Unexpected Structural Complexity in Cesium Thorium Molybdates

Article pubs.acs.org/crystal

Unexpected Structural Complexity in Cesium Thorium Molybdates Bin Xiao,†,‡ Jakob Dellen,† Hartmut Schlenz,† Dirk Bosbach,† Evgeny V. Suleimanov,§ and Evgeny V. Alekseev*,†,‡ †

Institute of Energy and Climate Research (IEK-6), Forschungszentrum Jülich GmbH, 52428 Jülich, Germany Institut für Kristallographie, RWTH Aachen University, 52066 Aachen, Germany § Department of Chemistry, Lobachevsky State University of Nizhny Novgorod, 603950 Nizhny Novgorod, Russia ‡

S Supporting Information *

ABSTRACT: Three cesium thorium molybdates [Cs2Th(MoO4)3, Cs2Th3(MoO4)7, and Cs4Th(MoO4)4] were synthesized via the high-temperature solid-state method. Cs2Th(MoO4)3 crystallizes in orthorhombic space group Pnnm, containing [Th(MoO4)3]2− chains extending along the [100] direction. Cs2Th3(MoO4)7 is based on an open framework with channels occupied by Cs+ ions and running parallel through the [100] direction. It is the first case in which thorium atoms coordinate simultaneously in three distinct environments, namely, 7-coordinated pentagonal bipyramid, 8-coordinated square antiprism, and 9-coordinated tricapped trigonal prism. Cs4Th(MoO4)4 is monoclinic, built from [Th(MoO4)4]4− sheets formed by vertex sharing of MoO4 tetrahedra and ThO8 antiprisms. The Raman and infrared spectra were recorded, and the vibrations related to internal and external Mo−O bonds in the MoO42− coordination geometry were discussed.

1. INTRODUCTION Thorium, as the most abundant radioactive element in nature, has long been used as a surrogate for studying the transuranic element behavior in the +4 oxidation state, especially Pu(IV).1 Because of the absence of 5f electrons, thorium exists predominantly in the tetravalent oxidation state, achieving high coordination numbers and exhibiting fascinating properties with a diverse topological configuration and rich coordination chemistry.2 [ThB5O6(OH)6][BO(OH)2]·2.5H2O (NDTB-1), e.g., with the crystal structure of a cationic framework, exhibits remarkable anion exchange capabilities.3−5 Molybdenum in many cases is one of the fission products in a nuclear reactor, and the reactions between molybdenum and actinides as well as nuclear fuel may affect the fuel properties.6 The Mo6+ cations are often coordinated with four to six oxygen atoms forming tetrahedral and square pyramidal and octahedral configurations, and therefore, the actinide molybdate compounds show a high structural flexibility and are capable of forming various structural motifs.7−12 The motifs range from simple finite clusters in K8Th(MoO4)6 to the complex framework in (NH4)4[(UO2)5(MoO4)7](H2O)5 that is built upon open [(UO2)5(MoO4)7]4− channels with a size of 4.8 Å × 4.8 Å.13,14 In the 1970s, Tabuteau et al. studied the A2MoO4−M(MoO4)2 (A = K+, Rb+, or Cs+; M = Np4+ or Pu4+) system by means of X-ray powder diffraction and microthermal analysis; compound A2Pu(MoO4)3 adopted the scheelite (CaWO4-type) structure with A and Pu sites on the 8-coordinated Ca sites.15,16 In the past few decades, the actinide molybdate compounds with high valence, especially U(VI), have been researched extensively.9,17−20 However, only a small amount of information about the structural chemistry of thorium molybdate compounds is available © 2014 American Chemical Society

compared to the amount available for the uranium molybdate family. Most of the studies of the thorium molybdates were conducted more than two decades ago.21−23 Until recently, because thorium is considered as a fuel in the new generation for nuclear fuel cycles, the thorium molybdates achieved renewed attention.24,25 For example, the thermodynamic properties of A2Th(MoO4)3 and A4Th(MoO4)4 (A = Na or Tl) have been investigated by Dahale et al.26,27 Cs2Th(MoO4)3 and Cs4Th(MoO4)4 were characterized by high-temperature X-ray powder diffraction and thermal expansion techniques, but the structural details are still not known.28 Given the importance of the structural characteristics of these thorium molybdates for turning chemical properties into specific applications, here, we report on the synthesis, structure, and vibrational spectra of a series of three cesium thorium molybdates with novel structural topologies and themes.

2. EXPERIMENTAL SECTION 2.1. Preparation of Single Crystals. The titled thorium molybdate crystals [Cs2Th(MoO4)3, Cs2Th3(MoO4)7, and Cs4Th(MoO4)4] were prepared by the solid-state reaction method using the A.R. grade chemicals of Th(NO3)4·5H2O (Merck), CsNO3 (AlfaAesar), and MoO3 (Alfa-Aesar) without further purification. Ten different Th:Cs:Mo ratios [1:1:2, 1:1:4, 1:1:5, 1:2:2, 1:2:4, 1:2:5, 1:3:4, 1:3:5, 1:4:4, and 1:4:5; 0.10 g or 0.15 mmol of Th(NO3)4·5H2O for each reaction] were carefully ground and loaded into 10 platinum crucibles. All mixtures were heated initially at 1050 °C in air for 5 h to produce a homogeneous melt. After this, all mixtures were cooled to 400 °C at a rate of 5 °C/h followed by rapid quenching. The obtained Received: March 25, 2014 Published: April 16, 2014 2677

dx.doi.org/10.1021/cg500418e | Cryst. Growth Des. 2014, 14, 2677−2684

Crystal Growth & Design

Article

step width of 0.02°). The aperture of the fixed divergence slit was set to 0.2 mm and that of the receiving slit to 8.0 mm. The discriminator of the detector was set to an interval of 0.16−0.25 V. The collected data were compared with the calculated data derived from singlecrystal data using Mercury version 3.1.30 2.5. Scanning Electron Microscopy−Energy-Dispersive Spectroscopy (SEM−EDS). Scanning electron microscopy images and energy-dispersive spectroscopy data were collected using a FEI Quanta 200F Environment Scanning Electron Microscope. The EDS results are provided in Figure SI2 and Table SI1 of the Supporting Information. 2.6. Infrared (IR) and Raman Spectra. A Bruker Equinox spectrometer was used for the IR experiment, utilizing the KBr pellet technique. Approximately 200 mg of KBr and 2 mg of each sample composition were mixed carefully. After that, a pressure of 10 tons was applied and held constantly for 3 min for each mixture to prepare pellets. The IR spectra were recorded from 400 to 1050 cm−1, as shown in Figure SI3 of the Supporting Information. A pure KBr pellet prepared under the same experimental conditions was used as the blank sample. The unpolarized Raman spectra were recorded using a Horiba LabRAM HR spectrometer with a Peltier-cooled multichannel CCD detector. An objective with a 50× magnification was linked to the spectrometer, allowing the analysis of samples as small as 2 μm in diameter. All the samples were in the form of polycrystalline powders. The incident radiation was performed with a He−Ne laser at a power of 17 mW (λ = 632.81 nm). The focal length of the spectrometer was 800 mm, and a 1800 gr/mm grating was used. The spectral resolution was ∼1 cm−1 with a slit of 100 μm. The spectra were recorded in the range of 90−1150 cm−1. No photoluminescence (PL) was observed. 2.7. Bond-Valence Analysis. Bond-valence sums (BVS) for all atom positions in the three cesium thorium molybdate phases were calculated, and the results for all atoms are in good agreement with their expected formal valences. The bond-valence parameters for Th(IV)-O, Cs(I)-O, and Mo(VI)-O were used according to the method of Brese and O’Keeffe.31

Table 1. Crystallographic Data of Cs2Th(MoO4)3, Cs2Th3(MoO4)7, and Cs4Th(MoO4)4 Cs2Th(MoO4)3 mass space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z λ (Å) F(000) μ (cm−1) ρcalcd (g cm−3) R(F) for F02 > 2σ(F02)a Rw(F02)b a

Cs2Th3(MoO4)7 Cs4Th(MoO4)4

977.68 Pnnm 5.2569(3) 9.7336(8) 26.8467(16) 90 90 90 1373.71(16) 4 0.71073 1688 18.74 4.727 0.0536

8326.08 P1 12.4439(5) 14.9301(8) 15.9617(9) 78.210(5) 78.697(4) 77.997(4) 2802.7(3) 1 0.71073 3592.0 21.532 4.933 0.0243

1403.44 P2/c 12.6902(6) 6.6202(3) 11.4593(7) 90 90.146(6) 90 962.71(9) 2 0.71073 1212.0 17.750 4.841 0.0321

0.1298

0.0473

0.0688

R(F) = ∑||F0| − |Fc||/∑|F0|. bR(F02) = [∑w(F02 − Fc2)2/∑w(F04)]1/2.

products consisted of colorless crystals in the form of prisms for each reaction. Because of the similar shapes of crystals and broken glass pieces, the yield for each reaction could not be defined. Powder X-ray diffraction studies revealed that Cs2Th(MoO4)3 crystallizes as the major product at Th:Cs:Mo molar ratios of 1:1:2, 1:2:2, 1:2:4, and 1:2:5. Crystals of Cs2Th3(MoO4)7 and Cs4Th(MoO4)4 could be found and easily isolated from 1:1:5 and 1:4:5 ratios, respectively. 2.2. Pure Phase Synthesis. The single crystals for each reaction obtained via the method described above always mixed with glass pieces. To obtain pure polycrystalline samples, the conventional solidstate reaction was performed for all three phases. A stoichiometric amount of Th(NO3)4·5H2O, CsNO3, and a 5% excess of MoO3 were mixed thoroughly for each phase and kept at 400 °C for 24 h in air. Then, the obtained samples were characterized by X-ray powder diffraction measurement. The described procedures were repeated several times with heating steps of 50 °C until the experimental XRD results agreed with the theoretical results. Pure phases of Cs2Th(MoO4)3, Cs2Th3(MoO4)7, and Cs4Th(MoO4)4 were finally obtained at 650, 700, and 500 °C, respectively. The XRD patterns for all three cesium thorium molybdates are provided in Figure SI1 of the Supporting Information. 2.3. Crystallographic Studies. Crystals of all titled thorium molybdates were picked and mounted on an Agilent SuperNova (Dual Source) diffractometer with optical alignment using a digital camera. The crystal data were collected by means of monochromatic Mo Kα1 radiation (0.71073 Å), equipped with microfocus X-ray tube technology, running at 50 kV and 0.8 mA, providing a beam size of approximately 30 μm. Standard CrysAlisPro software was used for calculating the dimensions of the unit cells as well as for controlling the data collection. More than a hemisphere of data was collected for each crystal. After collection, data were corrected for Lorentz, polarization, absorption, and background effects. SHELXL-97 was used for the determination and refinement of the structures.29 The detailed crystallographic information is listed in Table 1. The structures were determined by direct methods and refined to an R1 of 0.0536 for Cs2Th(MoO4)3, an R1 of 0.0243 for Cs2Th3(MoO4)7, and an R1 of 0.0321 for Cs4Th(MoO4)4. 2.4. Powder X-ray Diffraction. X-ray powder diffraction patterns of all pure phases were collected on a Bruker-AXS D4 Endeavor diffractometer in Bragg−Brentano geometry with a copper tube equipped with a primary nickel filter providing Cu Kα̅ radiation (λ = 1.54187 Å) at room temperature. Using a voltage equal to 40 kV and an electric current equal to 40 mA (1.6 kW), the one-dimensional silicon strip LynxEye detector (Bruker-AXS) was adopted to collect data in the 2θ range from 10° to 80° (total counting time of 0.5 s/step,

3. RESULTS AND DISCUSSION 3.1. Structural Descriptions. 3.1.1. Cs2Th(MoO4)3. The crystal structure of Cs2Th(MoO4)3 was determined and refined in space group Pnnm. It is based on [Th(MoO4)3]2− chains propagating parallel to the [100] direction, as depicted in panels a and b of Figure 1. Th(1) atoms adopted antiprism coordination geometry with coordination of eight O atoms. The average Th−O bond length, 2.42 Å, is consistent with the mean distance described for 8-coordinated thorium in oxygen surroundings.17 The linkages of two ThO8 antiprisms via sharing edges lead to Th−Th dimers. The dimers are interconnected through two symmetrically distinct Mo(1)O4 and Mo(2)O4 tetrahedra, with tridentate and bidentate binding modes, respectively. In both cases, the Mo−O bond lengths with the bridging oxygen atoms [1.79(1)−1.84(1) Å] are significantly longer than those with the terminal oxygen atoms [1.70(2)−1.73(1) Å]. The O−Mo−O bond angles are all close to the expected tetrahedral values, with a minimum of 102.8(7)° [O(2)−Mo(1)−O(2)] and a maximum of 115.7(5)° [O(3)− Mo(2)−O(5)]. The structure of Cs2Th(MoO4)3 is strongly related to those of its lanthanide counterparts. For instance, the [Th(MoO4)3]2− chains in Cs2Th(MoO4)3 can be seen as fundamental building units for the construction of the sheets observed in CsLn(MoO4)2 (Ln = Dy or Pr) series.32,33 From panels b and c of Figure 1, one can see that the sheets of lanthanum molybdates in the latter structure can be obtained by simply fusing the respective chains along the [001] direction. It is noteworthy that the formation of [Th(MoO4)3]2− chains in Cs2Th(MoO4)3 2678



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Table 2. Raman-Active and IR-Active Vibrational Shifts of Cs2Th(MoO4)3, Cs2Th3(MoO4)7, and Cs4Th(MoO4)4 Crystalsa Cs2Th(MoO4)3

Figure 1. (a) Projection of the structure of Cs2Th(MoO4)3 parallel to the [100] direction. (b) Chain of [Th(MoO4)3]2− extending along the [100] direction. (c) Two-dimensional sheets in CsLn(MoO4)2 (Ln = Dy or Pr). The chains in Cs2Th(MoO4)3 can be seen as the fundamental building units for the formation of this two-dimensional sheet. The yellow and light blue polyhedra represent Th and Ln, respectively, green tetrahedra Mo, and blue circles Cs atoms.

Figure 2. Chains in the structures of Cs2Th(MoO4)3 and K2Th(MoO4)3 are derived from the same assemblage of [Th2Mo12] dimers (fundamental building block). (a) [Th2Mo12] dimers. (b) Procedure for the topological connection of [Th2Mo12] dimers to the chain structure of Cs2Th(MoO4)3. (c) Procedure for the topological connection of [Th2Mo12] dimers to chains in K2Th(MoO4)3. Note that the dimers are also linked by edge sharing of ThO8 polyhedra, as highlighted in the figure.

is similar to the formation of those found in K2Th(MoO4)3. K2Th(MoO4)3 is a superstructure derived from CaWO4 scheelite via the simultaneous replacement of each divalent Ca2+ site with two-thirds of a monovalent K+ cation and onethird of a tetravalent Th4+ cation.34 The topological similarities of polyhedral linkages within Cs2Th(MoO4)3 and scheeliterelated K2Th(MoO4)3 can be better presented using black and white node representation.35 Via this approach, as seen in Figure 2, black and white nodes represent ThO8 antiprisms and

a

Cs2Th3(MoO4)7

IR

Raman

IR

941sh

959sh

972sh

933s 915s 890s 837s 764s 688sh 672s 582sh 557sh 472w 450w 411m

954s 948sh 942s 930s 925s 918s 911sh 885m 866s 828m 775m 751sh 693w 465m 421m 405w 373s 360s 344sh 334m 300m 292sh 276w 187w 168w 144m 136m 124m 110m 98m

954sh 934s 916s 891sh 872s 856sh 822s 795sh 759s 735sh 446sh 428m 404w

Raman 1131w, 1040w, 988sh 966sh 958sh 954s 939s 934sh 931sh 925sh 925s 917m 908sh 902sh 886sh 864sh 854m 843m 831m 824sh 818sh 808m 796s 787m 776sh, 768s 761sh 748m 743sh 730sh 722m 631w 539w 458w 401m 379sh 368sh 356sh 344sh 338s 334sh 323m 309m 298sh 289sh 285sh 231w 226w 199w 184w 172w 147w 129w 113w 104w 96w

Cs4Th(MoO4)4 IR 962m 888sh 847s 803s 788sh 750s 740s 622sh 449sh 424w

Raman 1077w, 1047w 960sh 952s 946w 939s 933sh 927m 915sh 922sh 906sh 899s 893sh 886w 884w 873sh 864m 840sh 834s 826sh 817w 804m 788s 776s 770s 766sh 627sh 574w 535w 418w 383m 368sh 360m 351s 344m 333m 320s 316s 306m 299s 287m 219w 176sh 163w 142s 119m 111w 103w

Abbreviations: s, strong; m, middle; w, weak; sh, shoulder.

MoO4 tetrahedra, respectively. The nodes in the graph are linked by a line segment if the corresponding polyhedra share 2679



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Figure 3. Coordination environments of the ThO7 pentagonal bipyramid, ThO8 square antiprisms, and ThO9 tricapped trigonal prisms in the structure of Cs2Th3(MoO4)7.

at least one common vertex. The double bonds in the graph indicate an edge-sharing linkage between two coordination polyhedra. From the crystal chemical point of view, both structures can be seen as built upon [Th2Mo12] dimers as the fundamental building blocks (FBBs) (the idealized version is shown in Figure 2a). Nevertheless, the linking topology of adjacent dimers is different. Unlike the structure of Cs2Th(MoO4)3, in which the neighboring dimers are connected solely by MoO4 tetrahedra (Figure 2b), the same dimers are united also by edge sharing of Th polyhedra within K2Th(MoO4)3 (highlighted in Figure 2c). The linkage through the common edges in K2Th(MoO4)3 results in corrugated Th chains with a denser configuration. This can be detected from the nearly similar structural densities [4.74(1) and 4.73(1) g/cm3 for K2Th(MoO4)3 and Cs2Th(MoO4)3, respectively] in spite of the potassium-based structure containing lighter alkaline cations. 3.1.2. Cs2Th3(MoO4)7. The Cs2Th3(MoO4)7 compound crystallizes in a novel three-dimensional structure type in acentric triclinic space group P1. The structure has an exceptional complex composition with 12 symmetrically nonequivalent Th sites as well as 28 unique Mo sites. To the best of our knowledge, this is the first occurrence of a thorium inorganic compound in which Th atoms have three distinct coordination geometries within a single crystal structure. With other inorganic actinide families, only four compounds were reported to simultaneously contain three different types of actinide coordinations in a single phase (including one uranium borate,36 one uranium tungstate,37 and two minerals38,39). The coordination environment of all Th atoms is shown in Figure 3. Thorium atoms normally form 8-fold (65%) or 9−12-fold (32%);40

however, in the case of Cs2Th3(MoO4)7, Th(1)−Th(4) sites present an unusual ThO7 polyhedral geometry. ThO7 can be described as a highly distorted pentagonal bipyramid with bond lengths ranging from 2.270(7) to 2.427(5) Å. In contrast, Th(5)−Th(8) exist as more frequently observed ThO8 square antiprisms with Th−O bond distances ranging from 2.300(6) to 2.753(6) Å. Until now, except for trigonal Th(MoO4)2, which contains thorium in the ThO6 and ThO9 coordination environments,41 the ThO8 square antiprism is the only thorium configuration common to all thorium molybdates found in the literature. The third thorium coordination is a distorted ThO9 tricapped trigonal prism for the Th(9)−Th(12) sites. Again, ThO9 is also a common thorium polyhedral formation particularly observed in a large number of thorium chalcogenides, such as tellurium [Th(VO2)2(TeO6)42], chromium [Th(CrO4)(IO3)243], and selenium [Th(SeO3)(SeO4)43]. The Th−O bond distances in ThO9 tricapped trigonal prisms range from 2.343(7) to 2.614(6) Å, distances less uniform than those in ThO7, but comparable to those in ThO8. In all cases, each thorium polyhedron shares all its vertices with adjacent MoO4 tetrahedral groups to form the framework structure. The thorium molybdate framework with corner-sharing coordination in Cs2Th3(MoO4)7 is shown in Figure 4a. This structure can be described as being based upon the main fragment of infinite channels that have elliptically shaped cross sections with dimensions of 9.26(5) Å × 2.89(5) Å. The channels are parallel to the [100] direction and encapsulated Cs+ cations. We unrolled the channels into chains to better interpret their composition. As shown in Figure 4b, the chains are constructed from consecutive connections of three types of 2680



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Figure 4. (a) Framework structure of Cs2Th3(MoO4)7 with channels propagating along the [100] direction. (b) The channels are expanded to onedimensional chains. They are built from thorium molybdate polyhedra in the following order: ThO7, ThO9, ThO8, ThO7, ThO8, ThO9. (c) Black and white representation of channels. (d and e) The channels can be transformed from simple six-membered rings by folding the corresponding latters at the opposite ends.

Th polyhedral building chains (ThO7, ThO8, and ThO9) in the following order: ThO7, ThO9, ThO8, ThO7, ThO8, ThO9. Note that the connection environment around each type of Th polyhedra is different. This is also the reason why the structure of Cs2Th3(MoO4)7 cannot be determined in a centrosymmetric space group. The neighboring Th polyhedra within ThO7 and ThO8 chains are linked by two MoO4 tetrahedra, while in the chains of ThO9, adjacent ThO9 tricapped trigonal prisms share three MoO4 tetrahedra (see Figure 4b). These ThOx (x = 7, 8, or 9) chains are further fused together by corner-sharing additional MoO4 tetrahedra and assume a rather bent linkage conformation, as one can imagine, that finally leads to closed cylinders. Cutting the channels into chains can give detailed demonstration about variant Th polyhedral geometries inside the channels; however, this is not enough to recognize the Th−Mo polyhedral junctions around the one-dimensional cylinders. Herein, to understand the interior structure of framework channels, we adopted the method of tubular units44 to describe the topology of the channel network. This method has been widely used to demonstrate the topology of tubular units observed in inorganic compounds, for example, in uranyl selenate nanotubules [(UO2)3(SeO4)5]4−.44 The black and white topological representation of the channels is shown in Figure 4c. It is solely assembled from six-membered rings in which each black (Th) and white (Mo) node is 3-connected. It can be transformed using idealized unfolding tape of regular honeycomb-shaped hexagons (Figure 4d) via the folding and gluing procedure.45 First, label the equivalent points on sides of the tape by letters a−h. Then fold the tape by joining the corresponding opposite sides (a−a′, b−b′, c−c′, d−d′, e−e′, f−f ′, g−g′, and h−h′) to make a cylinder. The idealized

Figure 5. Structure of Cs4Th(MoO4)4. (a) Projection along the [010] direction showing the sheets of Th(MoO4)42−. (b) Projection along the [100] direction.

topological structure for the tubular unit in Cs2Th3(MoO4)7 is shown in Figure 4e. This is a common topology observed in the graphitelike sheet.46 3.1.3. Cs4Th(MoO4)4. The last member, Cs4Th(MoO4)4, was first investigated by Bushuev et al. in 1975.47 By analyzing powder X-ray diffraction patterns, they obtained only three crystallographic axes (a = b = 6.627 Å, and c = 12.682 Å) but gave no other lattice information. Recently, on the basis of this inadequate crystallographic data, Keskar et al. concluded that the crystal lattice of Cs4Th(MoO4)4 is hexagonal.28 However, by means of single-crystal X-ray diffraction and symmetry analysis, we found that Cs4Th(MoO4)4 crystallized as a monoclinic system in space group P2/c [a = 12.6902(6) Å, b = 6.6202(3) Å, c = 11.4593(7) Å, and β = 90.146(6)°]. Cs4Th(MoO4)4 is built from two-dimensional sheets composed of ThO8 antiprisms and MoO4 tetrahedra (see Figure 5). 2681



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Figure 6. Raman spectra of Cs2Th(MoO4)3, Cs2Th3(MoO4)7, and Cs4Th(MoO4)4.

It is isostructural to Rb4Th(MoO4)4.48 However, because of the larger ionic radius of the Cs+ cations between neighboring sheets in comparison to that of Rb+ cations, 1.74 Å for Cs and 1.61 Å for Rb,49 the interlayer distance is larger for Cs4Th(MoO4)4 [12.69(5) Å] than that for Rb4Th(MoO4)4 [12.29(5) Å]. Both two-dimensional compounds can be seen as being derived from the same structural fragments with a [Th(MoO 4 ) 4 ] 4− composition. It is worth noting that the similar stoichiometric composition compounds A4Th(MoO4)4 (A = Na or K) with a three-dimensional framework, but containing smaller cations, can also be described as being constructed from a similar [Th(MoO4)4]4− fragment.50,51 Different structural dimensionality with similar building fragments demonstrates the flexibility of Th−O−Mo linkages. 3.2. Vibrational Spectral Analysis. The vibrational data on molybdate units are well-known.52−55 Nevertheless, very few vibrational spectra of thorium-containing molybdates have been published. As discussed above, the coordination geometries of Mo in all three thorium molybdates in this work are based on the MoO42− group. The ideal MoO42− has Td symmetry with the four normal motions being assigned to A1(ν1), E(ν2), F1(rot), and F2(trans,ν3,ν4), where A1, E, and F2 are Raman permitted while F2 is also IR-active.56 Modes ν1−ν4 are denoted as nondegenerated symmetric stretching, doubly degenerated symmetric bending, triply degenerated asymmetric stretching, and asymmetric bending vibrations, respectively. The stretching vibrations of ν1 and ν3 in free MoO42− are located in the region from 900 to 700 cm−1, while the bending vibrations (ν2 and ν4) are situated in the range from 500 to 300 cm−1.57 When it comes to specific structure, however, because of symmetry alteration, the MoO42− groups are distorted; thus, more than four modes will be observed in the spectra. As shown in Figure 6,

the vibrational spectra for all three thorium molybdates can be coarsely separated into two frequency ranges. The first is the low-frequency region from 250 to 100 cm−1, where the modes result from lattice vibrations. Because of the strong coupling between MoO4 translations and Cs transitional modes, the modes located in this zone can be assigned as a conjunction of T(MoO4) + T(Cs). The second is the higher-frequency region originating from the internal vibrational motions of MoO42− between 1000 and 250 cm−1. The Raman- and IR-active shifts are listed in Table 2. Some weak lines in the spectra are derived from electrical or optical background; hence, these peaks have not been listed on the basis of comparison of Figure 6 and Table 2. The complete Raman fitting results for each cesium thorium molybdate compound are shown in Figure SI4 of the Supporting Information. On the basis of group theoretical principles,58,59 the standard analysis of the crystal selection rules for vibrational modes and the complete correlational relationships between the mode symmetries for all three cesium thorium molybdates are given in Table 3 and Figure SI5 of the Supporting Information, respectively. For Cs2Th(MoO4)3, the unit cell (Z = 4) comprises 12 MoO42− groups located in two nonequivalent sites (Cs, Ci). Four Th atoms occupy the C2 sites, and the eight Cs atoms are located at sites of C2 symmetry. With a total of 72 atoms in the unit cell, group theory results in 216 zone-center degrees of freedom that are distributed among the irreducible representations as Γ(D212h ) = 27Ag + 24A u + 27B1g + 24B1u + 27B2g + 30B2u + 27B3g + 30B3u 2682



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Table 3. Vibrational Modes for Cs2Th(MoO4)3, Cs2Th3(MoO4)7, and Cs4Th(MoO4)4 Crystals internal modes

external modes

ν1 ν3 ν2 ν4 transitions librations Raman-active IR-active optical acoustic

Cs2Th(MoO4)3

Cs2Th3(MoO4)7

Cs4Th(MoO4)4

2Ag + Au + 2B1g + B1u + B2g + 2B2u + B3g + 2B3u 5Ag + 4Au + 5B1g + 4B1u + 4B2g + 5B2u + 4B3g + 5B3u 3Ag + 3Au + 3B1g + 3B1u + 3B2g + 3B2u + 3B3g + 3B3u 5Ag + 4Au + 5B1g + 4B1u + 4B2g + 5B2u + 4B3g + 5B3u 8Ag + 7Au + 8B1g + 7Blu + 10B2g + 11B2u + 10B3g + 11B3u 4Ag + 5Au + 4B1g + 5Blu + 5B2g + 4B2u + 5B3g + 4B3u 27Ag + 27B1g + 27B2g + 27B3g 23B1u + 29B2u + 29B3u 27Ag + 24Au + 27B1g + 23B1u + 27B2g + 29B2u + 27B3g + 29B3u B1u + B2u + B3u

28A 84A 56A 84A 144A 84A 477A 477A 477A 3A

2Ag + 2Au + 2Bg + 2Bu 6Ag + 6Au + 6Bg + 6Bu 4Ag + 4Au + 4Bg + 4Bu 6Ag + 6Au + 6Bg + 6Bu 13Ag + 13Au + 14Bg + 14Bu 3Ag + 3Au + 3Bg + 3Bu 37Ag + 38Bg 36Au + 36Bu 37Ag + 36Au + 38Bg + 36Bu Au + 2Bu

This modes can be divided into B1u + B2u + B3u acoustic modes, 7Ag + 5Au + 7B1g + 5B1u + 5B2g + 7B2u + 5B3g + 7B3u stretching modes, 8Ag + 7Au + 8B1g + 7B1u + 7B2g + 8B2u + 7B3g + 8B3u bending modes, 8Ag + 7Au + 8B1g + 7B1u + 10B2g + 11B2u + 10B3g + 11B3u translational modes, and 4Ag + 5Au + 4B1g + 5B1u + 6B2g + 4B2u + 5B3g + 4B3u librational modes. The Ag, B1g, B2g, and B3g modes are Raman-active, while the B1u, B2u, and B3u modes are IR-active. The Au modes are silent. The sharp and intense Raman bands appearing at slightly high frequency, from 980 to 840 cm−1, strongly indicate that the Mo atoms in Cs2Th(MoO4)3 are in the form of MoO42−. The most narrow and intensive peak, at 954 cm−1, should be associated with ν1 stretching mode MoO42−, while the peaks at 828 and 775 cm−1 are considered to be due to the asymmetric stretching vibrations (ν 3 ) of MoO 4 2− . The crystal structure of Cs2Th3(MoO4)7 is triclinic in space group P1. In this structure, all atoms occupy C1 sites. Because of the huge number of atoms in the unit cell and the comparatively low crystal symmetry, both Raman and IR spectra of Cs2Th3(MoO4)7 are rich in modes. The total 477A optical modes are both Raman- and IR-active. It is notable that the lines in Raman and IR spectra of Cs2Th3(MoO4)7 are not split well. They are broader and cannot be well recognized in comparison with those of two other thorium molybdates. This behavior may result from the strong coupling among MoO42− groups inside the lattices. The monoclinic Cs4Th(MoO4)4 compound crystallizes in space group P2/c (No. 13), where Th atoms occupy 2f sites of C2 symmetry and Mo atoms occupy 4g sites of C1 symmetry. All the O atoms are situated at general positions (4g). It has 37Ag + 36Au + 38Bg + 36Bu optical modes and Au + 2Bu acoustic modes. Lattice transitions of the Cs, Th, and Mo atoms give rise to 13Ag + 13Au + 14Bg + 14Bu modes. The Raman-active modes of Cs4Th(MoO4)4 occur at the 37Ag + 38Bg species. Because of the presence of distorted tetrahedra in the structure of Cs4Th(MoO4)4 [Mo−O bond distances in the range of 1.730(3)−1.838(3) Å], great splitting of ν1 and ν3 modes is observed, as shown by the considerable intensity of lines in the spectra. Because of the different chemical bonding configuration of MoO42− due to the presence of different cations, the stretching modes for Cs4Th(MoO4)4 move toward the high-energy side compared to the Raman spectrum of Rb4Th(MoO4)448 but are in agreement with the vibrational data of MoO42− in the literature.52,60

compounds. Compound Cs2Th3(MoO4)7, with three different Th polyhedral environments (ThO7, ThO8, and ThO9), is indeed a unique case in the chemistry of thorium. The channels that exist in Cs2Th3(MoO4)7 depicted in terms of tubular building units can be topologically transformed by regular honeycomb-shaped hexagons. The analysis of stretching and bending shifts in Raman and IR spectra shows that all molybdenum atoms in these three compounds are adopted solely in MoO42− tetrahedral geometries.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

* Supporting Information S

Additional figures and tables and CIF data. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Helmholtz Association for funding via Grant VH-NG-815. REFERENCES

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4. CONCLUSIONS The thorium molybdate [Th(MoO4)3]2− chains in Cs2Th(MoO4)3 are topologically related to those in K2Th(MoO4)3, an example of a scheelite-related CaWO4 structure that is a well-known structure type accommodated by a wide range of inorganic 2683



Article

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