Abstract
The isotypic triel borates Al5B12O25(OH) and Ga4InB12O25(OH) were synthesized in a Walker-type multianvil apparatus under high-pressure/high-temperature conditions of 12.0 GPa/1400 °C and 12.3 GPa/1200 °C, respectively. The crystal structures of both compounds, determined by single-crystal X-ray diffraction, constitute new representatives of the structure type M5B12O25(OH) (M = Ga, In) crystallizing in the space group I41/acd. The presence of the hydroxyl groups was confirmed via single-crystal IR spectroscopy.
1 Introduction
In the last decade, our research group focused on high-pressure gallium and indium borates represented by 11 different compounds in this field since 2015. Recently, we reported on the successful synthesis of M5B12O25(OH) (M = Ga, In) [1], [2], a structure type incorporating either gallium or indium as metal cations. This was an intriguing finding, given that until then only the well-known calcite-type ortho-borates MBO3 (M = Al, Ga, In, Tl) [3], [4] were known to form isotypic compounds in this system. Based on this success, we tried if the even smaller Al3+ ion could also function as metal cation in the structure type M5B12O25(OH) (M = Ga, In) and succeeded in synthesizing the isotypic compound Al5B12O25(OH). Obviously, the structure of M5B12O25(OH) (M = Ga, In) is quite flexible concerning the incorporation of different metal cations. In subsequent experiments, we were able to additionally synthesize a mixed-cation variation with the formula Ga4InB12O25(OH) which represents the second known borate hosting indium and gallium on different crystallographic positions besides the recently published Ga4In4B15O33(OH)3 [5]. Actually, the structural motif we found in M5B12O25(OH) (M = Ga, In) had previously been observed in different variations in other compounds like e. g. Ti5B12O26 [6] and CaMg2P6O3N10 [7]. In contrast to Ti5B12O26, where the charge of the missing hydrogen atom is simply compensated by titanium being partially oxidized to 4+, CaMg2P6O3N10 bears no resemblance regarding the formula at first glance. However, the oxonitridophosphate shows obvious structural similarities which have already been discussed in our original publication of M5B12O25(OH) (M = Ga, In) [1], [2].
In the following, the syntheses, crystal structures and IR spectrometric data of the new isotypic compounds Al5B12O25(OH) and Ga4InB12O25(OH) are described in detail.
2 Experimental section
2.1 Synthesis
For the synthesis of Al5B12O25(OH), the mixture of starting material consisted of Al2O3 (99.9%, Alfa Aesar) and H3BO3 (99.5%, Carl Roth) in a stoichiometric ratio of 5:24, for the synthesis of Ga4InB12O25(OH), the nitrates Ga(NO3)3·8H2O (99.99%, Strem Chemicals) and In(NO3)3·5H2O[1] (99.99% chemPUR) were weighed in with H3BO3 in a stoichiometric ratio of 4:1:12. Both mixtures were thoroughly ground and encapsulated in platinum foil. The capsules were placed in a crucible made of α-BN (Henze Boron Nitride Products AG) and put into a “14/8 assembly” surrounded by eight tungsten carbide cubes (Hawedia) and placed into a nest formed by six steel wedges. The high-pressure/high-temperature conditions were generated with a Walker-type multianvil press (Voggenreiter) and a graphite furnace for resistance heating. A detailed description of the experimental setup can be found in the literature [8], [9], [10]. During the synthesis of Al5B12O25(OH) the maximum pressure of 12.0 GPa was built up in 317 min, and the maximum temperature of 1400 °C was reached in 8 min, held for 7 min and then lowered to 800 °C within an hour. Afterwards, the heating was turned off and the decompression process of ∼16 h was started. The platinum capsule was freed from its surroundings and cracked open. The reaction product appeared clean-white revealing colorless single-crystals. The synthesis of Ga4InB12O25(OH) was carried out with a maximum pressure of 12.3 GPa which was built up within 334 min. The maximum temperature of 1200 °C was achieved within 10 min and held for another 10 min. After a stepwise down regulation to 800 °C in 30 min, the assembly was quenched to room temperature. The decompression to ambient conditions took ∼16.5 h. The reaction product appeared gray-white clearly containing byproducts besides the target compound Ga4InB12O25(OH).
2.2 X-ray structure determination
The reaction products were 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 MoKα1 radiation (λ = 0.7093 Å) in transmission geometry across a 2θ range of 2.0–60.5 °. Figure 1 shows a comparison of the experimental powder patterns with calculated patterns derived from single-crystal structure data. All reflections that belong to side phases and therefore do not match the theoretical pattern are marked with asterisks. In the synthesis attempt for Al5B12O25(OH), the desired phase could be obtained nearly phase pure, however the reaction product of the synthesis aiming at Ga4InB12O25(OH) showed prominent reflections of at least one byproduct. Further experiments varying the reaction conditions or the molar ratio of the starting materials did not improve the result which can probably be explained by difficulties in obtaining the exact mass ratio of the hygroscopic gallium and indium nitrates.
The colorless single crystals of Al5B12O25(OH) and Ga4InB12O25(OH) were measured with a Bruker D8 Quest diffractometer equipped with a Photon 100 CMOS detector. The intensity data was corrected by a multi-scan absorption correction with Sadabs 2014/5 [11]. For the structure solution and parameter refinement, the software tools Shelxs-2013/1 [12], [13] and Shelxl-2013/4 [14] implemented in the program WinGX-2018.1 [15] were used. According to the systematic reflection conditions, both compounds were solved and refined in the space group I41/acd (no. 142, origin choice 2). To allow an easy comparison of the isotypic compounds, the data sets were standardized with the routine Structure Tidy[16] as implemented in Platon[17](version 170613). In both compounds, the H atom position H5 was fixed with the DFIX command at a distance of 0.83 Å to O5. Due to charge neutrality reasons, H5 can only be occupied by 1/4th. Except the hydrogen atoms, all atoms in the presented structures could be refined anisotropically. Details of the data collection can be found in the synoptical Table 1. In Table 3 and Table 4, the positional and displacement parameters, the Wyckoff positions, and the site occupancy factors are given.
Empirical formula | Al5B12O25(OH) | Ga4InB12O25(OH) |
---|---|---|
Molar mass, g mol−1 | 681.63 | 940.43 |
Crystal system | tetragonal | |
Space group | I41/acd (no. 142) | |
Single-crystal diffractometer | Bruker D8 Quest Kappa | |
Radiation/wavelength λ, pm | MoKα/71.07 | |
a, Å | 10.872(2) | 11.1767(6) |
c, Å | 21.460(4) | 21.888(2) |
V, Å3 | 2536.5(9) | 2734.2(3) |
Formula units per cell Z | 8 | 8 |
Calculated density, g cm−3 | 3.57 | 4.57 |
Crystal size, mm3 | 0.050 × 0.045 × 0.035 | 0.030 × 0.030 × 0.030 |
Temperature, K | 293(2) | 213(2) |
Absorption coefficient, mm−1 | 0.7 | 9.6 |
F(000), e | 2672 | 3536 |
θ range, deg | 3.26–39.16 | 3.18–39.38 |
Range in hkl | ±15; ±15; ±31 | −19/+18; ±19; ±39 |
Refl. total/independent | 24,148/1014 | 45,785/2046 |
Rint/Rσ | 0.0572/0.0266 | 0.0454/0.0166 |
Refl. with I > 2 σ(I) | 868 | 1873 |
Data/ref. parameters | 1014/103 | 2046/104 |
Absorption correction | multiscan | multiscan |
Final R1/wR2 (I > 2 σ(I)) | 0.0381/0.1021 | 0.0214/0.0509 |
Final R1/wR2 (all data) | 0.0458/0.1069 | 0.0256/0.0526 |
Goodness-of-fit on F2 | 1.087 | 1.089 |
Largest diff. peak/hole, e Å−3 | 1.59/−0.60 | 3.55/−0.89 |
Compound | a | c | V | r |
---|---|---|---|---|
Al5B12O25(OH) | 10.872(2) | 21.460(4) | 2536.5(9) | 0.675 |
Ga5B12O25(OH) | 11.150(5) | 21.76(2) | 2705(3) | 0.760 |
Ga4InB12O25(OH) | 11.1767(6) | 21.888(2) | 2734.2(3) | |
Ti5B12O26 | 11.211(2) | 22.115(4) | 2779.5(8) | 0.810 |
In5B12O25(OH) | 11.639(2) | 22.282(5) | 3018(2) | 0.940 |
Atom | Wyckoff position | x | y | z | Ueq (Uiso for H5) | s.o.f. |
---|---|---|---|---|---|---|
Al5B12O25(OH) | ||||||
Al1 | 32g | 0.34388(4) | 0.07391(4) | 0.04473(2) | 0.0040(2) | 1 |
Al2 | 8b | 0 | 1/4 | 1/8 | 0.0076(2) | 1 |
B1 | 32g | 0.0761(2) | 0.3362(2) | 0.25099(8) | 0.0028(3) | 1 |
B2 | 32g | 0.2527(2) | 0.3213(2) | 0.08254(7) | 0.0030(3) | 1 |
B3 | 32g | 0.2598(2) | 0.1573(2) | 0.16829(7) | 0.0045(3) | 1 |
O1 | 32g | 0.0004(2) | 0.08635(9) | 0.29233(4) | 0.0026(3) | 1 |
O2 | 32g | 0.0119(2) | 0.0654(2) | 0.12463(4) | 0.0027(3) | 1 |
O3 | 32g | 0.1625(2) | 0.3930(2) | 0.04453(6) | 0.0032(3) | 1 |
O4 | 32g | 0.1625(2) | 0.2655(2) | 0.29132(5) | 0.0026(3) | 1 |
O5 | 32g | 0.1701(2) | 0.0666(2) | 0.04187(6) | 0.0035(3) | 1 |
O6 | 32g | 0.3430(2) | 0.0782(2) | 0.13080(6) | 0.0027(3) | 1 |
O7 | 16d | 0 | 1/4 | 0.03496(8) | 0.0022(3) | 1 |
H5 | 32g | 0.17(2) | 0.133(8) | 0.062(6) | 0.08(6) | 0.25 |
Ga4InB12O25(OH) | ||||||
In1 | 8b | 0 | 1/4 | 1/8 | 0.00561(4) | 1 |
Ga1 | 32g | 0.34740(2) | 0.06874(2) | 0.04495(2) | 0.00422(4) | 1 |
B1 | 32g | 0.0727(2) | 0.3382(2) | 0.25223(6) | 0.0033(2) | 1 |
B2 | 32g | 0.2546(2) | 0.3155(2) | 0.08221(6) | 0.0029(2) | 1 |
B3 | 32g | 0.2657(2) | 0.1538(2) | 0.16823(6) | 0.0043(2) | 1 |
O1 | 32g | 0.00110(8) | 0.08635(8) | 0.29240(4) | 0.0030(2) | 1 |
O2 | 32g | 0.01711(8) | 0.05319(8) | 0.12414(4) | 0.0033(2) | 1 |
O3 | 32g | 0.16067(8) | 0.38208(8) | 0.04600(4) | 0.0033(2) | 1 |
O4 | 32g | 0.16100(8) | 0.27201(8) | 0.29080(4) | 0.0032(2) | 1 |
O5 | 32g | 0.17028(9) | 0.06217(8) | 0.04311(5) | 0.0038(2) | 1 |
O6 | 32g | 0.34650(8) | 0.07511(8) | 0.13262(4) | 0.0031(2) | 1 |
O7 | 16d | 0 | 1/4 | 0.02915(6) | 0.0031(2) | 1 |
H5 | 32g | 0.16(2) | 0.135(3) | 0.050(6) | 0.03(3) | 0.25 |
Atom | U11 | U22 | U33 | U12 | U13 | U23 |
---|---|---|---|---|---|---|
Al5B12O25(OH) | ||||||
Al1 | 0.0047(3) | 0.0041(3) | 0.0033(3) | −0.0009(2) | −0.0005(2) | −0.0003(2) |
Al2 | 0.0098(3) | 0.0098(3) | 0.0034(4) | −0.0058(4) | 0 | 0 |
B1 | 0.0030(6) | 0.0031(6) | 0.0022(7) | −0.0013(5) | −0.0005(5) | 0.0003(5) |
B2 | 0.0035(7) | 0.0029(7) | 0.0027(7) | 0.0007(5) | 0.0000(5) | 0.0003(5) |
B3 | 0.0047(7) | 0.0054(7) | 0.0032(7) | 0.0016(5) | −0.0013(5) | −0.0017(5) |
O1 | 0.0026(5) | 0.0020(5) | 0.0031(5) | −0.0004(3) | 0.0006(4) | 0.0009(3) |
O2 | 0.0029(5) | 0.0018(5) | 0.0033(5) | 0.0003(4) | 0.0013(4) | −0.0002(3) |
O3 | 0.0024(5) | 0.0036(5) | 0.0035(5) | −0.0003(4) | −0.0019(4) | −0.0007(4) |
O4 | 0.0019(5) | 0.0029(5) | 0.0029(5) | −0.0001(4) | −0.0013(3) | −0.0005(4) |
O5 | 0.0034(5) | 0.0037(5) | 0.0033(5) | −0.0012(4) | 0.0002(4) | 0.0002(4) |
O6 | 0.0019(5) | 0.0031(5) | 0.0030(5) | 0.0004(4) | −0.0005(4) | 0.0000(3) |
O7 | 0.0027(6) | 0.0025(6) | 0.0015(6) | −0.0016(5) | 0 | 0 |
Ga4InB12O25(OH) | ||||||
In1 | 0.00700(5) | 0.00700(5) | 0.00284(7) | −0.00337(5) | 0 | 0 |
Ga1 | 0.00570(6) | 0.00377(6) | 0.00321(6) | −0.00113(4) | −0.00100(4) | −0.00005(4) |
B1 | 0.0031(4) | 0.0033(4) | 0.0033(5) | −0.0004(3) | 0.0002(4) | −0.0003(4) |
B2 | 0.0025(4) | 0.0027(4) | 0.0035(5) | −0.0002(3) | 0.0000(4) | 0.0003(4) |
B3 | 0.0035(5) | 0.0049(5) | 0.0047(5) | 0.0007(4) | −0.0013(4) | −0.0007(4) |
O1 | 0.0025(3) | 0.0032(3) | 0.0032(3) | −0.0002(2) | −0.0001(2) | 0.0010(2) |
O2 | 0.0033(3) | 0.0025(3) | 0.0042(3) | 0.0006(2) | 0.0016(3) | 0.0004(2) |
O3 | 0.0036(3) | 0.0027(3) | 0.0036(3) | 0.0003(2) | −0.0018(2) | −0.0004(3) |
O4 | 0.0029(3) | 0.0033(3) | 0.0036(3) | −0.0001(2) | −0.0009(3) | −0.0008(2) |
O5 | 0.0034(3) | 0.0038(3) | 0.0043(3) | −0.0010(2) | −0.0003(3) | 0.0007(3) |
O6 | 0.0032(3) | 0.0029(3) | 0.0032(3) | 0.0012(2) | 0.0000(3) | 0.0002(2) |
O7 | 0.0031(4) | 0.0035(5) | 0.0027(5) | −0.0015(3) | 0 | 0 |
CSD-2002397 (Al5B12O25(OH)) and CSD-2002398 (Ga4InB12O25(OH)) contain the supplementary crystallographic data for this paper. The data can be obtained free of charge from The Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/data_request/cif.
2.3 IR spectroscopy
The transmission FT-IR spectra of single crystals of Ga4InB12O25(OH) and Al5B12O25(OH) were 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, liquid nitrogen cooled (Mercury Cadmium Telluride (MCT) detector and a Hyperion 3000 microscope (Bruker). 180 scans of the samples were acquired using a Globar (silicon carbide) rod as mid-IR source and a 15× IR objective as focus. During the measurements, the samples were positioned on a BaF2 window. Atmospheric influences were corrected with the software Opus 6.5.
3 Results and discussion
3.1 Crystal structure
The title compounds crystallize in the tetragonal space group I41/acd (no. 142, origin choice 2) with eight formula units (Z = 8). Al5B12O25(OH) and Ga4InB12O25(OH) show exactly the same site symmetry and are clearly isotypic to each other. In the two other known triel borates with the formula M5B12O25(OH) (M = Ga, In) [1], [2], the second metal cation position showed large displacement ellipsoids leading to a preferred refinement of a disordered position slightly shifted from the octahedral center. Both Al5B12O25(OH) and Ga4InB12O25(OH) did not show this kind of disorder and the second metal cation could be refined in the octahedral center. Despite this discrepancy Al5B12O25(OH), Ga4InB12O25(OH), Ga5B12O25(OH), and In5B12O25(OH) can be regarded as isotypic to each other. Table 2 gives a comparison of the lattice parameters of these four compounds and the related borate Ti5B12O26. As expected, Al5B12O25(OH) features the smallest unit cell and the lattice parameters of Ga4InB12O25(OH) lie between those of Ga5B12O25(OH) and In5B12O25(OH). The augmentation of the unit cells with increasing ionic radii of the metal cations is illustrated in Figure 2. The M5B12O25(OH) (Al, Ga, In, Ga/In) structure type features two crystallographically different metal cation positions in octahedral coordination, three different boron positions forming a complex network of corner-sharing BO4 tetrahedra, and one disordered proton site. Figure 3 gives an overview of the arrangement of the metal cation octahedra. Both polyhedra shown in green and orange represent the dinuclear M2O10 units in Al5B12O25(OH) and Ga4InB12O25(OH) centered by Ga or Al1. Along the crystallographic c axis, every second unit is dislocated along b or rotated by 90° pertaining to the corresponding unit. The In or Al2 atoms form 12 isolated MO6 octahedra per unit cell, which fill the holes in the structure along the
Al5B12O25(OH) | Ga4InB12O25(OH) | Al5B12O25(OH) | Ga4InB12O25(OH) | ||||
---|---|---|---|---|---|---|---|
M1– | O6 | 1.848(2) | 1.920(2) | B1– | O7 | 1.469(2) | 1.450(2) |
O5 | 1.866(2) | 1.939(2) | O3 | 1.474(2) | 1.487(2) | ||
O4 | 1.873(2) | 1.967(2) | O1 | 1.479(2) | 1.471(2) | ||
O5 | 1.893(2) | 1.981(2) | O4 | 1.492(2) | 1.495(2) | ||
O1 | 1.913(2) | 2.0182(9) | Ø B1–O | 1.48 | 1.48 | ||
O3 | 1.968(2) | 2.0883(9) | |||||
Ø M1–O | 1.89 | 1.99 | B2– | O6 | 1.470(2) | 1.477(2) | |
O1 | 1.476(2) | 1.466(2) | |||||
M2– | O7 | 1.932(2) 2× | 2.098(2) 2× | O2 | 1.487(2) | 1.477(2) | |
O2 | 2.011(2) 4× | 2.2081(9) 4× | O3 | 1.496(2) | 1.511(2) | ||
Ø M2–O | 1.98 | 2.17 | Ø B2–O | 1.48 | 1.48 | ||
B3– | O5 | 1.459(2) | 1.469(2) | ||||
O4 | 1.473(2) | 1.471(2) | |||||
O6 | 1.485(2) | 1.483(2) | |||||
O2 | 1.513(2) | 1.494(2) | |||||
Ø B3–O | 1.48 | 1.48 |
Al5B12O25(OH) | Ga4InB12O25(OH) | Al5B12O25(OH) | Ga4InB12O25(OH) | ||
---|---|---|---|---|---|
O5–M1–O5 | 83.39(6) | 82.91(4) | O2–M2–O2 | 82.64(7) 2× | 80.07(5) 2× |
O5–M1–O3 | 85.56(5) | 85.42(4) | O7–M2–O2 | 89.77(3) 4× | 90.49(2) 4× |
O5–M1–O3 | 87.26(5) | 88.20(4) | O7–M2–O2 | 90.23(3) 4× | 89.51(2) 4× |
O4–M1–O1 | 87.88(5) | 86.82(4) | O2–M2–O2 | 97.36(7) 2× | 99.94(5) 2× |
O5–M1–O1 | 89.92(5) | 88.58(4) | Ø O–M2–O90 | 90.0 | 90.0 |
O6–M1–O1 | 90.08(5) | 89.44(4) | |||
O5–M1–O1 | 90.72(5) | 90.24(4) | O2–M2–O2 | 179.54(5) 2× | 179.02(5) 2× |
O6–M1–O3 | 91.57(5) | 91.48(4) | O7–M2–O7 | 180.0 | 180.0 |
O6–M1–O5 | 91.62(5) | 90.94(4) | Ø O–M2–O180 | 179.7 | 179.3 |
O6–M1–O4 | 92.42(5) | 92.70(4) | |||
O5–M1–O4 | 92.59(5) | 93.42(4) | |||
O4–M1–O3 | 96.51(5) | 99.11(4) | O6–B2–O1 | 107.7(2) | 108.4(2) |
Ø O–M1–O90 | 90.0 | 89.9 | O6–B2–O3 | 109.1(2) | 108.5(2) |
O2–B2–O3 | 109.1(2) | 110.1(2) | |||
O6–M1–O5 | 174.95(6) | 173.85(4) | O6–B2–O2 | 109.4(2) | 108.2(2) |
O1–M1–O3 | 175.24(5) | 173.94(4) | O1–B2–O2 | 110.3(2) | 109.8(2) |
O4–M1–O5 | 175.40(5) | 174.11(4) | O1–B2–O3 | 111.1(2) | 111.8(2) |
Ø O–M1–O180 | 175.2 | 174.0 | Ø O–B2–O | 109.5 | 109.5 |
O7–B1–O3 | 106.8(2) | 106.5(2) | O4–B3–O6 | 107.5(2) | 108.4(2) |
O1–B1–O4 | 107.4(2) | 108.4(2) | O5–B3–O4 | 108.1(2) | 107.2(2) |
O7–B1–O4 | 109.3(2) | 107.50(9) | O5–B3–O6 | 109.3(2) | 109.2(2) |
O3–B1–O1 | 110.8(2) | 111.3(2) | O6–B3–O2 | 109.6(2) | 109.8(2) |
O7–B1–O1 | 111.2(2) | 111.0(2) | O4–B3–O2 | 109.7(2) | 109.4(2) |
O3–B1–O4 | 111.3(2) | 112.1(2) | O5–B3–O2 | 112.6(2) | 112.7(2) |
Ø O–B1–O | 109.5 | 109.5 | Ø O–B3–O | 109.5 | 109.5 |
d(D–H) | d(H···A) | d(D···A) | ∠(D–H···A) | |
---|---|---|---|---|
Al5B12O25(OH) | ||||
O5–H5···O2 | 0.84(2) | 1.78(5) | 2.591(2) | 161(15) |
Ga4InB12O25(OH) | ||||
O5–H5···O2 | 0.83(2) | 2.0(2) | 2.648(2) | 130(11) |
Al5B12O25(OH) | Ga4InB12O25(OH) | Ga5B12O25(OH) | In5B12O25(OH) | Ti5B12O26 [6] | |||||
---|---|---|---|---|---|---|---|---|---|
Al1 | 3.13 | Ga | 3.04 | Ga1 | 3.14 | In1 | 3.08 | Ti1 | 3.31 |
Al2 | 2.45 | In | 2.93 | Ga2 | 2.37 | In2 | 2.14 | Ti2 | 2.43 |
B1 | 2.99 | B1 | 3.02 | B1 | 2.94 | B1 | 2.98 | B1 | 3.00 |
B2 | 2.96 | B2 | 2.96 | B2 | 2.93 | B2 | 2.96 | B2 | 2.97 |
B3 | 2.96 | B3 | 2.99 | B3 | 3.02 | B3 | 3.02 | B3 | 2.93 |
O1 | −1.99 | O1 | −2.00 | O1 | −1.99 | O1 | −1.96 | O1 | −2.03 |
O2 | −1.79 | O2 | −1.90 | O2 | −1.87 | O2 | −1.92 | O2 | −2.06 |
O3 | −1.90 | O3 | −1.80 | O3 | −1.88 | O3 | −1.86 | O3 | −2.10 |
O4 | −2.03 | O4 | −2.01 | O4* | −1.87 | O4 | −1.93 | O4 | −1.88 |
O5* | −1.87 | O5* | −1.84 | O5 | −2.09 | O5 | −2.18 | O5 | −1.85 |
O6 | −2.09 | O6 | −2.09 | O6 | −1.96 | O6 | −1.96 | O6 | −2.04 |
O7 | −2.00 | O7 | −2.20 | O7 | −1.98 | O7 | −2.01 | O7 | −1.80 |
H** | 0.91 | H** | 0.85 | H** | 0.93 |
* This oxygen atom is the donor atom of the hydrogen bond. The bond valence sum was calculated without the consideration of the hydrogen atom because it would get disproportionally high if calculated with a full contribution of the proton as it only has a site occupancy factor of 0.25.
** To calculate the valence sum of the hydrogen atom, a distance of 0.97 Å to the donor oxygen atom was assumed and on the basis of that the distance to the acceptor atom(s) was determined via a second refinement process.
3.2 IR spectroscopy
Figure 8 displays the single-crystal IR spectra of Ga4InB12O25(OH) and Al5B12O25(OH) in the spectral region of 600–4000 cm−1. In the broad band in the range 600–1400 cm−1 many different vibrations of M–O and B–O bonds overlap. The position of the O–H stretching vibration depends on the strength of the hydrogen bond. Hammer et al. classify RO···O distances between 2.5 and 2.7 Å as strong hydrogen bonds which result in IR bands between 1600 and 3200 cm−1 [29]. Apparently, this is the case for the hydrogen bonds in Al5B12O25(OH) (RO5···O2 = 2.59 Å) and Ga4InB12O25(OH) (RO5···O2 = 2.65 Å). Besides this strong hydrogen bond in both structures, H5 presumably is also engaged in other hydrogen bonds to e. g. O1 and O7. While those RO···O distances are only slightly longer in Al5B12O25(OH) (RO5···O1 = 2.69 Å and RO5···O7 = 2.72 Å) and therefore in the crossover between strong and weak hydrogen bonds, they should clearly be classified as weak in Ga4InB12O25(OH) (RO5···O1 = 2.80 Å and RO5···O7 = 2.85 Å). The strong band for Ga4InB12O25(OH) near 3500 cm−1 and the jump near 3200 cm−1 thus indicate both poor hydrogen bonding (non-contacted hydroxyl groups) along with stronger hydrogen bonding. The latter prevails for Al5B12O25(OH) (Figure 8).
4 Conclusion
The isotypic compounds Al5B12O25(OH) and Ga4InB12O25(OH) were synthesized in high-pressure/high-temperature experiments in a multianvil press. Both structures could be determined from single-crystal data and comprise a complex network of BO4 tetrahedra with single- and double-octahedral units centered by the metal cations. IR spectroscopic studies confirmed the presence of hydroxyl groups in both borates. With Al5B12O25(OH) and Ga4InB12O25(OH), two new representatives of the structure type M5B12O25(OH) (M = Ga, In) could be discovered. Whether this structure type is also stable with other metal cations remains to be investigated in future experiments.
Acknowledgments
We thank Assoc. Univ. Prof. Dr. Gunter Heymann for the recording of the single-crystal data, Freia Ruegenberg for the IR measurements and Univ. Prof. Dr. Roland Stalder for giving us access to the single-crystal IR spectrometer.
Author contribution: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.
Research funding: None declared.
Conflict of interest statement: The authors declare no conflicts of interest regarding this article.
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