High-Pressure Synthesis and Characterization of New Actinide Borates, AnB4O8 (An=Th, U)

New actinide borates ThB4O8 and UB4O8 were synthesized under high-pressure, high-temperature conditions (5.5 GPa/1100 °C for thorium borate, 10.5 GPa/1100 °C for the isotypic uranium borate) in a Walker-type multianvil apparatus from their corresponding actinide oxide and boron oxide. The crystal structure was determined on basis of single-crystal X-ray diffraction data that were collected at room temperature. Both compounds crystallized in the monoclinic space group C2/c (Z=4). Lattice parameters for ThB4O8: a=1611.3(3), b=419.86(8), c=730.6(2) pm; β=114.70(3)°; V=449.0(2) Å3; R1=0.0255, wR2=0.0653 (all data). Lattice parameters for UB4O8: a=1589.7(3), b=422.14(8), c=723.4(2) pm; β=114.13(3)°; V=443.1(2) Å3; R1=0.0227, wR2=0.0372 (all data). The new AnB4O8 (An=Th, U) structure type is constructed from corner-sharing BO4 tetrahedra, which form layers in the bc plane. One of the four independent oxygen atoms is threefold-coordinated. The actinide cations are located between the boron–oxygen layers. In addition to Raman spectroscopic investigations, DFT calculations were performed to support the assignment of the vibrational bands.


Introduction
Over the last decade, our research into the high-pressure chemistry of borates has led to the synthesis of several new compounds with fascinating structures, owing to the efficient use of the multianvil technique. [1] For example, we discovered the rare-earth borate Dy 4 B 6 O 15 , [2] which was the first borate that exhibited edge-sharing BO 4 tetrahedra. Later on, HP-NiB 2 O 4 was synthesized, [3] which was the first borate in which all of the [BO 4 ] 5À tetrahedra showed a linkage through a common edge to a second tetrahedron, as well as HP-KB 3 O 5 , [4] which simultaneously contained all three pos-sible conjunction modes, that is, corner-sharing BO 3 groups, corner-sharing BO 4 units, and edge-sharing BO 4 tetrahedra.
Following our interest in the high-pressure chemistry of alkali, alkaline-earth, transition-metal, and rare-earth borates, we decided to broaden our research activities into the field of actinide borates. This field of structural chemistry is highly topical, as reflected by the considerable number of new actinide borates with interesting structures and properties that have been synthesized within the last few years. [5][6][7][8][9][10][11] A closer look at the existing compounds with An-B-O ternary systems only showed a few phases. Just six compounds with the actinide cations thorium, uranium, and americium are known, namely: ThA C H T U N G T R E N N U N G (B 2 O 5 ), [12] ThB 66.8 O 0.36 , [13] UA C H T U N G T R E N N U N G (BO 3 ) 2 , [14] (UO 2 )A C H T U N G T R E N N U N G (B 2 O 4 ), [12] AmB 9 O 18 , [11] and AmBO 3 . [15] To the best of our knowledge, no ternary actinide borates have been synthesized under high-pressure conditions so far. However, recent studies on the chemistry of high-pressure alkaline uranyl borates demonstrated the feasibility of this approach. [10] Historically, Berzelius has already reported the possible presence of a thorium borate in a mineral that was found in Norway in 1824. [16] Furthermore, there are several existing hydrated actinide borates of the actinides thorium, uranium, neptunium, plutonium, and americium. Research into actinide borates is of urgent importance in the question of the storage of nuclear waste. Owing to the high stability and insolubility of borates, they are of interest for the immobilization of nuclear waste. In this context, borates in which the metal cation is in the oxidation state 4+ have a special position, because the cation (especially cerium) can be regarded as a "dummy" for plutonium, owing to their comparable ionic radii. Herein, we describe the syntheses, single- type is constructed from corner-sharing BO 4 tetrahedra, which form layers in the bc plane. One of the four indepen-A C H T U N G T R E N N U N G dent oxygen atoms is threefold-coordinated. The actinide cations are located between the boron-oxygen layers. In addition to Raman spectroscopic investigations, DFT calculations were performed to support the assignment of the vibrational bands.
Keywords: actinides · borates · density functional theory · highpressure chemistry · Raman spectroscopy crystal structural determinations, and Raman spectroscopic investigations of AnB 4 O 8 A C H T U N G T R E N N U N G (An = Th, U), as well as quantumchemical calculations of the harmonic vibrational frequencies of ThB 4 O 8 .

Results and Discussion
Synthesis and crystal-structure analysis: The compounds ThB 4 O 8 and UB 4 O 8 were synthesized from their corresponding actinide oxides and B 2 O 3 under high-pressure, high-temperature conditions (5.5 GPa and 1100 8C for ThB 4 O 8 ; 10.5 GPa and 1100 8C for UB 4 O 8 ) in a 1000 ton multianvil press that was fitted with a Walker-type module. A detailed description of the syntheses is provided in the Experimental Section. Figure 1 shows the diffraction patterns of ThB 4 O 8 (top) and UB 4 O 8 (bottom), as well as reflections of the corresponding actinide oxide (marked with lines) and reflections of another still-unknown side product (marked with circles). The single-crystal intensity data were collected at room temperature on a Nonius Kappa-CCD diffractometer with graphite-monochromated Mo Ka radiation (l = 71.073 pm). Tables 1, Table 2, and Table 3 list the details of the data collection and evaluation, as well as the positional parameters of the refinement. Interatomic distances and interatomic angles are listed in Table 4 and Table 5.    Figure 2 shows the crystal structure of  [2,17] ), a-RE 2 B 4 O 9 (RE = Sm-Tb, Ho [18][19][20][21] ), and the rare-earth meta-borates d-REA C H T U N G T R E N N U N G (BO 2 ) 3 (RE = Ce, La [22,23] ), this structure is exclusively built up from tetrahedral borate groups. Figure 3 shows the composition of the borate layers. A closer look at the layers exhibits infinite chains along the b axis, which consist of [B2O 4 ] 5À tetrahedra that are connected through one common oxygen atom, O3 ( Figure 3, large spheres). These chains of [B2O 4 ] 5À tetrahedra and antiparallel-orientated chains alternate along the c axis and are linked together through the common oxygen atoms of the [B1O 4 ] 5À and [B2O 4 ] 5À tetrahedra (O1, O2, and O3). Figure 4 shows the layers along the a, b, and c directions. The BO 4 groups form a central "dreier ring", a "vierer ring", and different "sechser rings". [24] The corners of the "dreier rings" are formed from two O3 atoms and one O2 atom that are located along the b axis. The "vierer rings" are composed of two [B1O 4 ] 5À groups and two [B2O 4 ] 5À groups that are linked together through the O2 and O3 atoms; these "vierer rings" form empty channels along the b axis, as shown in Figure 4. These rings can be represented by a unit that is comprised of five [B2O 4 ] 5À groups and four  (2) Table 5. Interatomic angles [8] in AnB 4 O 8 (An = Th, U; space group C2/ c), as calculated from the single-crystal lattice parameters; standard deviations are given in parentheses. [B1O 4 ] 5À groups, as shown in Figure 5. The crystal structure of AnB 4 O 8 A C H T U N G T R E N N U N G (An = Th, U) contains four crystallographically distinguishable oxygen atoms: Oxygen atoms O1 and O2 are twofold-coordinated by boron atoms. Oxygen atom O4 is a terminal oxygen atom that is orientated towards the cation layer; a view along the c axis shows the terminal O4 oxygen atoms of the borate layers. The O3 oxygen atom is exceptional, in that it is a threefold-coordinated oxygen atom. Figure 6 shows the [O3 [3] A C H T U N G T R E N N U N G (BO 3 [25][26][27] The oxygenboron-oxygen angles in the tetrahedral [BO 4 ] 5À groups in Table 5 and correspond well with the expected angles of tetrahedrally coordinated groups. Figure 7 shows the coordination sphere of the actinide cations in AnB 4 O 8 A C H T U N G T R E N N U N G (An = Th, U). Ten oxygen atoms coordinate to both the thorium and uranium cations. Owing to the two longest actinideÀoxygen distances (Th1ÀO4'' = 285.9(5) pm ( 2), U1ÀO4'' = 300.2(4) pm ( 2)) and the large difference between the third-longest AnÀO distances (Th1 À O3 = 259.7(5) pm, U1 À O3 = 252.2(4) pm), the description as an 8+2 coordination mode for the Th1 and U1 atoms is reasonable. These coordination spheres result in thorium À oxygen distances of between 240.3(5) and   The threefold-coordinated oxygen atoms O3 [3] are denoted as large spheres.  [28,29] and CHARDI concepts (charge distribution in solids, SQ). [30] The results of these calculations are listed in Table 6. All of the calculated values correspond well with the expected values of the formal ionic charges.
Owing to the fact that the structure type of AnB 4 O 8 A C H T U N G T R E N N U N G (An=Th, U) is exclusively built up from BO 4 tetrahedra, one could imagine a structural relationship with the structures of well-known uranium and thorium silicates. However, both of the ThB 4 O 8 and UB 4 O 8 structures exhibit threefold-coordinated oxygen atoms, a structural motif that is unknown in the chemistry of silicates. Therefore, there is no visible direct structural relationship.
Vibrational spectroscopy: Figure 8 shows the Raman spectra of single crystals of the actinide borates ThB 4 O 8 and UB 4 O 8 within the range 100-1500 cm À1 . No OH or water bands could be detected within the range 3000-3600 cm À1 . Bands at about 900 cm À1 in borate compounds are usually assigned to the stretching modes of the [BO 4 ] 5À groups. However, trigonal [BO 3 ] 3À groups are expected at wavenumbers above 1150 cm À1 . [37][38][39][40] No bands are observed above 1200 cm À1 , as expected from the crystal structure due to the absence of trigonal [BO 3 ] 3À groups. Bands at smaller wavenumbers than 500 cm À1 can be assigned to An À O (An = Th, U) bonds, to lower-wavenumber-shifted bending and stretching modes of tetrahedral [BO 4 ] 5À groups, and to lattice vibrations. The large variation in B À O distances and in the linkage of the tetrahedral [BO 4 ] 5À groups led to various experimentally observed modes.
FTIR-ATR measurements of the products (mixture of the actinide borate, unreacted actinide oxide, and a still-unknown phase) were performed to exclude water or hydrated borates. The spectra showed no bands within the region 3000-4000 cm À1 .
Quantum-mechanical calculations of the harmonic vibrational frequencies: To validate the quality of the basis sets and the functional, a geometry optimization of ThB 4 O 8 was performed. Starting from the single-crystal structure, the geometry optimization yielded deviations in the lattice parameters and the atomic positions of less than 1 %. The calculations of the harmonic vibrational frequencies were per-  Table 6. Charge distribution in AnB 4 O 8 (An = Th, U; space group C2/c), as calculated by using the bond-length/bond-strength (AEV) [28,29] and Chardi concepts (AEQ). [30] T were not possible. Moreover, the calculation did not consider the temperature (297 K for the experiment) and the addition of two Gaussian peaks in the experimental spectrum led to a shift of the maxima. Table 7 lists the modes above 600 cm À1 . These bands can be assigned to boron À oxygen bending or stretching modes. However, in the assignment, the highly condensed boron-oxygen layers must be considered. An exclusive stretching or bending motion inside a tetrahedral [BO 4 ] 5À group was not possible. Each stretching or bending motion induced motions of neighboring atoms. As expected, the calculation yielded no vibrational modes above 1200 cm À1 . The evaluation of the theoretical modes exclusively showed the impossibility of the assignment of one band to a particular stretching or bending mode inside the [BO 4 ] 5À group. For example, the two modes at 1179 and 1182 cm À1 derive their origin from a various number of vibrational modes in the boron-oxygen layer.

Conclusion
Herein, we have described the high-pressure, high-temperature syntheses, single-crystal structural determinations, spectroscopic investigations, and theoretical calculations of the new actinide borates ThB 4 O 8 and UB 4 O 8 . The crystal structures are constructed from layers of linked BO 4 tetrahedra. These layers contain threefold-coordinated oxygen atoms. The actinide cations are located between the boron-oxygen layers. In the future, we will attempt the synthesis of isotypic compounds with other cations in the oxidation state 4+ that have similar ionic radii, such as Ce 4+ , by using the multianvil high-pressure technique. Furthermore, this research into actinide borates will be a good starting point for synthesizing the first actinide fluoride borate, in analogy to our work in the field of rare-earth fluoride borates.

Experimental Section
Caution: Working with UO 3 and ThO 2 requires appropriate precautions for the handling of radioactive and toxic substances. 9 %) in a 1:2 molar ratio were finely ground together, added into a platinum capsule, and placed in a boron-nitride crucible (Henze BNP GmbH, HeBo-Sint S100, Kempten, Germany). Then, the crucibles were placed into the center of an 18/11 assembly (for the thorium borate) or into the center of a 14/8 assembly (for the uranium borate). All of the synthetic steps were performed inside a glove box. The assemblies were compressed by using eight tungsten-carbide cubes (TSM-10 Ceratizit, Reutte, Austria). To apply the pressure, a 1000 ton multianvil press with a Walker-type module (both devices were purchased from Voggenreiter, Mainleus, Germany) was used. The assembly and its preparation are described in refs. [41][42][43][44][45]. For the synthesis of ThB 4 O 8 , the 18/11 assembly was compressed up to 5.5 GPa over 160 min, then heated at 1100 8C (in a cylindrical graphite furnace) over 10 min, kept at that temperature for 10  Crystal-structure analysis: The powder X-ray diffraction pattern of were obtained in transmission geometry from flat samples of the reaction product on a STOE STADI P powder diffractometer with Mo Ka1 radiation (Ge monochromator, l = 70.93 pm). Figure 1 shows the experimental powder X-ray diffraction patterns of  Figure 1), and, in both cases, a still-unknown phase (denoted with circles in Figure 1). Small single crystals of ThB 4 O 8 and UB 4 O 8 were isolated by mechanical fragmentation. The single-crystal intensity data were collected at RT on a Nonius Kappa-CCD diffractometer with graphite-monochromated Mo Ka radiation (l = 71.073 pm). A semiempirical absorption correction based on equivalent and redundant intensities (Scalepack [46] ) was applied to the intensity data. All of the relevant details of the data collection and evaluation are listed in Table 1 for both compounds. The structure solution and parameter refinement [a] s = Stretching mode, b = bending mode; pairs of bonded atoms with a large relative motion between them are given in parentheses. (full-matrix least-squares against F 2 ) were performed by using the SHELX-97 software suite. [47,48] According to the systematic extinctions, the monoclinic space group C2/c was derived in both cases. All of the atoms were refined with anisotropic displacement parameters and the final difference Fourier syntheses did not reveal any significant peaks in both refinements. Tables 2-6 list the positional parameters, anisotropic displacement parameters, interatomic distances, and angles in these structures.
Vibrational spectra: Confocal Raman spectra of single crystals of AnB 4 O 8 A C H T U N G T R E N N U N G (An = Th, U) within the range 50-4000 cm À1 were recorded on a Horiba Jobin Yvon Labram-HR 800 Raman microspectrometer. The samples were excited by using the 532 nm emission line of a frequencydoubled 100 mW Nd:YAG laser and by using the 633 nm emission line of a 17 mW HeNe laser with an Olympus 50 objective lens. The diameter of the laser spot on the surface was approximately 1 mm. The scattered light was dispersed by using an optical grating with 1800 lines mm À1 and collected by using a 1024 256 open-electrode CCD detector. The spectroscopic resolution, as determined by measuring the Rayleigh line, was less than 2 cm À1 . The spectra were recorded unpolarized. The accuracy of the Raman line shifts, as calibrated by regularly measuring the Rayleigh line, was on the order of 0.5 cm À1 . Background and Raman bands were fitted by using the built-in spectrometer software LabSpec to a secondorder polynomial and convoluted Gaussian-Lorentzian functions, respectively.
The FTIR-ATR (Attenuated Total Reflection) spectra of powdered samples were measured on a Bruker Alpha-P spectrometer with a diamond ATR-crystal (2 2 mm) that was equipped with a DTGS detector within the spectroscopic range 400-4000 cm À1 (resolution: 4 cm À1 ). 24 scans of the sample were acquired. A correction for atmospheric conditions was performed by using OPUS 7.0 software.
DFT calculations: In addition to the experimentally recorded IR and Raman spectra of ThB 4 O 8 , quantum-chemical computations of harmonic vibrational frequencies were performed by using the Crystal 09 program. [49][50][51] An important step in any quantum-mechanical calculation is the choice of an adequate basis set and a compromise must often be found between balancing computational effort and the accuracy of the results. To decrease the computational effort, a basis set with an effective core potential (ECP) for thorium was chosen. A suitable basis set for the actinide atom was identified based on geometry optimizations of ThB 4 O 8 . All-electron basis sets were employed for boron [52] and oxygen atoms. [53] Out of the results on the geometry optimization of ThB 4 O 8 , the welltested ECP78MWB GUESS [54] basis set was chosen for the thorium atom. All of the calculations were performed by using the PBESOL functional [55] for the correlation and exchange functionals and the SCF convergence for the energy was set at 10 À12 E h . The overall computation time for the calculations of the harmonic vibrational frequencies of ThB 4 O 8 took 168 h on a cluster with 12 Intel Xeon CPU X5670 2.93 GHz processors.