High-pressure synthesis and crystal structure of HP-Al2B3O7(OH)

Orthorhombic HP-Al2B3O7(OH) was synthesized in a Walker-type multianvil apparatus under highpressure/high-temperature conditions of 12.4 GPa and 1200 °C, respectively. Its structure is isotypic to that of Ga2B3O7(OH) and has been determined via single-crystal X-ray diffraction at room temperature. HP-Al2B3O7(OH) crystallizes in the space group Cmce (Z = 8) with the lattice parameters a = 10.3124(4), b = 7.3313(3), c = 10.4801(5) Å, and V = 792.33(6) Å. The compound has also been characterized by IR and Raman spectroscopy.


Introduction
Regarding the system Al-B-O-H, the literature research yields five compounds so far. Besides the well-known mineral Jeremejevite (Al 6 (BO 3 ) 5 (F,OH) 3 ) [1], our working group recently prepared the compound Al 5 B 12 O 25 (OH) which represents a new member of the high-pressure structure type M 5 B 12 O 25 (OH) (M = Al, Ga, In, Ga/In) [2,3]. Jianhua Lin et al. who are working on hydrothermally synthesized zeolite-like borate structures, published the remaining three compounds in the system Al-B-O-H. In detail, this research working group found three microporous aluminum borates, which they named PKU-1 [4], PKU-2 [5], and PKU-6 [6] (PKU stands for Peking University). PKU-1 with the sum formula HAl 3 B 6 O 12 (OH) 4 was published in 2003. In this compound all Al atoms are octahedrally coordinated building up a porous structure with 10-and 18-membered ring windows. PKU-2 (Al 2 B 5 O 9 (OH) 3 ·nH 2 O) is even more porous featuring 24-membered rings around channels of edge-sharing AlO 6 octahedra. PKU-6 (Al 2 B 3 O 7 (OH)designated as HAl 2 B 3 O 8 in reference [6]) was published in 2007 and shares the sum formula and point group with our title compound but shows a completely different structure. Infinite chains of square-pyramidally coordinated Al atoms are interlinked by three corner-sharing trigonal planar BO 3 groups. Thus, in the hydrothermally synthesized PKU-6, the Al atoms are pentacoordinated and the B atoms show a three-fold coordination. In our title compound HP-Al 2 B 3 O 7 (OH), the coordination numbers of the aluminum and boron atoms are increased by one as one would expect for a highpressure phase due to the pressure-coordination rule in solid state chemistry. However, the condensed structure of HP-Al 2 B 3 O 7 (OH) is not unprecedented as it has already been described in 2015 for the gallium containing isotype Ga 2 B 3 O 7 (OH) [7].
The present report contains a detailed description of the synthesis and crystal structure of our title compound as well as vibrational spectroscopic data of HP-Al 2 B 3 O 7 (OH) single crystals.

Experimental section 2.1 Synthesis
The starting materials Al(NO 3 ) 3 ·9H 2 O (≥98%, Sigma-Aldrich) and H 3 BO 3 (99.5% Carl Roth) were mixed in a stoichiometric ratio of 2:3, ground in an agate mortar and encapsulated in platinum foil (0.027 mm, 99.9%, Chem-PUR). The platinum capsule was then placed into a crucible with a lid, both made of α-BN (Henze Boron Nitride Products AG) and centered in a "14/8" assembly which includes, amongst others, a graphite furnace for resistance heating. Inside a uniaxially compressed Walker-type module, six steel wedges and eight tungsten carbide cubes surround the octahedral assembly to generate a quasihydrostatic pressure on the sample. A detailed description of the experimental setup is given in the literature [8][9][10]. HP-Al 2 B 3 O 7 (OH) was formed under the extreme reaction conditions of 12.4 GPa and 1200°C. The required pressure and temperature were adjusted within 6 h and 10 min, respectively. The heating power was maintained for 10 min and then gradually lowered to T = 800°C within 30 min. Finally, the sample was quenched to room temperature and slowly decompressed to ambient conditions. After freeing the platinum capsule from its surroundings and cutting it open with a scalpel, a clean-white reaction product with colorless single crystals appeared. Compared to our starting mixture, the product quantity was obviously diminished, which we explain with the great amount of volatile parts in aluminum nitrate nonahydrate. HP-Al 2 B 3 O 7 (OH) did also form as a byproduct in syntheses performed with Al 2 O 3 (99.9%, Alfa Aesar) instead of Al(NO 3 ) 3 ·9H 2 O, slightly varying ratios of Al to B, higher temperatures up to 1400°C, or a lower pressure of 12.0 GPa.

Crystal structure determination by X-ray diffraction
The reaction product was analyzed with a STOE Stadi P powder diffractometer equipped with a Mythen 1 K detector (Dectris, Switzerland). The measurement was carried out with Ge(111)-monochromatized MoKα 1 radiation (λ = 0.7093 Å) in transmission geometry across a 2θ range of 2.0-50.0°. Figure 1 shows a comparison of the experimental powder pattern with a simulation derived from single-crystal structure data. HP-Al 2 B 3 O 7 (OH) clearly constitutes the main phase, but nevertheless the experimental powder pattern shows additional reflections of at least one byproduct. The colorless single crystals of HP-Al 2 B 3 O 7 (OH) were measured with a Bruker D8 Quest diffractometer equipped with a Photon 100 CMOS detector. For structure solution and parameter refinement, the software tools SHELXS-2013/1 [11,12] and SHELXL-2013/4 [13] implemented in the program WINGX-2018.1 [14] were used. According to the systematic reflection conditions, HP-Al 2 B 3 O 7 (OH) was solved and refined in the space group Cmce (no. 64) and afterwards standardized with the routine STRUCTURE TIDY [15] implemented in PLATON [16] (version 170613). Except for the proton, all atoms could be refined anisotropically. Details of the data acquisition can be found in the synoptical Table 1. Table 2 contains the positional parameters and Wyckoff positions and Table 3 the displacement parameters of all atoms in HP-Al 2 B 3 O 7 (OH).
CSD-2027675 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_ request/cif.

Vibrational spectroscopy
The transmission FT-IR spectrum of an HP-Al 2 B 3 O 7 (OH) single crystal was measured in the spectral range of 600-4000 cm −1 with a Vertex 70 FT-IR spectrometer (spectral resolution 4 cm −1 ) equipped with a KBr beam splitter, a liquid nitrogen cooled MCT (Mercury Cadmium Telluride) detector and a Hyperion 3000 microscope (Bruker). 320 scans of the sample were acquired using a Globar (silicon carbide) rod as mid-IR source and a 15× IR objective as focus. During the measurement, the sample was positioned on a BaF 2 window. Atmospheric influences were corrected with the software OPUS 6.5 [17].
A single-crystal Raman spectroscopic analysis was performed using a HORIBA JOBIN-YVON LabRam-HR800 spectrometer with a focal length of 800 mm. The spectrum was recorded unpolarized at ambient conditions. The analyzed area was excited using a (green) Nd-YAG laser with an excitation wavelength of 532 nm. Scattered light was dispersed by an optical grating with 1800 grooves per mm resulting in a spectral resolution of 1.92 cm −1 , and collected with a cooled Andor™ CCD collector. An Olympus 50× objective, a confocal pinhole of 1000 µm and a slit of 100 µm was used for the measurement to maximize the intensity at a lateral resolution of ∼5 µm 2 . Before the analyses, the spectral position of the Raman mode of a Si standard wafer was measured against the position of the Rayleigh line, resulting in the expected Raman shift of 520.7 cm −1 . The spectrum was obtained in multi-window acquisition mode provided by the LABSPEC (version 5.93. 20) [18] software package in the frequency range of 200-1500 cm −1 and represents the average of two single measurements with an acquisition time of 250 s Table : Wyckoff positions, atomic coordinates, isotropic U iso or equivalent isotropic displacement parameters U eq (Å  ) for HP-Al  B  O  (OH). U eq is defined as one third of the trace of the orthogonalized U ij tensor (standard deviations in parentheses).

Atom
Wyckoff position x y z U eq (U iso for H)   (Figure 2a), and BB2 is composed of 12 corner-sharing BO 4 tetrahedra forming a flexed unit of two sechser rings [19] that are interlinked by two vierer rings ( Figure 2b). As depicted in Figure 3, each BB1 is encircled by two BB2s. The flexed BB2 units are interconnected forming endless zig-zag chains along the crystallographic b axis. Between these zig-zag chains, the BB1 clusters fill the space in alternating orientations in relation to c (visualized in Figure 4). All oxygen atoms belong to both building blocks except O4, which connects to three Al atoms and the proton in HP-Al 2 B 3 O 7 (OH). While the proton position was geometrically constructed in Ga 2 B 3 O 7 (OH), it was possible to freely refine it in the aluminum compound. The proton is located in the middle of a BO 4 sechser ring at a distance of 0.94(3) Å to O4 and forms hydrogen bonds to two O2 atoms and to O5. Figure 5 shows the hydrogen bonding situation in HP-Al 2 B 3 O 7 (OH  [20][21][22]. Likewise, the B-O distances (Ø = 1.49 Å) and tetrahedral angles (Ø = 109.5°) are as expected [23]. All interatomic distances and angles are given in Tables 4-6. To support the crystal structure solution, the Madelung part of lattice energy (MAPLE value) [24,25] of HP-Al 2 B 3 O 7 (OH) was calculated and compared to the stoichiometric sum of the compounds α-Al 2 O 3 , HP-B 2 O 3 , and H 3 BO 3 . The results showed only a minimal deviation of 0.15% (see Table 7). The bond valence sums (BVS) for HP-Al 2 B 3 O 7 (OH) were calculated with the bond-length/bond-strength concept [26] and the charge distribution was derived by using the charge distribution in solids (CHARDI) concept [27]. The results are listed in Table 8. Both calculations led to reasonable values with only one noticeable BVS deviation for O4 being the donor of the hydrogen bond.

Vibrational spectroscopy
For the isotypic compound Ga 2 B 3 O 7 (OH) [7], the IR and Raman bands were assigned based on DFT calculations and therefore provide a good basis for the interpretation of the spectra of HP-Al 2 B 3 O 7 (OH). The FT-IR spectrum of an HP-Al 2 B 3 O 7 (OH) single crystal is shown in Figure 6           crystal of HP-Al 2 B 3 O 7 (OH) does not show any contamination with organic compounds. The Raman spectrum is presented in Figure 7 and shows only the frequency range between 200 and 1500 cm −1 due to strong luminescence at higher Raman shifts that becomes even more pronounced when exciting with a red (633 nm) laser, so that OH-related vibrations (expected between 3000 and 4000 cm −1 ) are not displayed. The DFT calculations for the isotypic compound Ga 2 B 3 O 7 (OH) also allow assignments for the aluminum analog. The Al 3+ = Ga 3+ exchange mainly results in higher vibrational frequencies and consequently increasing Raman shifts of Al-related bands if compared to the respective Ga-related counterparts. Bands in the 250-700 cm −1 frequency range can be mainly assigned to vibrations of the AlO 6 units. The Al-O-H, Al-O-Al bending vibrations are expected to appear in a wide range between 250 and 1150 cm −1 , while the stretching vibrations for the BO 4 tetrahedra appear in the 800-1150 cm −1 region.

Conclusion
The present report presents a detailed synthesis protocol and the determination and discussion of the crystal structure of the new aluminum borate HP-Al 2 B 3 O 7 (OH), which is isotypic to Ga 2 B 3 O 7 (OH). While the proton position in the gallium compound had to be calculated geometrically, it was possible to locate it in the electron density map and refine it via difference Fourier analysis of the aluminum analog. IR data additionally confirm the presence of the hydroxyl group. Attempts to synthesize the isotypic indium compound have not been successful yet.