Thorium Chemistry in Oxo-Tellurium System under Extreme Conditions

Article pubs.acs.org/IC

Thorium Chemistry in Oxo-Tellurium System under Extreme Conditions Bin Xiao,† Philip Kegler,† Dirk Bosbach,† 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

‡

S Supporting Information *

ABSTRACT: Through the use of a high-temperature/highpressure synthesis method, four thorium oxo-tellurium compounds with different tellurium valence states were isolated. The novel inorganic phases illustrate the intrinsic complexity of the actinide tellurium chemistry under extreme conditions of pressure and temperature. Th2Te3O11 is the first instance of a mixed-valent oxo-tellurium compound, and at the same time, Te exhibits three different coordination environments (TeIVO3, TeIVO4, and TeVIO6) within a single structure. These three types of Te polyhedra are further fused together, resulting in a [Te3O11]8− fragment. Na4Th2(TeVI3O15) and K2Th(TeVIO4)3 are the first alkaline thorium tellurates described in the literature. Both compounds are constructed from ThO9 tricapped trigonal prisms and TeVIO6 octahedra. Na4Th2(TeVI3O15) is a three-dimensional framework based on Th2O15 and Te2O10 dimers, while K2Th(TeVIO4)3 contains tungsten oxide bronze like Te layers linked by ThO9 polyhedra. The structure of β-Th(TeIVO3)(SO4) is built from infinite thorium chains cross-linked by TeIVO32− and SO42− anions. Close structural analysis suggests that β-Th(TeIVO3)(SO4) is highly related to the structure of α-Th(SeO4)2. Additionally, the Raman spectra are recorded and the characteristic peaks are assigned based on a comparison of reported tellurites or tellurates. TeIV or TeVI, the tellurium compounds can be classified into two series, tellurite or tellurate, respectively. TeIV can exist in diverse coordination geometries such as TeIVO3 trigonal pyramidal, TeIVO4 disphenoid, and TeIVO5 square pyramidal.15−17 Attributable to the second-order Jahn−Teller (SOJT) distortion, the lone-pair electrons of TeIV are able to result in asymmetric units, producing the noncentrosymmetric or polar structures with interesting physical properties. The coordination geometry of TeVI is simpler than that of TeIV. TeVI exists in two forms, metatellurate TeVIO42− and orthotellurate TeVIO66−. The TeVIO42− ion that is isostructural to SO42− is very rare. Therefore, TeVI is normally found in the octahedral TeVIO6 configuration. These TeOx (x = 3−6) tellurite/tellurate units can further polymerize to produce more complicated polytellurites or polytellurates building units by sharing common corners or edges. The inexhaustible topological arrangements of these tellurium building units allow the tellurium compounds to expand nearly without limitation, forming a fascinating group of zero-dimensional (0D) clusters,18,19 one-dimensional (1D) chains,15,20 two-dimensional (2D) sheets,21 and three-dimensional (3D) framework22 materials.

1. INTRODUCTION The actinide oxo-tellurium family have gathered a wide attention primarily due to the rich redox behavior of actinides and tellurium resulting in chemical and structural complexity of the system.1−4 For instance, in higher oxidation states (+5 or +6), actinides tend to exhibit nearly linear actinyl [OAnO] groups with 2+ or 1+ positive charge.5 Here, the coordination geometry is generally observed to tetragonal, pentagonal, or hexagonal bipyramids, and the corresponding coordination environment is strongly anisotropic, attributable to the terminal oxygen atoms. On the other hand, actinides in lower valence states (from +2 to +4) usually form a more isotropic coordination environment demonstrating more similarities to the Ln series.6,7 Tetravalent thorium, for example, not possessing 5f electrons, can coordinate with up to 15 ligands to result in fascinating topological geometries and properties.8 [ThB5O6(OH)6][BO(OH)2]·2.5H2O (NDTB-1), as a noticeable representative, comprising a supertetrahedral cationic framework, exhibits a remarkable anion exchange capability that is particularly highlighted by selectively removing TcO4− from nuclear waste streams.9−11 Moreover, Th is only stable in the 4+ oxidation state in oxo-phases, a property which makes it as an ideal modeling element for the study of crystal chemistry of more toxic and radioactive NpIV/PuIV compounds.12−14 Tellurium also exhibits diverse structural properties. According to the different oxidation states of tellurium, namely, © XXXX American Chemical Society

Received: December 12, 2016

A

DOI: 10.1021/acs.inorgchem.6b03030 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

report the syntheses, crystal structures, and characterizations of four new thorium tellurites, tellurates, and mixed-valent oxotellurate/tellurites.

The combination of the actinides with tellurium has already resulted in numerous materials with novel structural topologies, especially in the uranium oxo-tellurium family. The actinide tellurium families are known from a handful of uranyl tellurium minerals, including cliffordite (UO2(Te3O7)),23 moctezumite (Pb(UO2)(TeO3)2),24 and schmitterite (UO2TeO3),25 from the deposition site in west Sonora, Mexico, near the town of Moctezuma, and also Markcooperite (Pb2(UO2)TeO6) from Otto Mountain, California.26 Among them, UO2(Te3O7) is based on a complex U-Te network that possesses Te6O18 hexagonal rings constructed from six corner-sharing TeO5 square pyramids. Following this, the uranium tellurium family is largely expanded in the past decade through the application of a variety of synthesis methodologies, together with the active explorations of many research scientists. With study, it is possible to subdivide these synthetic uranium tellurium compounds into several categories according to their different synthesis conditions (the known uranium tellurium compounds are listed in the Supporting Information). The first group is a series of uranyl tellurites prepared via a high-temperature route (around 800 °C) using molten alkaline salts as fluxes. These compounds share a common motif that they all form in 2D zigzag layers with charge-compensating cations located in between, a typical structural type that dominates hexavalent uranium chemistry.27 Compounds in the second group are derived from the hydrothermal method at relatively low temperature (around 200 °C). Compared to the first group, compounds under hydrothermal condition exhibit more diverse structural architectures, which can be exemplified from the 1D chains observed in β-Tl2(UO2)(TeO3)228 to the 3D framework found in Na8(UO2)6(TeO3)10.29 The third group is isolated from the recently developed high-temperature/high-pressure technique. Our recent studies show that the elevated pressure and temperatures have a crucial influence on chemical, structural, and material properties of the chemical elements and their compounds.30 Under the extreme condition, the compounds tend to crystallize in unconventional structure types which are dramatically different from those synthesized via conventional high-temperature or hydrothermal conditions, as demonstrated in the high probability of noncentrosymmetric observed in the Na-U-Te oxo-system and the structural complexity of the K-U-Te group.31 However, compared to the tremendous success achieved from the uranium oxo-tellurium family, very few details are known about the formation and crystal structures of thorium counterparts. The only representative in the thorium oxotellurium family with a refined crystal structure is ThTe2O6, which was obtained more than 30 years ago from a solid-state method at a temperature of 700 °C.32−34 This compound adopts the CeSe2O635 structural type, which has also been found in the compounds of CeTe2O6,33,36 PuTe2O6,37 PuSe2O6,38 and α-ThSe2O6.39 Following this, recently, another thorium tellurium compound Th13Te24O74 was reported from the reaction of ThO2 and TeO2 with a molar ratio of 13:24 in a sealed ampule at 700 °C.40 However, only the powder diffraction patterns were characterized, and no single structural analysis was performed on this material. Encouraged by the success from the uranium oxo-tellurium family, recently we undertook a series of studies on the thorium oxo-tellurium under high-temperature/high-pressure conditions. Our aim is to understand the chemistry of these compounds under the extreme condition that may lay a basis for nuclear safety and environmental application. Herein, we

2. EXPERIMENTAL SECTION 2.1. Syntheses. Syntheses. Caution! The thorium nitrate used in this study is a radioactive material due to α-emission. Therefore, standard precautions were performed, and all studies were conducted in a laboratory dedicated to studies on radioactive elements. All high-temperature/high-pressure experiments were performed using the piston cylinder module of a Voggenreiter LP 1000-540/50 at Forschungszentrum Jülich, IEK-6 in a 1/2 in. piston cylinder talc-pyrex assembly. The calibration procedure of the piston cylinder module is described in detail in ref 41. Crystal Growth of Th2Te3O11. Th2Te3O11 was synthesized by mixing 20.0 mg of Th(NO3)4·5H2O, 30.8 mg of TeO3, and 6.1 mg of Na2S2O4. This leads to an approximate ratio of Th:Te:S to 1:5:2. The mixture was thoroughly ground and transferred into a platinum capsule (outer diameter: 4 mm, wall thickness: 0.2 mm, length: 7 mm). The capsule was sealed on both sides with an impulse micro welding device (Lampert PUK U4) and placed into the center of a 1/2 in. piston cylinder talc-pyrex assembly. The experiment was performed at 3.5 GPa. The pressure was applied within 30 min and was kept constant for the complete experimental run. After the desired pressure was reached, the temperature program was started. First, the temperature was increased to 1100 °C within 30 min, and then held at this temperature for 180 min. After this, the temperature was decreased to 900 °C within 60 min. Then, the temperature was slowly decreased to 350 °C in a rate of 0.14 °C/min, followed by quenching to room temperature. The quenching time of the sample is about 2−3 s. After quenching, the setup was decompressed in a period of 30 min. Finally, the platinum capsule was crashed in order to extract the reaction products out of the high-pressure assembly. After synthesis, a few colorless blocks were isolated for crystallographic studies. Crystal Growth of Na4Th2(TeVI3O15). Na4Th2(TeVI3O15) was obtained using the same temperature steps as those of Th2Te3O11. Loading 20.0 mg of Th(NO3)4·5H2O, 11.2 mg of TeO2, and 6.0 mg of NaNO3 into a platinum capsule (outer diameter: 4 mm, wall thickness: 0.2 mm, length: 7 mm). This results in the Th:Te:Na ratio of 1:2:2. Several of the colorless blocks were suitable for single crystallographic studies. Crystal Growth of K2Th(TeVIO4)3. K2Th(TeVIO3)3 was prepared through mixing 20.0 mg of Th(NO3)4·5H2O, 61.6 mg of TeO3, and 78.5 mg of KCl. This leads to an approximate ratio of reactants of Th:Te:K = 1:10:30. The mixture was fully ground and transferred into a platinum capsule (outer diameter: 4 mm, wall thickness: 0.2 mm, length: 7 mm). The experimental pressure and temperature procedures adopted for K2Th(TeVIO4)3 were very similar to those of Na4Th2(TeVI3O15). The only difference is the cooling rate. Here, we used a final rate of 0.10 °C/min to decrease the temperature form 900 to 350 °C. Crystals in the form of colorless tablets were isolated for crystallographic studies. Crystal Growth of β-Th(TeIVO3)(SO4). Th(TeIVO3)(SO4) was prepared through mixing 20.0 mg of Th(NO3)4·5H2O, 5.6 mg of TeO2, and 6.1 mg of Na2S2O4. This leads to the ratio of Th:Te:S equals 1:1:2. β-Th(TeIVO3)(SO4) was synthesized by following the similar procedures for making the K2Th(TeVIO4)3. The colorless crystals were isolated for crystallographic studies. 2.2. Crystallographic Studies. The as-obtained thorium tellurium crystals were selected for data collection. The crystals were mounted on glass fibers and optically aligned on a single crystal diffractometer (Agilent SuperNova, Dual Source). The data collection was done using a monochromatic Mo−Kα tube which has the incident wavelength of 0.71073 Å and runs at 50 kV and 0.8 mA providing a beam size of approximately 30 μm. The unit-cell dimensions for these crystals were refined using least-squares techniques against the positions of all measured reflections. More than a hemisphere of data were collected for each crystal, and the three-dimensional data were reduced and filtered for statistical outliers using the standard B


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Inorganic Chemistry

Table 1. Crystallographic Data of Th2Te3O11, Na4Th2(TeVI3O15), K2Th(TeVIO4)3, and β-Th(TeIVO3)(SO4), Respectively

a

compound

Th2Te3O11

Na4Th2(TeVI3O15)

K2Th(TeVIO4)3

β-Th(TeIVO3)(SO4)

mass space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z λ (Å) ρcalcd (g cm−3) R(F) for Fo2 > 2σ(Fo2)a wR2(Fo2)b

1022.88 P21/c 12.4379(6) 5.2898(2) 17.3601(9) 90 130.678(3) 90 866.22(8) 4 0.71073 7.844 0.0323 0.1437

1178.83 R3c̅ 9.9109(3) 9.9109(3) 45.1306(11) 90 90 120 3839.1(3) 12 0.71073 6.119 0.0248 0.0968

885.04 Pnma 11.4511(8) 7.3352(5) 12.4911(13) 90 90 90 1049.20(15) 4 0.71073 5.603 0.0425 0.1066

503.70 P21/c 7.0324(5) 9.6314(6) 8.8053(6) 90 103.724(7) 90 579.37(7) 4 0.71073 5.775 0.0483 0.1271

R(F) = ∑||Fo| − |Fc||/∑|Fo|. bR(F2o) = [∑w(F2o − F2c )2/∑w(F4o)]1/2.

CrysAlisPro program. Data were corrected for Lorentz, polarization, absorption, and background effects. The crystal structure determination and refinement were carried out using the SHELXL-97 program.42 The data and crystallographic information are given in Table 1. The structures were solved by direct methods and refined to R1 = 0.0323 for Th2Te3O11, R1 = 0.0248 for Na4Th2(TeVI3O15), R1 = 0.0425 for K2Th(TeVIO4)3, and R1 = 0.0483 for β-Th(TeIVO3)(SO4), respectively. 2.3. Raman Studies. Utilizing a Peltier cooled multichannel CCD detector, the unpolarized Raman spectra were recorded with a Horiba LabRAM HR spectrometer. All the samples were in the form of single crystals. An objective with a 50× magnification was linked to the spectrometer, allowing the analysis of samples as small as 2 μm in diameter. The incident radiation was produced by 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 around 1 cm−1 with a slit of 100 μm. The Raman spectroscopic investigations for all samples were executed at room temperature in the range of 100−1050 cm−1. 2.4. Scanning Electron Microscopy/Energy-Dispersive Spectroscopy (SEM/EDS). Scanning electron microscopy images and energy-dispersive spectroscopy (SEM/EDS) data were collected using a FEI Quanta 200F Environment Scanning Electron Microscope. The EDS results are in good agreement with the proposed chemical compositions for all four thorium tellurium compounds. 2.5. Bond-Valence Analysis. Bond-valence sums (BVS) for all atom positions in the four thorium tellurium compounds were calculated. The bond-valence parameters for ThIV−O, TeIV−O, TeVI− O, and SVI−O are used according to Brese and O’Keeffe.43 The BVS for all atoms are consonant with their expected formal valences.

alkali source. Our efforts in this direction did not yield the intended products. Instead, two new thorium tellurate phases, that is, Na4Th2(TeVI3O15) and K2Th(TeVIO4)3, were obtained. The TeVI is found in both cation-involved phases, even though Na4Th2(TeVI3O15) was synthesized from the TeO2 as the starting chemical. This might suggest that the sealed-condition environment with the presence of oxo nitrates could drive the valence transition from TeIV to TeVI.31,45 3.2. Structure Descriptions. Th2Te3O11. The Th2Te3O11 crystallizes in the P21/c space group, exhibiting a complicated 3D framework in which the thorium layers are interconnected by tellurium fragments (see Figure 1a). There are two crystallographically different Th and three Te sites in an asymmetric unit. Both Th sites are nine-fold coordinated, forming in a distorted monocapped square antiprismatic configuration. The Th(1)−O bond distances range from 2.207(9) to 2.640(9) Å, and are slightly longer than those of Th(2), which range from 2.254(9) to 2.630(10) Å, but are still comparable with those in other thorium compounds.33,46,47 These two ThO9 polyhedra are connected with each other through common edges, resulting in corrugated Th layers that stretch along the bc plane (Figure 1b). It is of interest that such layers with the same topological linkages are also observed in a series of lanthanide-bearing tellurium compounds, M2Te4O11 (M = La−Nd and Sm−Yb).48 Similar to the structure of Th2Te3O11, the tellurium units in these latter compounds are also found being embedded between lanthanide layers. In the tellurium-based trimer, the Te(1) and Te(2) are tetravalent with lone-pair electrons, while Te(3) is observed to be hexavalent. The TeIV(1) site is coordinated by three O atoms, resulting in a more frequently observed TeO3 trigonal geometry with the lone pairs occupying the pyramidal site. The bond distances and angles in TeO3 are from 1.860(10) to 1.910(1) Å and from 90.1(4) to 97.1(3)°, respectively. The TeIV(2) is found to be surrounded by four oxygen atoms, forming in a disphenoidal geometry. The Te−O lengths are in the range of 1.869(10)−2.315(10) Å, which are consistent with reported TeIV in four-fold coordinated geometry.20,49 The sixfold coordinated TeVI(3) site is in a distorted octahedral geometry. The TeVI(3)−O bond distances exhibit three short (from 1.860(9) to 1.876(10) Å) and three long Te−O (from 1.927(9) to 2.070(10) Å) bonds. It is noteworthy that Te(3)O6 is highly polarized, attributable to the lone-pair electrons from TeIV(1) cations near to this octahedral site. The distortion can

3. RESULTS AND DISCUSSION 3.1. Synthesis. The reaction of thorium nitrate with a series of tellurium sources under extreme conditions generates four thorium oxo-tellurium compounds containing different oxidation states of Te. The tellurium can be tetravalent and/or hexavalent. The standard redox potential of TeVI/TeIV is 1.02 V in aqueous solution.44 The mixed-valence (TeVI/TeIV) TeTh2Te3O11 can be isolated as a major phase using TeO3 and Na2S2O4. Here, the Na2S2O4 was used as a reducing agent. We found in our synthesis that, if TeO3 was replaced by TeO2, only Th(TeIVO3)(SO4) can be isolated. We also tried to synthesize the mixed-valent oxo-tellurium phases by using TeO2/TeO3 as the starting materials without Na2S2O4 but failed. This indicates the essential role of Na2S2O4 as phase forming agent. Initially, we tried to synthesize the alkali-cations dependent thorium mixed-valent oxo-tellurium by adding the NaNO3 or KCl as C


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Inorganic Chemistry

shown in Figure 2a,b. These pseudo-channels, with ellipticshaped cross sections, are constructed from a skeleton of ThO9 and TeO6 polyhedra with a pair of either TeO3 or TeO4 filled in the center (see Figure 2b). In order to describe the interior polyhedral linkages around the channel structure, here we cut and unfold the associated cylindrical channel into the 2D plane, exhibited in Figure 2c,d. The planes shown in Figure 2d are formed from an assemblage of Th2O16 dimers and TeO6 octahedra, which are connected by means of both cornerand edge-sharing manners. The corresponding nodal presentation of planar topology is shown in Figure 2e. It is assembled solely from eight-membered rings. All the white (Te) nodes are three-connected, while the black (Th) nodes are two- or threeconnected. Using the method of building stripes,51 the idealized topological structure of Th2Te3O11 can be converted via the folding and gluing procedure. First, label the equivalent points on sides of the stripe by letters a, b, c, and d. Second, fold the stripe by jointing the corresponding opposite sides (a − a′, b − b′, c − c′, and d − d′) to make a cylinder. It is important to note that the method of building stripes is also helpful to understand the symmetry inside the channels.52 As can be easily detected from Figure 2f, the idealized channel structure is related by a two-fold screw axis along the b-axis, as required by the space group P21/c. Therefore, the corresponding cylindroid channels feature an achiral topology in this structure. Such a method was also adopted by Krivovichev et al. to present the linkage of U and Mo polyhedra inside the channels of a rare openframework structure of (NH4)4[(UO2)5(MoO4)7](H2O)5.53 In contrast to Th2Te3O11, the equivalent points on the side of tapes for (NH4)4[(UO2)5(MoO4)7](H2O)5 are not opposite with each other; thus, their folding procedure results in a chiral U-Mo channel topology. Na4Th2(TeVI3O15). The structure of Na4Th2(TeVI3O15) with space group of R3̅c, as determined by single crystal X-ray diffraction study, demonstrates two crystallographically distinct Th sites and one Te site in an asymmetric unit. It is based upon a 3D [Th2Te3O15]4− anionic framework which consists of Th2O15 and Te2O10 dimers (see Figure 3a), and is charge balanced by Na+ cations. Both Th(1) and Th(2) sites are coordinated by nine oxygen atoms. They share a common face to give rise to a Th2O15 dimer. This Th2O15 dimer that resides on a three-fold rotational axis is further bounded with three Te2O10 dimers, forming a [Th2Te6O33]22− fragment, as shown in Figure 3b. Each Te2O10 dimer in the resulting thorium tellurate fragment is composed by two Te(1) in an edgesharing manner, similar to the one in the reported AgUO2(HTeO5)54 and (NH4)4(VO2)2[Te2O8(OH)2]·2H2O.15 Two of such thorium tellurate fragments which are related by an inversion center are connected through sharing of corners with another three Te2O 10 dimers to compose a so-called fundamental building block (FBB) (see Figure 3c). The 3D framework of Na4Th2(TeVI3O15) is finally complete by bridging these FBBs, shown in Figure 3d. The ThO9 tricapped trigonal prisms in Na4Th2(TeVI3O15) are distorted with one short (Th−O(3)) and one rather long Th−O bond (Th−O(4)) with distances of 2.316(7) and 2.900(7) Å, respectively. In this case, the Th−O bond lengths in Na4Th2(TeVI3O15) are less uniform compared to those in ThO2 (Th−O bond distances are fixed to 2.42 Å) but comparable to those in Th(TeO3)2 (between about 2.20 and 2.53 Å). The values of Te(1)−O bond lengths vary from 1.871(7) to 2.066(6) Å, with the shortest lengths being the edging-sharing oxygen atoms (O(1) and O(4)). The BVS

Figure 1. View of the structure of Th2Te3O11. (a) Projection of the Th2Te3O11 along the [010] direction. The structure can be considered as composed of thorium sheets with tellurium fragments in between. (b) View of the thorium sheets along the [100] direction. To better understand the structural complexity, the two crystallographically unique Th sites are distinguished by different colors. Orange and yellow stand for Th(1) and Th(2), respectively. (c) The local coordination environment of tellurium fragment with the formula of [Te3O11]8−. (d) The [Te3O8]4− anion fragment observed in the lanthanide alkaline-earth tellurite of La2Ba(Te3O8)(TeO3)2 bears an analogue to that found in Th2Te3O11. Legends: the Th polyhedra are in yellow, and the TeIVO3, TeIVO4, and TeVIO6 in the compound Th2Te3O11 are in pink, green, and blue, respectively. The TeO3 in the La2Ba(Te3O8)(TeO3)2 are in dark yellow.

be especially observed from the bonded oxygen atoms, which is denoted by the longest Te−O distance (2.070(10) Å) in this direction. Bond-valence calculations on these three Te sites result in values of 3.91, 3.93, and 5.92 v.u. for Te(1), Te(2), and Te(3), respectively. A unique feature of Th2Te3O11 is the presence of mixedvalent tellurium in three different polyhedral geometries (TeO4, TeO5, and TeO6). These Te polyhedra are fused together by sharing trans-corners of the TeVI(3)O6 octahedron, leading into a [Te3O11]8− trimetric fragment, shown in Figure 1c. In a construction perspective, the resulting [Te3O11]8− fragment demonstrates a certain similarity to the [Te3O8]4− anion found in the structure of La2Ba(Te3O8)(TeO3)2, a lanthanide tellurite (see Figure 1d).50 In the latter structure, these anion fragments also play a role of linkage to cross-connect adjacent lanthanum−tellurium layers. It is noteworthy that the mixedvalent tellurium with three kinds of coordination geometries is the first time to be observed with respect to the tellurium chemistry. Most reported tellurium compounds contain Te cations only in one or two coordination geometries with the corresponding oxidation state of either tetravalent or hexavalent. Though there has been a report of two isostructural tellurium compounds, NH4ATe4O9·2H2O (A = Rb or Cs), which simultaneously embrace three different coordinations of TeIV, pyramidal TeO3, disphenoid TeO4, and square pyramidal TeO5,49 they only contain one kind of Te oxidation state (TeIV). The structure of Th2Te3O11 can also be portrayed as based on the fundamental building block of infinite pseudo-channels, D


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Inorganic Chemistry

Figure 2. (a) Structure of Th2Te3O11 projected along the b-axis. (b) The elliptic-shaped pseudochannel is formed of ThO9 and TeO6 polyhedra. (c) The channel extends along the b-axis. (d) The channel is unfolded into the 2D plane. (e) Nodal presentation of the plane topology shows the building stripes of the channel structure. (f) Fold the stripe by jointing the corresponding opposite sides (a − a′, b − b′, c − c′, and d − d′) to make a cylinder inTh2Te3O11.

Figure 3. Process of building the structure of Na4Th2(TeVI3O15). (a) The Th2O15 and Te2O10 dimers. (b) One Th2O15 dimer that resides on a three-fold rotational axis is further bounded with three Te2O10 dimers, forming a [Th2Te6O33]22− fragment. (c) Two of such thorium tellurate fragments related by an inversion center are connected with another three Te2O10 dimers to compose a so-called fundamental building block (FBB). (d) The 3D framework of Na4Th2(TeVI3O15) is finally complete by bridging these FBBs. Legends: Th and Te polyhedra are shown in yellow and blue. Na+ ions are shown in black nodes. Figure 4. (a) View of the structure of K2Th(TeVIO4)3 along the b-axis. (b) The projection along the [100] direction shows the HTO-like topology arrangement of TeO6 octahedra. (c) The local coordination environment of ThO9 capped square antiprism. (d) The local coordination of Te centers.

calculation for only Te site results in 5.77 v.u., confirming the hexavalent oxidation state of tellurium in Na4Th2(TeVI3O15). K2Th(TeVIO4)3. The 3D compound K2Th(TeVIO4)3 crystallizes in the Pnma space group with one Th and two Te atoms per asymmetric unit. The fundamental building block of this structure can be considered as an undulated hexagonal tellurium layer interlinked by ThO9 polyhedra, shown in Figure 4a. The charge compensating K+ cations are found resided between the intervals of adjacent tellurium layers. The Th atoms, serving as an interlayer linker, achieve a capped square antiprismatic coordination geometry by attaching nine

O atoms from the tellurium layers. More specifically, each ThO9 polyhedron shares three edges of its square face with three TeO6 octahedra from one tellurium layer, and also shares one corner and two edges with the other three TeO6 octahedra from the adjacent oxo-tellurium layer. The variation of Th−O distances is appreciable, from 2.325(12) to 2.691(8) Å; the E


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Inorganic Chemistry average distance is 2.480 Å. As the case of two crystallographically different Te sites, both are octahedrally coordinated by O with Te−O bond distances between 1.851(12) and 2.009(8) Å. Within each TeO6 octahedron, a bond asymmetry is observed in the local C2 direction. This out-of-center distortion results in two short, two normal, and two long Te−O bonds. The short bond distances occur particularly in terminal O, which has Te−O bond distances of 1.851(12) and 1.873(13) Å for Te(1) and 1.858(8) and 1.883(7) Å for Te(2), respectively. In fact, these intra-octahedral distortions are quite common among those reported for HTO-type (hexagonal-tungsten-oxide-type) compounds.55 Bond-valence sum calculations of K2Th(TeVIO4)3 lead to values of 5.82, 5.84, and 4.07 v.u. for Te(1), Te(2), and Th(1), respectively, confirming the presence of hexavalent tellurium and tetravalent thorium. The local coordination geometries for Th and Te in K2Th(TeVIO4)3 are shown in Figure 4c,d. Inside the tellurium layers, all the TeO6 are corner-shared with four other tellurium octahedra in their equatorial planes, leaving two trans-corners which are not involved in interplane linkage. As a result, the resulting tellurium layer has the composition of [TeO4]2− and is propagating parallel to the (100) plane (see Figure 4b. These tellurium layers stack along the crystallographic a-axis in a way that the hexagonal cavity of one layer is facing the triangular cavity of its next layer, forming in an ···ABABAB··· configuration. The neighboring “A” and “B” layers are related with each other by translation of the length of a ThO9 wide along the crystallographic c-axis. The occurrence of tellurium layers with the corner-sharing TeO6 octahedra is usually observed among the family called hexagonal tungsten oxide bronzes (HTO).56−59 Most compounds in this family are composed of three- and six-membered tunnels constructed by corner-sharing MO6 octahedra (M = W, Mo, and Te). Typical examples are hexagonal WO3 and alkali tungsten bronzes AxWO3.60,61 In these compounds, the neighboring layers are further connected by sharing two trans apical O atoms of the WO6 octahedra, forming 3D tungsten networks. In K2Th(TeVIO4)3, however, the tellurium layers are separated, and they are further linked by ThO9 polyhedra. It is also noted that the structure of K2Th(TeVIO4)3 also bears striking similarities with that of RbTe2O6, which may also be considered as a derivation of the HTO family.62 In RbTe2O6, each WO6 octahedron shares all its corner O atoms with surrounding WO6 octahedra to create a 3D network with complex triangle and hexagonal cavities. Structure of β-Th(TeIVO3)(SO4). α-Th(TeIVO3)(SO4) (P21/ c) was recently reported to be obtained from hydrothermal synthesis with amorphous Th(OH)4, H2SO4 solution, and TeO2 as starting materials.63 Using the high-temperature/highpressure method, we successfully isolated its second modification, β-Th(TeIVO3)(SO4). β-Th(TeIVO3)(SO4) also crystallizes in the P21/c space group. α- and β-Th(TeIVO3)(SO4) represent a structural example where similar building blocks combine to give rise to compounds with the same stoichiometry but different architecture. Both polymorphs are composed of ThO9, TeO3, and SO4 polyhedra. The structure of α-Th(TeIVO3)(SO4) is slightly different with that of βTh(TeIVO3)(SO4), and the densities of α-Th(TeIVO3)(SO4) and β-Th(TeIVO3)(SO4) are 6.02 and 5.78 g/cm3, respectively. This difference stems from the structural dimensionality of these two polymorphs, where the α-Th(TeIVO3)(SO4) is formed in a 2D layered structure, while the β-Th(TeIVO3)(SO4) is a 3D framework, shown in Figure 5a.

Figure 5. Comparison between β-Th(TeIVO3)(SO4) and α-Th(SeO3)2. (a, d) View of the structure of β-Th(TeIVO3)(SO4) and αTh(SeO3)2, respectively. (b, e) The thorium-tellurite chains in βTh(TeIVO3)(SO4) and α-Th(SeO3)2, respectively. (c, f) The local Th coordination environment between β-Th(TeIVO3)(SO4) and αTh(SeO3)2, respectively.

The structure of β-Th(TeIVO3)(SO4) is dominated by Th chains that propagate along the b-axis. There are one crystallographically distinct Th, one Te, and one S site in the asymmetric unit. The Th ion, in tricapped trigonal geometry (ThO9), is simultaneously bonded to TeO32− and SO42− anions. The TeO32− units chelate and bridge with the Th ions while the SO42− groups coordinate with Th ions only in a corner-sharing manner. The Th−O bond lengths range from 2.29(1) to 2.58(1) Å with an average value of 2.46 Å. This value is consistent well with that in other compounds with ninecoordinated Th.64 Each Te4+ cation in the TeO3 trigonal pyramid exhibits an asymmetric coordination environment attributable to the lone pairs. The Te−O bond lengths for βTh(TeIVO3)(SO4) are between 1.83(1) and 1.89(1) Å. The S6+ cation is linked by four O atoms and shows SO4 tetrahedral geometry with the bond lengths and the O−S−O bond angles ranging from 1.42(1) to 1.53(1) Å and from 103.9(7)° to 114.6(9)°, respectively. Bond-valence calculations for the Th4+, Te4+, and S6+ result in values of 4.2, 4.1, and 5.7 v.u., respectively, agreeing well with the expected valence states in βTh(TeIVO3)(SO4). It is noteworthy that the crystal structure of β-Th(TeIVO3)(SO4) is very similar to that of α-Th(SeO3)2.39 Both compounds contain edge-shared Th ribbons which are connected together through TeO32−/SO42− or SeO32− units. The long pairs of TeO32− or SeO32− units are distributed perpendicular to the Th ribbons in β-Th(TeIVO3)(SO4) and αTh(SeO3)2, respectively. However, the coordination geometry of Th atoms is fundamentally different in both compounds. In order to full understand the differences involved, these ribbons are dissected into simpler units, as shown in Figure 5. Each Th4+ cation in β-Th(TeIVO3)(SO4) shares one edge and two corners with the SeO32− trigonal pyramids and another four corners with SO4 tetrahedra. This leads to a Th coordination number of 9 (Figure 5c). In contrast, each Th achieves eightfold coordination with O atoms in a bicapped trigonal prism F


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Inorganic Chemistry

The Raman spectra for all the above-discussed thorium tellurium compounds are characterized by motions from tellurite (TeIVO3 trigonal pyramid), or tellurate (TeVIO6 octahedron). For TeIVO32− units in C3v symmetry, there are four vibrational modes, including v1 (symmetric stretching vibration), v2 (symmetric bending vibration), v3 (doubly degenerate antisymmetric stretching vibration), and v4 (doubly degenerate bending vibration). In an aqueous solution, the free TeO32− units have the frequencies of v1 at 758 cm−1, v2 at 364 cm−1, v3 at 703 cm−1, and v4 at 326 cm−1, respectively.66 All of these vibrations are both Raman and infrared active.67,68 The ideal TeVIO6 octahedron has three kinds of Raman active vibrational bands, v1(A1g), v2(Eg), and v5(F2g), and the stretching and bending vibrations for mostly tellurate compounds are distributed in the range of 550−750 cm−1 and 350−450 cm−1, respectively.69 As mentioned above, compound Th2Te3O11 is constructed from both tellurite and tellurate units, which is expected to be reflected in its Raman vibrations. Due to the heavily overlapping bands of different kinds of Te-O vibrations in Th2Te3O11, the assignment of the vibrational peaks is difficult. Therefore, only the selected assignment was made based on the comparison of the reported Raman studies of tellurites and tellurates.67,70,71 The spectrum of Th2Te3O11 is dominated by the presence of intense and narrow bands, among which are overlapping bands and bands with a high intensity. This is primarily derived from the strong interactions between coordination geometries of different types (TeO3, TeO4, and TeO6). The spectral profile of Th2Te3O11 bears considerable resemblance to that of the mixed tellurite-tellurate mineral yecoraite (Bi5Fe3O9(TeIVO3)(TeVIO4)2·9H2O).72 Frost et al. reported that the Raman bands of mixed-valent tellurium compounds in this region roughly follow a basic frame: tellurate unit (TeO62−) has higher wavenumber bands than those of the tellurite unit (TeO32−).72 Thus, the band located at 813 cm−1 in Th2Te3O11 may be attributed to the v1 of TeO66−, while the band located at 790 cm−1 may be assigned to v1 of TeO32− (Table 2). It is interesting to compare the spectra of Na4Th2(TeVI3O15) and K2Th(TeVIO4)3, since these two compounds have only octahedral TeO6 units. The spectra of Na4Th2(TeVI3O15) and K2Th(TeVIO4)3 match very well in the region from 600 to 900 cm−1. The spectrum of the former compound shows five well separated bands at 619, 656, 680, 714, and 813 cm−1. These bands are located at slightly higher frequencies than those for the potassium-based compound (606, 637, 654, 699, and 767 cm −1 ). The first four bands might be assigned to v 3 antisymmetric stretching modes and the last one band to the v1 symmetric stretching mode. For the spectrum of β-Th(TeIVO3)(SO4), the bands around 600−750 cm−1 might be due to one of or both Te-O and S-O vibrations because this is an overlapping zone for these vibrations. The intense peak at 817 cm−1 can be ascribed to the symmetric Te-O stretch, while the sharp peak located at 1018 cm−1 is attributed to v1 of SO42−. The positions of these bands are in good agreement with those reported for tlapallite, a mixed tellurite-sulfate oxo-anion mineral.73

(Figure 5f), by attachment of six corners and one edge with SeO32− units. Role of Counter Cations and Mixed Oxo-Anions on the Structure of Actinide Tellurium Compounds. As can be seen from the above description, all four reported thorium tellurium compounds, Th2Te3O11, Na4Th2(TeVI3O15), K2Th(TeVIO4)3, and β-Th(TeIVO3)(SO4), crystallize in different structures, dependent on the participation of alkali cations or SO42− oxoanions. A closer examination shows that, under the extreme condition, these third party ions play an important role in determining the architectures of crystalline actinides bearing phases, including formation of polarity and chirality, increase of structural dimensionality, and creation of cavities and channels. Specifically, the influence of different Na+ cationic content on the acentric structure formation is particularly apparent in the sodium uranyl tellurium family.45 In terms of dimensional increase, it has already been exhibited that the uranyl structures increased from commonly observed 2D sheets to 3D frameworks in the potassium-bearing uranyl tellurium family.31 Besides, it is noted that the ratio of Te/Th also plays a significantly role to affect the structure architectures. Especially, in Th(TeIVO3)(SO4), due to the smaller ratio of Te/Th (Te:Te = 1:1) attributed to the dilution of SO42− oxo-anions, the TeIV oxo-anions are isolated. However, the Te polyhedra are polymerized into polynuclear clusters or extended chains in the other three thorium tellurium compounds. Undoubtedly, more flexible structure architectures are left to be discovered under the extreme condition by using variations of the alkaline cations and mixed oxo-anions. 3.3. Raman Spectral Analysis. Raman spectra in the range of 100−1050 cm−1 of all studied thorium tellurium compounds are presented in Figure 6. The Raman vibrational data of tellurites or tellurate compounds are well-established in the literature;65 however, only a few studies have been undertaken on the vibrational spectroscopy of thorium tellurates or tellurites. Here, this work may help to fill up the research blankness in this regard.

4. CONCLUSION In summary, the complex structural chemistry of the Th-Te oxo-family obtained under extreme condition is elucidated for the first time. We demonstrated that the high-temperature/ high-pressure synthesis method can serve as a facile route to

Figure 6. Raman spectra for Th2Te3O11, Na4Th2(TeVI3O15), K2Th(TeVIO4)3, and β-Th(TeIVO3)(SO4), respectively. G


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Inorganic Chemistry Table 2. Observed Raman Bands (cm−1) for Th2Te3O11, Na4Th2(TeVI3O15), K2Th(TeVIO4)3, and β-Th(TeIVO3)(SO4), Respectivelya Th2Te3O11

Na4Th2(TeVI3O15)

K2Th(TeVIO4)3

β-Th(TeIVO3)(SO4)

105(m) 113(s) 126(s) 138(m) 144(m) 150(m) 154(m) 174(s) 196(m) 211(w) 219(w) 237(w) 248(s) 282(w) 298(m) 299(m) 327(m) 341(m) 364(w) 387(s) 401(m) 449(m) 456(s) 509(m) 583(m) 647(sh) 651(sh) 659(m) 668(sh) 686(s) 697(sh) 753(s) 763(sh) 775(s) 790(s) 813(s)

122(w) 142(w) 164(m) 240(m) 255(sh) 283(sh) 384(m) 412(m) 439(m) 462(m) 485(m) 492(m) 498(m) 526(m) 580(sh) 619(sh) 656(s) 680(s) 714(sh) 735(sh) 767(s) 820(sh) 865(m) 899(m) 935(m)

134(m) 163(w) 207(s) 255(w) 275(w) 329(m) 341(m) 398(m) 422(s) 466(s) 384(s) 394(s) 413(m) 440(s) 606(w) 637(m) 654(m) 699(s) 735(w) 759(w) 821(s) 914(w) 933(w)

102(sh) 118(sh) 142(w) 153(m) 178(m) 197(m) 222(m) 310(w) 314(w) 346(m) 456(s) 477(s) 615(m) 645(m) 693(s) 714(s) 736(sh) 748(sh) 817(s) 1018(s)

condition on the coordination flexibility of actinides, allowing the formation of new polymorphs with different structural topologies.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b03030. EDS measurement results, BSE images, and known uranyl tellurium phases (PDF) Crystallographic data for K2Th(TeVIO4)3 (CIF) Crystallographic data for Na4Th2(TeVI3O15) (CIF) Crystallographic data for Th2Te3O11 (CIF) Crystallographic data for β-Th(TeIVO3)(SO4) (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Evgeny V. Alekseev: 0000-0002-4919-5211 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors are grateful to Dr. Martina Klinkenberg and Jakob Dellen (IEK-6, Forschungszentrum Jülich) for the kind help in electron microscopy, EDX experiments, and Raman experiment. We are grateful to the Helmholtz Association for supporting within the VH-NG-815 grant.

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REFERENCES

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a

Abbreviation: s, strong; m, medium; w, weak; sh, shoulder. The accuracy of all values is assumed to be ±1 cm−1.

produce novel actinide tellurium oxo-phases. The thorium tellurium compounds possess interesting features such as multivalence and multicoordination geometries. Th2Te3O11 is the first actinide tellurium structure that contains both tetravalent and hexavalent oxidation states of Te in three different coordination configurations (TeIVO3, TeIVO4, and TeVIO6). Na4Th2(TeVI3O15) and K2Th(TeVIO4)3 are the first thorium tellurates containing alkaline cations. The structure difference between these two tellurates seems to be due to the size and dimensions of the alkali metals, which have exemplified to be an important factor for synthesizing actinides. Our further studies will include more other alkaline metals such as Li, Rb, and Cs to understand these effects on the electronic structure of the actinides under extreme condition. The high-pressure compound of β-Th(TeIVO3)(SO4) crystallizing in a 3D framework is based on nine-fold ThO9 polyhedra, which is completely different from its room-pressure counterpart αTh(TeIVO3)(SO4), a 2D structure building from eight-fold coordinated ThO8. This shows the influence of extreme H


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Inorganic Chemistry

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