High-Pressure Borates High-Pressure Synthesis and Crystal Structure of the Highly Condensed Yttrium Borate YB 7 O 12

: The yttrium borate YB 7 O 12 was obtained in a high-pressure/high-temperature experiment at 10 GPa and 1573 K. The crystal structure was clarified by X-ray single-crystal diffraction, where the compound was found to crystallize in the orthorhombic space group Pccn (no. 56) with the lattice parameters a = 11.985(2), b = 6.717(6), c = 7.867(7) Å, and a volume of fruitful discussions with respect to the characterization of the compound.


Introduction
Since the beginning of our investigations of high-pressure borates, one of our main goals was the synthesis of anionic backbones as highly condensed as possible. Previous attempts of our group to accomplish that, led to the discovery of various high-pressure borates with highly connected boron-oxygen frameworks, for example CsB 5 O 8 , [1] HP-MB 3 O 5 , (M = K, Rb, Tl), [2] HP-Cs 1-x (H 3 O) x B 3 O 5 (x = 0.5-0.7), [3] and HP-(NH 4 )B 3 O 5 , [4] all of them containing simultaneously planar BO 3 groups as well as edge-and corner-sharing BO 4 tetrahedra. The presence of these different structural groups and connections between them enriched by threefold-coordinated oxygen atoms makes it difficult to find a quantitative parameter for the comparison of these borates with respect to a degree of condensation expressed by the ratio B/O. However, concerning high-pressure borates which are exclusively built up from corner-sharing BO 4 tetrahedra, our efforts resulted in highly condensed compounds like -MB 4 O 7 (M = Mn-Zn) [5] and RE 2 B 8 O 15 (RE = La, Pr, Nd) [6] possessing a degree of condensation of B/O = 0.57 and 0.53, respectively. These values are quite high in comparison to the highest reachable value of 0.66 in the high-pressure modification of boron oxide B 2 O 3 -II, [7] which is exclusively built up from corner-sharing BO 4 tetrahedra including threefold coordinated oxygen atoms.
Furthermore, there exist no ternary compounds of the prelanthanide cations scandium and yttrium with such highly condensed networks. In the case of yttrium, the ICSD (Inorganic Crystal Structure Database) lists only four ternary borates, two V = 633.0(1) Å 3 . Built up exclusively from BO 4 tetrahedra, YB 7 O 12 features an exceptionally high degree of condensation, which originates from the finding that every tetrahedron contains two threefold-coordinated oxygen atoms. The compound was further characterized by X-ray powder diffraction. of them, YBO 3 [8] and Y 17.33 (BO 3 ) 4 (B 2 O 5 ) 2 O 16 [9] (revised formula of Y 3 BO 6 ), were synthesized at ambient pressure, while the other two phases -Y(BO 2 ) 3 [10] and -Y 2 B 4 O 9 [11] were recently found under high-pressure conditions by our group.
In this article, we report the synthesis of a new highly condensed high-pressure yttrium borate with the composition YB 7 O 12 . The compound exhibits an extremely high condensed borate framework exclusively built up from corner-sharing BO 4 tetrahedra including threefold coordinated oxygen atoms with a boron-oxygen ration of B/O = 0.58. Additionally, the boron to metal ratio of 7:1 is exceptionally high, which was, to the best of our knowledge, never found before in any high-pressure borate. By searching the ICSD for comparably condensed borates, we found only three examples of ternary phases with higher boron to metal ratios namely CsB 9 O 14 [12] with a ratio of 9:1 as well as BaB 8 O 13 [13] and Sr 2 B 16 O 26 [14] with a ratio of 8:1. Therefore, our new yttrium borate possesses the third highest boron to metal ratio of all ternary borates to date.

Results and Discussion
Crystal Structure YB 7 O 12 crystallizes in the orthorhombic space group Pccn (no. 56) with four formula units (Z = 4) and the lattice parameters a = 11.985(2), b = 6.717(6), c = 7.864(7) Å leading to a cell volume of V = 633.0(1) Å 3 . As depicted in Figure 1, the compound is built up of structural units of five BO 4 tetrahedra that are connected via common threefold-coordinated oxygen atoms O6, forming a merged "double-windmill". Two additional tetrahedra with B4 in their centres are connected to this structural unit on each side via the oxygen atoms O2 and O3, forming two three membered rings. This leads to the fundamental building block, which consists of seven BO 4 tetrahedra. This fundamental building block is arranged alternatingly with its mirror image, thereby forming two three-membered rings and one four-membered ring (see Figure 2). Thus, infinite bands are   formed along the c-axis. Each unit cell contains two of these bands, which run in opposing directions. The chains are interconnected via the aforementioned B4O 4 tetrahedra, where each tetrahedron links three bands. Thus, a three-dimensional network with channels parallel to the c-axis is developed. In these channels, the Y 3+ cations are located ( Figure 3). Figure 4 shows the tenfold coordination of the yttrium cations by oxygen anions, forming a distorted bicapped square antiprism. The Y-O bond lengths vary between 2.308(8) and 2.659(8) Å, coinciding with values reported in other high-pressure yttrium borates [2.383-2.419 Å in -Y(BO 2 ) 3 [10] and 2.401-2.602 Å in α-Y 2 B 4 O 7 [11] ]. The B-O distances lie within a rather wide range of 1.397(2)-1.562(2) Å (Table 1). Especially the oxygen atoms O3 and O6 feature comparatively long B-O bond lengths [≥ 1.506(2) Å] owing to the fact that they are threefold coordinated by boron atoms. Additionally, the O-B-O angles also vary widely between 100.2(7) and 119.8(8)° (Table 2), the latter angle being slightly above the reported range of 95.7-119.4°. [15] Despite this distortion, the average values for the bond lengths and angles, 1.481 Å and 109.9°, respectively, are in good agreement with those reported by Zobetz [1.476(35) Å/109.44°]. [15] The positional parameters are listed in Table 3.
To support the determined crystal structure, the bond valences according to the bond length/bond-strength (ΣV) [16] as well as the CHARDI (ΣQ) [17] concept were calculated. Table 4 shows the obtained values from both calculations, which fit well to the expected formal ionic charges of +3 for yttrium and boron and -2 for oxygen.
During our research, we found the compound KZn 4 (PO 4 ) 3 to be homeotypic to our new yttrium borate YB 7 O 12 . In contrast to our yttrium borate, where the anionic framework consists exclusively from BO 4 tetrahedra, the backbone in the zincophosphate is built up of ZnO 4 and PO 4 tetrahedra, where the PO 4 tetrahedra are set in the middle of the chains, equal to B3O4 in the structure of YB 7 O 12 . The interconnection of the two opposing chains also solely consists of PO 4 tetrahedra, taking the position of the B4O 4 unit in YB 7 O 12 . Similar to our structure, the zincophosphate exhibits channels parallel to the c-axis. In these channels, the large cations (in this case potassium) exhibit a tenfold coordination by oxygen atoms. In analogy to the zincophosphate KZn 4 (PO 4 ) 3 , the yttrium borate can be written as YB 4 (BO 4 ) 3 . Figure 5 shows a comparison of our yttrium borate with the zincophosphate, highlighting the similarities in the wavelike framework and the two fundamental building blocks. The comparison of the lattice parameters of the two structures shows that the zincophosphate is approximately 1.2 times larger than the borate, which is expected due to the larger atoms building up this compound. A detailed list of the standardized crystal data and positional parameters in comparison to our yttrium borate can be found in the Table 5 and  Table 6. [18] Figure 6 shows a comparison of the measured powder pattern and the theoretical powder pattern calculated from the singlecrystal data. As can be seen clearly, the reaction product contains primarily the presented compound YB 7 O 12 with a few inexplicable peaks indicating the presence of at least one unknown compound. The large amorphous halo at low 2θ angles presumably results from excess B 2 O 3 .  Figure 6. Comparison of the experimental X-ray powder diffraction data (top) and the theoretical X-ray powder pattern simulated from the single-crystal data (bottom). Reflections marked with an asterisk derive from an unknown impurity.

Conclusions
In this work, we report the high-pressure synthesis of YB 7 O 12 under high-pressure/high-temperature conditions of 10 GPa/ 1573 K using a Walker-type multianvil apparatus. Like the two hitherto known high-pressure yttrium borates -Y(BO 2 ) 3 and -Y 2 B 4 O 7 , which are both formed at slightly lower temperatures of 1273 K and 1293 K, respectively, the crystal structure of YB 7 O 12 is exclusively built up from BO 4 tetrahedra and, as in -Y(BO 2 ) 3 , contains threefold-coordinated oxygen atoms. With a degree of condensation of B/O = 0.58, YB 7 O 12 exhibits a higher value than previous examples of high-pressure borates featuring solely BO 4 tetrahedra. The yttrium atoms are tenfold coordinated by oxygen anions and located in channels parallel to the c-axis.
Being homeotypic to KZn 4 (PO 4 ) 3 , where the ZnO 4 and PO 4 tetrahedra occupy the positions of the BO 4 tetrahedra, the presented yttrium borate YB 7 O 12 constitutes the highest condensed high-pressure borate to date. It also exhibits the third highest boron to metal ratio recorded in any ternary borate, whether one considers ambient/moderate or high-pressure conditions.

Experimental Section
Synthesis: The new high-pressure borate YB 7 O 12 was synthesized under high-pressure/high-temperature conditions in a Walker-type multianvil module in combination with a 1000 t downstroke press (both Max Voggenreiter GmbH, Mainleus, Germany). As starting materials, the binary oxides Y 2 O 3 (Sigma-Aldrich, USA, 99.99 %) and B 2 O 3 (Strem Chemicals, Newburyport, USA, 99.9+ %) at a molar ratio of 1:7 were used. After grinding, the starting mixture was filled into a crucible, closed with a lid (both α-BN; Henze Boron Nitride Products AG, Lauben, Germany), placed into an 14/8 assembly and compressed by eight tungsten carbide cubes (Hawedia, Marklkofen, Germany). Further details on the assembly and its preparation can be found in the literature. [19] The sample was compressed to 10 GPa within 230 min, heated up to 1573 K in 10 min and kept at these conditions for further 30 min. After cooling the sample to 573 K within 10 min, the heating was switched off and the 960 min decompression process started. After separation from the surroundings, the product YB 7 O 12 was recovered nearly phase-pure as colorless air-stable crystals.

Single-Crystal X-ray Diffraction Analysis:
The single-crystal intensity data was collected with a Bruker D8 Quest Kappa diffractometer equipped with a Photon 100 CMOS detector. Multi-scan absorption corrections were carried out with the program SADABS-2014/5. [20] For the structure solution and parameter refinement, the software SHELXS/L-2013 [21] was used. According to the systematic extinctions, the space group Pccn (no. 56) was derived. The starting positional parameters were deduced from an automatic interpretation of direct methods. The structure of YB 7 O 12 was refined using anisotropic displacement parameters for all atoms (full-matrix leastsquares on F 2 ). Table 7 shows all the relevant details of the data collection and refinement.
CCDC 1874643 (for YB 7 O 12 ) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre.
For the comparison of our yttrium borate with the already known zincophosphate KZn 4 (PO 4 ) 3 , the atomic coordinates of both struc- tures were standardized employing STRUCTURE TIDY [22] as implemented in PLATON. [23] X-ray Powder Diffraction: The powder diffraction pattern of a flat sample of the reaction product YB 7 O 12 was collected in transmission geometry on a Stoe Stadi P powder diffractometer (Stoe & Cie GmbH, Darmstadt, Germany). The measurement was performed with Ge(111)-monochromatized Mo-K α1 radiation (λ = 70.93 pm) in the 2θ range of 2-52°with a step size of 0.015°.