High-Pressure Synthesis and Characterization of the Actinide Borate Phosphate U2[BO4][PO4]

A new actinide borate phosphate, U2[BO4][PO4], was synthesized in a Walker-type multianvil apparatus at 12.5 GPa and 1000 °C. The crystal structure was determined from single-crystal X-ray diffraction data collected at room temperature. U2[BO4][PO4] crystallizes in the monoclinic space group P21/c with four formula units per unit cell and the lattice parameters a = 854.6(2), b = 775.3(2), c = 816.3(2) pm, and β = 102.52(3)°. The structure consists of double layers of linked uranium–oxygen polyhedra parallel to [100]. The borate tetrahedra are located between the uranium–oxygen layers inside the double layer. The phosphate groups link the double layers.


Introduction
The structural chemistry of borates exhibits a respectable diversity, which comes from the ability of the boron atom to form trigonal-planar [BO 3 ] 3groups and tetrahedral [BO 4 ] 5groups. Moreover, links between these groups to form chains, layers, or highly condensed three-dimensional networks enlarge the amount of possible compounds. Phosphates show similar properties and can form isolated ions such as the diphosphate anion [P 2 O 7 ] 4or the triphosphate anion [P 3 O 10 ] 5-, infinite chains such as those in the mercury(II) polyphosphate Hg(PO 3 ) 2 , [1] or layers as in Ag 2 PdP 2 O 7 . [2] Therefore, the combination of borate and phosphate groups leads to an enormous amount of possible structures. Although the ternary U-B-O system is represented by only two different oxoborates with the compositions U(BO 3 ) 2 [3] and (UO 2 )(B 2 O 4 ), [4] more than ten compounds are known in the U-P-O system [e.g., U(PO 3 ) 4 , [5] U(P 2 O 7 ), [6] and U 2 (PO 4 )(P 3 O 10 )]. [7] Interestingly, there are no quaternary uranium compounds that contain both borate and phosphate groups. In general, compounds with bor-parameters a = 854.6(2), b = 775.3 (2), c = 816.3(2) pm, and β = 102.52(3)°. The structure consists of double layers of linked uranium-oxygen polyhedra parallel to [100]. The borate tetrahedra are located between the uranium-oxygen layers inside the double layer. The phosphate groups link the double layers.
ate and phosphate groups can be categorized in the classes of borophosphates and borate phosphates. In borophosphates, the [BO 3 ] 3-, [BO 4 ] 5-, and [PO 4 ] 3groups are connected among each other. If the borate and phosphate groups are isolated, the compounds are designated as borate phosphates. In recent years, the field of borophosphate chemistry has been greatly extended. Selected examples of this class are the compounds M II [BPO 4 (OH) 2 ] (M II = Mn, Fe, Co), [8] Mg 3 (H 2 O) 6 [BPO 4 (OH) 3 ] 2 , [9] and Ba 3 (BP 3 O 12 ). [10] In contrast to the large number of borophosphates, borate phosphates are rare, particularly those that exclusively exhibit tetrahedral borate and phosphate groups such as the mineral seamanite Mn 3 (OH) 2 [14] which possesses disordered borate tetrahedra in the channels of the structure that can be exchanged by (TcO 4 )groups. The topicality of this research in actinide borates is demonstrated by work recently published by Wu et al. [15] Here, the application of high-pressure/high-temperature conditions enabled the synthesis of a uranium borate phosphate with the composition U 2

Results and Discussion
Synthesis and Crystal Structure Analysis 4 ] was synthesized from UO 3 , H 3 BO 3 , and P 4 O 10 under high-pressure/high-temperature conditions of 12.5 GPa and 1000°C in a 1000 ton multianvil press with a Walker-type module. A detailed description of the synthesis is provided in the Experimental Section. The single-crystal intensity data were collected at room temperature with a Nonius KappaCCD diffractometer with graphite-monochromated Mo-K α radiation (λ = 71.073 pm). In contrast to the structural refinement of the isotypic Th 2 [BO 4 ][PO 4 ] phase by Lipp et al., [12] we found split positions of the uranium atoms with a ratio of 0.93(1):0.07(1) for both sites of the heavy atoms U1a/U1b and U2a/U2b. The reason for  (2) the presence of these split positions is presumably a boron/ phosphorus disorder at the tetrahedral centers of the [BO 4 ] 5and [PO 4 ] 3groups. The omission of the refinement of split positions led to significant residual peaks near the uranium atoms and to the impossibility to calculate the anisotropic atomic displacement parameters of the boron atoms. The single-crystal measurement of a second single crystal led to the same disorder with the same ratio. It was impossible to refine the oxygen atoms with split positions; therefore, the minor part of the split uranium atoms was not described with a similar coordination environment as the major part. Therefore, the calculation of the coordination spheres of U1b, U2b, P1b, and B1b was not practicable. Owing to the high standard deviations of the boronoxygen distances in the isotypic Th[BO 4 ][PO 4 ] phase, we as-   4 ] was performed on the basis of the main split positions U1a and U2a, which are denoted as U1 and U2 respectively, in the following. Tables 1, 2, and 7 list details of the data collection and evaluation as well as the positional parameters of the refinement. Interatomic distances and angles are listed in Tables 3 and 4.

Crystal Structure
The  4 ], the valence of the uranium cations calculates to 4+ owing to the principle of electroneutrality. This is confirmed by the intense emerald green color of the crystals and additionally verified by calculations of the bond valence sums by using the bond length/bond strength (ΣV) [16,17] and the charge distribution in solids concept (CHARDI, ΣQ). [18] The results of these calculations for all atoms are listed in  (6) pm], the description as an 8+2 coordination for U1 and an 8+1 coordination for U2 is reasonable. Calculations of the effective coordination number (δ-ECoN) values by using the program MAPLE (madelung part of lattice energy) [19][20][21] resulted in a value of 0.09 for O3 [U1-O3 298.4 (2) pm] in the coordination sphere of U1. Owing to the low value, the coordination sphere of U1 can also be described as an 8+1 coordination; however, following the structure description of Th 2 [BO 4 ][PO 4 ], [12] we describe the U1 atom with a tenfold (8+2) coordination. These coordination spheres result in uranium-oxygen distances between 215.2(6) and 298.4(2) pm with a mean value of 256.3 pm for U1 and between 234.0(6) and 287.5(6) pm with a mean value of 245.0 pm for U2. Figure 2 displays the coordination spheres of the uranium ions.

With the successful synthesis of U 2 [BO 4 ][PO 4 ], the first isotypic compound to Th 2 [BO 4 ][PO 4 ] was found and characterized. The structure is built up by isolated [BO 4 ] 5and [PO 4 ] 3groups and double layers formed by linked
uranium-oxygen polyhedra. The application of similar synthetic conditions to other tetravalent cations with similar ionic radii, such as Ce 4+ , could lead to additional isotypic compounds and will be studied in the future.

Experimental Section
Caution: Working with UO 3 requires precautions for the handling of radioactive and toxic substances. 4 ] was synthesized by a two-stage synthesis. The synthesis of the precursor was achieved under high-pressure/high-temperature conditions of 7.0 GPa and 700°C from a nonstoichiometric mixture of UO 3 [46.63 mg, synthesized by pyrolysis of UO 2 (NO 3 ) 2 ·6H 2 O at 300°C], H 3 BO 3 (30.13 mg; Carl Roth GmbH + Co. KG, Karlsruhe, Germany, 99.8+%), and partially hydrolyzed P 4 O 10 (23.14 mg; Merck GmbH, Darmstadt, Germany, p.a.). The starting materials were finely ground, inserted into a gold capsule, and placed in a boron nitride crucible (Henze BNP GmbH, HeBoSint® S100, Kempten, Germany). The boron nitride crucible was placed into an 18/11assembly and compressed by 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 from Voggenreiter, Mainleus, Germany) was used. A detailed description of the assembly preparation can be found in refs. [32][33][34][35][36] In detail, the 18/11-assembly was compressed to 7.0 GPa in 200 min and heated to 700°C (cylindrical graphite furnace) in the following 10 min, kept there for 10 min, and cooled down to 300°C in 30 min at constant pressure. After natural cooling to room temperature by switching off the heating, a decompression period of 10 h was required. The recovered MgO octahedron (pressure transmitting medium, Ceramic Substrates & Components Ltd., Newport, Isle of Wight, UK) was broken apart, and the sample was carefully separated from the surrounding graphite and boron nitride cruci-ble. The precursor was gained in the form of a yellow tough bulk. The bulk was dried for 3 h in a compartment dryer at 80°C. The X-ray powder diffraction analysis showed an amorphous phase. The yellow color might indicate a hexavalent uranium compound. This amorphous precursor was the starting material for the synthesis of U 2 [BO 4 ][ PO 4 ]. For the synthesis, high-pressure/high-temperature conditions of 12.5 GPa and 1000°C were required by using an 14/11-assembly. The preparation of the assembly was equal to the preparation described before. In detail, the 14/11 assembly was compressed to 12.5 GPa in 380 min and heated to 1000°C (cylindrical graphite furnace) in the following 10 min, kept there for 5 min, and cooled to 450°C in 25 min at constant pressure. After natural cooling to room temperature by switching off the heating, a decompression period of 12 h was required.  4 ] were collected at room temperature by using a Nonius KappaCCD diffractometer with graphite-monochromated Mo-K α radiation (λ = 71.073 pm). A semiempirical absorption correction based on equivalent and redundant intensities (Scalepack) [37]  was applied to the intensity data. All relevant details of the data collection and evaluation are listed in Table 7.

Synthesis: The uranium borate phosphate U 2 [BO 4 ][PO
According to the systematic extinctions, the monoclinic space group P2 1 /c was derived. The structure solution and parameter refinement (full-matrix least-squares against F 2 ) were performed by using the SHELX-97 software suite [38,39] with anisotropic atomic displacement parameters for the atoms of the main split position. The final difference Fourier syntheses did not reveal any significant residual peaks in all refinements. The ratio of the occupation disorder was confirmed by two methods. First, the refinement with a free variable x for the major part and (1x) for the minor part was done automatically by the program and converged to a value of 0.93. Second, the manual variation of the occupation factors to equal isotropic displacement parameters for the positions at P1a and B1a. This method works with the simple assumption of a nearly equal mobility of the phosphate and borate anions in the crystal lattice. Therefore, the anisotropic displacement parameters of the phosphorus and boron atoms were reset to isotropic values and the refinement astonishingly leads to the same occupation factor of 0.93 for P1a and B1a. The positional parameters of the refinements, anisotropic displacement parameters, interatomic distances, and interatomic angles are listed in the Table 1, Table 2, Table 3, and Table 4.
Vibrational Spectroscopy: The confocal Raman spectra of the single crystals of U 2 [BO 4 ][PO 4 ] in the range 50-4000 cm -1 were recorded with a Horiba Jobin Yvon Labram-HR 800 Raman microspectrometer. The samples were excited by using the 532 nm emission line of a frequency-doubled 100 mW Nd:YAG laser and the 633 nm emission line of a 17 mW helium neon laser under an Olympus 50 ϫ objective lens. The size of the laser spot on the surface was approximately 1 μm. The scattered light was dispersed by an optical grating with 1800 lines mm -1 and collected by a 1024 ϫ 256 open electrode CCD detector. The spectral resolution, determined by measuring the Rayleigh line, was less than 2 cm -1 . The spectra were recorded unpolarized. The accuracy of the Raman line shifts, calibrated by regularly measuring the Rayleigh line, was in the order of 0.5 cm -1 . Background and Raman bands were fitted by the builtin spectrometer software LabSpec to second-order polynomial and convoluted Gaussian-Lorentzian functions, respectively.