High-Pressure Synthesis and Crystal Structure of Ce4B14O27

Ce4B14O27 was synthesized under conditions of 2.6 GPa and 750 °C in a Walker-type multianvil apparatus. The crystal structure was determined on the basis of single-crystal X-ray diffraction data, collected at room temperature, revealing that Ce4B14O27 is isotypic to La4B14O27. Ce4B14O27 crystallizes monoclinically with four formula units in the space group C2/c (No. 15) and the lattice parameters a = 1117.8(2), b = 640.9(2), c = 2531.7(5) pm, and β = 100.2(1)°. The three-dimensional boron-oxygen framework consists of [BO4]5– tetrahedra and trigonal-planar [BO3]3– groups. The structure contains two crystallographically different cerium ions. Furthermore, Raman spectroscopy was performed on single crystals of Ce4B14O27.


Introduction
The structural chemistry of oxoborates exhibits a respectable diversity, which yields from the ability of the boron atom to form trigonal-planar [BO 3 ] 3groups and tetrahedral [BO 4 ] 5groups. These groups can occur isolated or linked to highlycondensed three-dimensional networks. In the majority of cases, the trigonal-planar [BO 3 ] 3groups disappear with increasing pressure, so in high-pressure oxoborates, the boron atoms favor the fourfold coordination forming [BO 4 ] 5groups. Above a pressure of 10 GPa, only a few compounds are known, which contain trigonal-planar [BO 3 ] 3groups, e.g. Ho 31 O 27 (BO 3 ) 3 (BO 4 ) 6 . [1] The linking of the tetrahedral [BO 4 ] 5groups follows normally via common corners. In the past, we observed that these boron-oxygen tetrahedra can share common edges to realize denser structures like the polyborates RE 4 B 6 O 15 (RE = Dy, Ho), [2,3] and α-RE 2 B 4 O 9 (RE = Sm-Ho). [4][5][6] Moreover, high-pressure/high-temperature syntheses led to increased coordination numbers (CN) of the rareearth ions, and also the coordination numbers of the oxygen atoms could be partially enhanced from twofold (O [2] ) to threefold coordinated (O [3] ).
Mainleus, Germany) was used. The assembly and its preparation are described in the literature. [20][21][22][23][24] The 18/11 assembly was compressed up to 2.6 GPa in 65 min and heated to 750°C (cylindrical graphite furnace) in the following 10 min, kept there for 15 min, and cooled down to 450°C in 25 min at constant pressure. After natural cooling down to room temperature by switching off the heating, a decompression period of 3.5 h was required. The recovered octahedral pressure medium (MgO, 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. The compound Ce 4 B 14 O 27 was found in the form of colorless air-resistant crystals.
Two corresponding experiments under ambient pressure conditions at 700°C and 850°C using CeO 2 , B 2 O 3 , and CeF 3 (flux material) in a boron-nitride crucible did not lead to the desired polyborate Ce 4 B 14 O 27 . Instead of, the syntheses led to the monoclinic meta-borate α-Ce(BO 2 ) 3 . [1,12] Crystal Structure Analysis: The sample was characterized by powder X-ray diffraction, which was performed in transmission geometry on a flat sample of the reaction product, using a STOE STADI P powder diffractometer with Ge(111)-monochromatized Mo-K α1 (λ = 70.93 pm) radiation. The diffraction pattern showed reflections of Ce 4 B 14 O 27 and CeF 3 . Figure 1 shows the experimental powder pattern that matches well with the theoretical pattern simulated from the single-crystal data. Small single-crystals of Ce 4 B 14 O 27 were isolated by mechanical fragmentation. The single crystal intensity data were collected at room temperature with a Nonius Kappa-CCD diffractometer with graphitemonochromatized Mo-K α radiation (λ = 71.073 pm). A semiempirical absorption correction based on equivalent and redundant intensities (SCALEPACK) [25] was applied to the intensity data. All relevant details of the data collection and evaluation are listed in Table 1. According to the systematic extinctions, the monoclinic space group C2/c (no. 15) was derived. Because of the fact that Ce 4 B 14 O 27 is isotypic to La 4 B 14 O 27 , [16] the structural refinement was performed using the positional parameters of La 4 B 14 O 27 as starting values [SHELXL-97 [26][27][28] (full-matrix least-squares on F 2 )]. In comparison with the structure data of Schleid et al., in which only the lanthanum atoms could be refined with anisotropic displacement parameters, an anisotropic refinement for all atoms of Ce 4 B 14 O 27 was possible. The final difference Fourier syntheses did not reveal any significant peaks in the refine-   Table 3, Table 4, and Table 5 list the positional parameters, anisotropic displacement parameters, interatomic distances, and angles.

Crystal Structure of Ce 4 B 14 O 27
The structure of Ce 4 B 14 O 27 consists of a highly condensed boron-oxygen network and trivalent cerium ions.  Table 4. The mean values of the boronoxygen distances (146.7-147.8 pm for tetrahedral coordinated boron atoms and 136.6-137.9 pm for trigonal-planar boron atoms) correspond well with the known average values for  Table 5 and correspond well to the expected angles of tetrahedral-and trigonal groups. Three tetrahedral [BO 4 ] 5groups (B1-B3) are condensed via shared corners to so called "dreier" rings. [32] .  -0.0016(9) 0.0017 (8) 0.0007 (9) contains two crystallographically distinguishable rare-earth ions. The rare-earth ion Ce1 is surrounded by ten oxygen atoms between 238.4(2) and 285.9(2) pm with a mean value of 263.1 pm, whereas Ce2 is coordinated by eight oxygen atoms between 231.9(2) and 284.1(2) pm with a mean value of 252.6 pm. Figure 7 displays the coordination spheres of the cerium ions. For a more detailed description of the structure, the reader is referred to the description of the isotypic compound La 4 B 14 O 27 . [16] In this paper, we briefly compare  [33][34][35] were calculated from the crystal structure to compare them with the MAPLE values received from the summation of the  [36] and the high-pressure modification B 2 O 3 -II. [37] The value of 182961 kJ·mol -1 was obtained in comparison to 181866 kJ·mol -1 (deviation = 0.6 %), starting from the binary oxides [2 ϫ Ce 2 O 3 (14150 kJ·mol -1 ) + 7ϫ B 2 O 3 -II (21938 kJ·mol -1 )].
Furthermore, the bond-valence sums of Ce 4 B 14 O 27 were calculated from the crystal structure for the rare-earth ions, using the bond length/bond-strength concept (ΣV). [38,39] The calculation revealed values of: +2.95 (Ce1) and +3.12 (Ce2), which fit well for the formal ionic charges. For the boron ions, the values vary between +2.91 and +3.09. The oxygen ions show values of -1.90 to -2.15.    the slightly larger size of La 3+ compared to Ce 3+ . No greater deviations of the bond lengths and angles are observed. The coordination numbers of the rare-earth ions are equivalent.

Raman Spectroscopy
Confocal Raman spectra of single crystals of Ce 4 B 14 O 27 were measured in the range of 100-6000 cm -1 with a HORIBA www.zaac.wiley-vch.de    Figure 8 shows the Raman spectrum of Ce 4 B 14 O 27 from 100 to 4000 cm -1 . In the range of 3000 to 3600 cm -1 , no OH or water bands could be detected. Bands around 900 cm -1 in oxoborates are usually assigned to stretching modes of the [BO 4 ] 5groups. However, the trigonal [BO 3 ] 3groups are ex-pected at wavenumbers above 1100 cm -1 . [5,[40][41][42][43] The range between 100 and 1500 cm -1 is displayed in Figure 9 (top) and the range between 1500 and 3000 cm -1 in the bottom of Figure 9. Bands at wavenumbers smaller than 500 cm -1 can be assigned to Ce-O bonds, to lower wavenumbers shifted bending and stretching modes of the tetrahedral [BO 4 ] 5groups, as well as lattice vibrations. As expected, bands between 800 and 1800 cm -1 are observed due to the presence of trigonal [BO 3 ] 3and tetrahedral [BO 4 ] 5groups, whereas vibrational modes above 1200 cm -1 generally refer to trigonal [BO 3 ] 3groups. The large variation of B-O distances inside the [BO 4 ] 5and [BO 3 ] 3groups leads to various modes and based on the deviation of the distances from the ideal B-O distance to a large shift of the bands.

Conclusions
With the successful synthesis of Ce 4 B 14 O 27 , a new cerium polyborate and furthermore the first isotypic compound to La 4 B 14 O 27 was synthesized and characterized. In accordance with the relatively low applied pressure of 2.6 GPa, the boron oxygen network is built up by trigonal [BO 3 ] 3and tetrahedral [BO 4 ] 5groups. Four of the seven crystallographically difwww.zaac.wiley-vch.de 273 ferent boron atoms are coordinated by four oxygen ions. The application of similar synthetic conditions to heavier rare-earth elements could lead to additional isotypic compounds and will be studied in the future.