U2–O15 1.813(15) Rb1–O17 2.902(3)U2–O7 2.250(8) Ag1–O3 2.559(8) U2–O7 2.241(15) Rb1–O9 2.972(1)U2–O11 2.315(8) U2–O11 2.364(16) Rb1–O15 2.975(1)U2–O13 2.350(8) U2–O13 2.365(14) Rb1–O6 3.033(1)U2–O12 2.422(8) U2–O12 2.452(16) Rb1–O3 3.077(1)U2–O4 2.476(8) U2–O4 2.481(14) Rb1–O12 3.265(1)

Rb1–O2 3.418(1)U3–O3 1.801(8) U3–O3 1.798(15)U3–O5 1.838(8) U3–O5 1.850(15)U3–O7 2.232(8) U3–O7 2.180(16)U3–O10 2.275(8) U3–O11 2.251(16)U3–O11 2.315(8) U3–O10 2.304(14)U3–O12 2.406(8) U3–O1 2.402(14)U3–O1 2.452(8) U3–O12 2.440(16)Compound 3 Compound 5U1–O9 1.769(11) W1–O13 1.788(10) U1–O2 1.713(18) W1–O13 1.785(17)U1–O2 1.788(11) W1–O10 1.813(9) U1–O9 1.792(19) W1–O10 1.861(16)U1–O7 2.182(9) W1–O4 1.874(10) U1–O7 2.199(17) W1–O4 1.883(15)U1–O4 2.382(10) W1–O1 1.911(9) U1–O5 2.379(17) W1–O1 1.960(17)U1–O5 2.406(10) W1–O11 1.985(9) U1–O1 2.397(17) W1–O11 1.987(18)U1–O1 2.422(9) W1–O8 2.383(11) U1–O4 2.400(16) W1–O8 2.322(18)U1–O14 2.531(11) U1–O14 2.50(2)

K1–O12 2.651(1) Cs1–O9 3.023(1)U2–O15 1.784(11) K1–O9 2.679(1) U2–O15 1.78(2) Cs1–O14 3.024(2)U2–O6 1.809(11) K1–O6 2.688(1) U2–O6 1.781(18) Cs1–O15 3.062(1)U2–O7 2.227(10) K1–O3 2.738(1) U2–O7 2.199(17) Cs1–O6 3.161(1)U2–O13 2.340(10) K1–O8 2.835(1) U2–O11 2.341(18) Cs1–O3 3.209(1)U2–O11 2.343(10) K1–O13 3.049(1) U2–O13 2.346(18) Cs1–O12 3.222(1)U2–O4 2.468(10) K1–O14 3.072(1) U2–O12 2.429(19) Cs1–O2 3.442(1)U2–O12 2.495(10) U2–O4 2.467(15) Cs1–O8 3.462(1)U3–O3 1.792(10) U3–O3 1.777(18)U3–O5 1.833(10) U3–O5 1.868(17)U3–O7 2.194(9) U3–O7 2.208(17)U3–O11 2.278(10) U3–O10 2.281(16)U3–O10 2.321(9) U3–O11 2.282(18)U3–O1 2.414(10) U3–O1 2.368(17)U3–O12 2.481(10) U3–O12 2.446(19)

E. Balboni, P.C. Burns / Journal of Solid State Chemistry 213 (2014) 1–84

which is exemplary, include a short bond length of 1.766(8) Å, fourintermediate bond lengths ranging from 1.8338(8) to 2.012(7) Å,and one longer bond length of 2.327(8) Å.

Compounds 2–5 contain monovalent cations that balance thecharge of the uranyl tungstate framework as detailed below. Theyare coordinated by various combinations of ligands that are mostlypart of the uranyl or tungstate polyhedra, as well as H2O groupscontained within the channels in some cases. The correspondinginteratomic distances are provided in Table 3.

3.3. Structure connectivity

Each of the three symmetrically distinct uranyl pentagonalbipyramids share two equatorial edges with two other pentagonalbipyramids, forming a trimer sharing a μ3-O. The trimers arelinked into chains extending along [1 1 0] and [�1 1 0] by thesharing of one vertex between the U1 and U3 bipyamids ofadjacent trimers, and WO6 octahedra are linked to these chainsby sharing one edge with each of the U2 and U3 bipyramids ofadjacent trimers (Fig. 1). These uranyl tungstate chains are linkedinto a novel and complex framework by CCIs (Fig. 2). The CCI isdonated by the U3 uranyl ion, and accepted by the U1 uranyl ion(Fig. 3). The U3–O5 and U1–O5 bond lengths in 1 are 1.849(8) and2.418(8) Å, respectively, which are slightly elongated as expectedrelative to typical uranyl ions in pentagonal bipyramids in theabsence of CCIs. This arrangement yields a complex system ofintersecting channels that extend along [0 0 1] and [�1 1 0].According to the crystallographic data of compound 5 the channelsalong [0 0 1] (designated A) are 5.3�7.9 Å, as measured betweenthe centers of the bounding O atoms, whereas those along[�1 1 0] (channel B) are 5.5�5.8 Å.

Only H2O is located in the channels of 1, and the uranyltungstate framework is electroneutral. In compounds 2–5, amonovalent cation is located in one of the channels of the frame-work, together with H2O groups. Ag and K are at the center ofchannel B, and Rb and Cs are at the center of channel A.

In 1, O12 bridges the U2 and U3 cations with bond lengths of2.520(8) and 2.525(8) Å, respectively, giving a bond-valence sumof 0.8 at the O site, consistent with occupancy by H2O (Fig. 3). Thissite provides the charge-balance mechanism necessary for theinsertion of cations into the channel sites. The O12 site corre-sponds to a hydroxyl group, rather than water, in all of compounds2–5. For example, in 2 the U2–O12 and U3–O12 bond lengths are2.422(8) and 2.406(8) Å, respectively, and O12 is also bonded to Agin the channel with a bond length of 2.412(8) Å. The resultingbond-valence sum at O12 is 1.20 valence units, consistent withoccupancy by hydroxyl.

3.4. Chemical analysis, TGA and spectroscopy

The energy dispersive spectral analyses of U, W, Ag, K and Rb in1–5 gave elemental ratios within 2–3% of the values correspondingto the crystal structures determined from X-ray data, although themeasured value of Cs is 5% lower than the expected value(Supplementary information). The thermal data collected for 1and 2 reveals weight losses of 8.04% and �5.84%, respectively,with the major weight loss occurring over the temperature rangeof 100–400 1C (Supplementary information). These weight lossescorrespond to the exit of the H2O groups located within thechannels of the framework structure, and are in good agreementwith the expected 7.6% (compound 1) and 5.5% (compound 2)weight loss based on the crystallographically derived formulae.Powder X-ray diffraction data collected for the TGA residualindicate that the phases decomposed into uranium and tungstateoxides for 1 and uranium, tungsten and silver oxides for 2(Supplementary information).

The infrared spectra of 1–5 are similar. The spectra present twopeaks in the region between 700 and 800 cm�1, one between 735and 743 cm�1, and one between 771 and 788 cm�1. These peaksare attributed to the asymmetric stretching of the equatorial U–Obonds, and to the symmetric stretching of the apical W–O bond [45].Several sharp bands in the range of 800–950 cm�1 are attributableto both the asymmetric stretching of the uranyl unit and tothe asymmetric stretching of the apical W–O [45]. The sharp peakat �1600 cm�1 and the broad peak at 3500 cm�1 are attributed tothe vibrations of H2O and H bonds (Supplementary information).

The typical UV–vis–NIR spectra of a solid compound containingUO2

2þ uranyl ions displays two typical transitions at 310 and420 nm [41]. Weng et al. [41] and Morrison et al. [10] report thatcompounds containing cation–cation interactions display UV–vistransitions that are red-shifted, with values between 330 and550 nm. Compounds 1–5 display similar UV–vis–NIR spectra thatcontain, in all cases, two broad peaks at 330 and 430 nm. Thesespectra are consistent with UV–vis–NIR spectra of uranyl com-pounds containing cation–cation interactions reported in theliterature [10,41] (Supplementary information).

3.5. Ion exchange experiments

Following treatment in high ionic strength solutions, thestructures of 3Na_RT, 3Tl_RT, 2Na_140, 2Tl_140 and 3Tl_140remain in space group Cc and the details of the uranyl tungstateframework remain similar to 1–5. Consider first the exchange ofvarious cations for K in the channels of 3. For 3Na_RT, the K cationsof 3 were completely replaced by Na cations, but not at the samecrystallographic site. The Na cations are distributed over two sitesthat are near the center of channel B. In the case of 3Tl_RT, no

Fig. 1. Polyhedral representation of the chain of uranyl pentagonal bipyramids and WO6 octahedra in compounds 1–5 (Colors: Uranium pentagonal bipyraminds¼yellow,Tungsten octahedra¼blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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detectable Tl exchanged into the crystal studied. In contrast, in thecrystal of 3 heated at 140 1C, 3Tl_140, �50% of the K cations of 3were replaced by Tl. In this structure the K and Tl cations occupytwo distinct crystallographic sites. The K site is �50% occupiedand is at the center of B, whereas the Tl site is �50% occupied atthe center of A.

Consider next the exchange of cations relative to Ag containedwithin the channels of 2. For 2Na_140, no detectable exchangeoccurred. In the case of 2Tl_140,�80% of the Ag cations in 2 were

replaced by Tl. In this compound Tl and Ag occupy the samecrystallographic position at the center of A.

For crystals of 3Na_RT, 3Tl_RT, 2Na_140, 2Tl_140 and 3Tl_140,chemical analysis of U and W gave weight percents within 4% and2% of those expected from the crystal structure, respectively. Thedata of Na, K, Tl and Ag differ by up to 6% relative to the value fromthe structure determinations, perhaps reflecting incompleteexchange and crystal zonation (Supplementary information). Thisis consistent with heterogeneous cation exchange through thecrystals. The crystals of 2 and 3 present a prismatic habit, and thuscation exchange may be fastest and more extensive near thesurfaces of the crystals.

4. Discussion

Various studies over the past decade have demonstrated theextraordinary structural complexity of uranyl compounds containinghexavalent S, Se, Cr, Mo or W. This diversity is due to the variability ofcation coordination geometries, the linkages of the polyhedra, andgeometry isomerism [46]. To date, the crystal structures of at least 21uranyl tungstate compounds have been reported [15,47–57]. Twentyof these were synthesized at temperatures of several hundreddegrees Celsius, in the absence of water, and only one is a hydrousphase that was synthesized under hydrothermal conditions [56]. Asexpected, uranyl tungstates exhibit a broad range of structural units,specifically a variety of chains, sheets, and frameworks. Only one ofthese, Na2Li8[(UO2)11O12(WO5)2], contains CCIs [15]. Its structureconsists of complex anionic slabs of uranyl square, pentagonal, andhexagonal bipyramids, as well as WO6 octahedra, with Na and Lications in the interlayers.

The framework of polyhedra found for compounds 1–5, as wellas for crystals subsequent to ion exchange experiments, are new inuranyl tungstates but have been observed in an isotypical series ofuranyl periodates [17]. Although other uranyl tungstates haveframework structures, some with channels, 1–5 have the largestchannels observed to date. These channels are mostly void, whichresults in low densities such as 5.89 g/cm3 for the Cs compound, ascompared to the framework compounds Li2(UO2)(WO4)2 andLi2(UO2)4(WO4)4O with densities of 6.89 and 7.16 g/cm3, respec-tively [54].

Where CCIs occur in U(VI) compounds, the corresponding U–Oof the donor uranyl ion tends to be �4% longer than that of thetypical uranyl ion. In compound 1, which corresponds to anelectroneutral framework with no interstitual cations, the U3–O5length is 1.849(8) Å. In compounds 2 through 5, the correspondinglengths range from 1.833(10) to 1.868(17) Å. The longer U3–O5distances correspond to the presence of the larger Rb and Cscations in the channel sites, suggesting that despite its expectedrigid nature, the CCI is sensitive to steric constraints.

The relative ease of exchange of cations such as Na and Tl forthose contained in the channel sites of 2–5 is not unexpected, giventhe dimensions of the channels passing through the uranyl tungstateframework. The exchange of the channel cations demonstrates thatthe framework of compounds 1–5 is robust to cation exchange andheat treatment. In contrast, cation exchange experiments performedfor the framework compound (NH4)3(H2O)2{[(UO2)10O10][(UO4)(H2O)2]} [58] yielded the formation of a new layered structure incompound Cs3[(UO2)3O2(OH)32Cl](H2O)3. In the current work, Nareadily replaced K in 3 during soaking at room temperature, but notAg in 2 during soaking at 140 1C. Contact of 3 with a Tl-bearingsolution at room temperature did not result in the replacement of K,but increasing the temperature to 140 1C resulted in exchange ofmost of the K by Tl. About 80% of the Ag in 2 was replaced by Tlwhen the crystals were held at 140 1C in contact with a Tl-bearingsolution.

Fig. 2. Polyhedral representations of the framework structure of compounds 1-5.Chains extend along [1 1 0] and [�1 1 0] forming the extended framework.Channels extend along [0 0 1] (channel A) and along [�1 1 0] (channel B). Legendas in Fig. 1.

E. Balboni, P.C. Burns / Journal of Solid State Chemistry 213 (2014) 1–86

Acknowledgments

This research was supported by the Chemical Sciences, Geos-ciences and Biosciences Division, Office of Basic Energy Sciences,Office of Science, U.S. Department of Energy, Grant no. DE-FG02-07ER15880.

Appendix A. Supporting information

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.jssc.2014.02.001.

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