Synthesis and Crystal Structure of the Acentric Indium Borate InB6O9

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Synthesis and Crystal Structure of the Acentric Indium Borate InB6O9(OH)3 Daniela Vitzthum,Δ Lkhamsuren Bayarjargal,# Björn Winkler,# and Hubert Huppertz*,Δ Δ

Institut für Allgemeine, Anorganische und Theoretische Chemie, Universität Innsbruck, Innrain 80-82, A-6020 Innsbruck, Austria Institut für Geowissenschaften, Universität Frankfurt, Altenhöferallee 1, 60438 Frankfurt am Main, Germany

#

S Supporting Information *

ABSTRACT: The new acentric indium borate InB6O9(OH)3 was synthesized in a Walkertype multianvil apparatus at extreme pressure and temperature conditions of 12.3 GPa and 1500 °C. Single-crystal X-ray diffraction provided the data for the crystal structure solution and refinement. InB6O9(OH)3 crystallizes in the orthorhombic space group Fdd2 (Z = 8) with the lattice parameters a = 39.011(8), b = 4.4820(9), c = 7.740(2) Å, and V = 1353.3(5) Å3. The structure of InB6O9(OH)3 is basically built of corner-sharing BO4 tetrahedra and isolated InO6 octahedra. The presence of hydroxyl groups was confirmed with vibrational spectroscopic methods (IR and Raman). Furthermore, the second harmonic signal of an InB6O9(OH)3 powder sample yielded more than twice the intensity of quartz.

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INTRODUCTION Optical second harmonic generation (SHG) was first discovered in 1961 by Franken et al. in a quartz crystal.1 In 1975, Dewey et al. reported on the first nonlinear optical (NLO) borate KB5O8·4H2O that allowed phase-matched SHG to shorter wavelengths than any other compound known at this time.2 The intensive research on NLO borates began with the discovery of β-BaB2O43 (BBO) and LiB3O54 (LBO), which are still in use as NLO materials.5 Borates are not only superior to other NLO materials in UV and deep-UV applications but also more frequent to come by.6 The probability that a newly discovered crystal structure is acentric (which is a requirement for SHG) is more than twice as high in borates as in other inorganic compounds.5,7 Most of the interesting NLO borates such as β-BaB2O4,3 LiB3O5,4 CsLiB6O10,8 K2Al2B2O7,9 or βRb2Al2B2O710 feature isolated or adjunctive BO3 groups as structural units.11 Because of their planar and asymmetrical shape, BO3 groups and cyclic 3B rings are believed to be beneficial for the SHG properties of a compound.11 However, there also exist borates solely built of BO4 tetrahedra that show SHG intensity, like δ-BiB3O612 and γ-NiB4O7.13 As expected for high-pressure borates, our newly discovered compound InB6O9(OH)3 solely consists of fourfold coordinated boron atoms. To the best of our knowledge, InB6O9(OH)3 is the only acentric high-pressure borate in the system In−B−O(−H) currently. With the discovery of this compound, the family of indium borate hydroxides was enlarged by another representative. Besides H2InB5O1014 and In19B34O74(OH)11,15 InB6O9(OH)3 is now the third known compound in this system. In the following, we report on the synthesis, crystal structure, and SHG properties of InB6O9(OH)3. In addition, this work © XXXX American Chemical Society

contains Raman and IR spectroscopic studies of the title compound.

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EXPERIMENTAL SECTION

Synthesis. For the synthesis of InB6O9(OH)3, a light-yellow powder of In2O3 (99.9% ChemPUR) and H3BO3 (99.5%, Carl Roth) in a stoichiometric ratio of 1:12 (eq 1) was weighed and ground in an agate mortar. The homogenized mixture of the starting materials was filled into a crucible and closed with a lid both made of α-BN (Henze Boron Nitride Products AG). This reaction crucible was embedded into a “14/8 assembly”, which itself was surrounded by eight bevelled tungsten carbide cubes (Hawedia) forming a bigger cube. For the high-pressure synthesis, this cube was placed in a Walker-type module consisting of six steel wedges surrounded by steel rings and compressed and heated with a multianvil device (Voggenreiter). A detailed description of this setup can be found in the literature.16−18 12.3 GPa, 1500 ° C

In2O3 + 12 H3BO3 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 2 InB6O9(OH)3 + 15 H 2O (1) The synthesis of InB6O9(OH)3 was performed under extreme conditions of ∼12.3 GPa and 1500 °C. As soon as the maximum pressure was built up, the heating process started. In 7 min, a temperature of 1500 °C was reached and held for 5 min. Subsequently, a continuous controlled cooling to 1300 °C in 15 min followed before the sample was temperature-quenched to room temperature. Last, the decompression process was started and took ∼17 h. After the synthesis, the setup was disassembled, and the reaction crucible was opened. The product appeared as a white, firm chunk that could be easily separated from the surrounding BN with a scalpel. The single crystals of InB6O9(OH)3 were colorless. Received: February 27, 2018

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DOI: 10.1021/acs.inorgchem.8b00518 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry The synthesis described above represents only one example of several different high-pressure syntheses, which led to the formation of InB6O9(OH)3. In Table S1, the synthesis conditions of our three best experiments leading to nearly pure products of InB6O9(OH)3 can be found. We noticed that either the stoichiometric mixture of In/B = 1:6 or a slight excess of boric acid (1:8) worked best, although InB6O9(OH)3 also formed in the syntheses with less boric acid, when an additional boron-poor compound formed. X-ray Powder Diffraction. A flat powder sample of the reaction product was analyzed via X-ray powder diffraction in transmission geometry on a Stoe Stadi P powder diffractometer (STOE & Cie GmbH). The measurement was performed with Ge(111)-monochromatized Mo Kα1 radiation (λ = 70.93 pm) across a 2θ range of 2− 60.5° with a step size of 0.015° (2θ). The reflections were detected with a Mythen 1K detector (Dectris). Single-Crystal Structure Analysis. Suitable single-crystals of InB6O9(OH)3 were chosen and isolated with the help of a polarization contrast microscope and then prepared for single-crystal X-ray diffraction measurements on a Bruker D8 Quest diffractometer equipped with a Photon 100 CMOS detector. A multiscan absorption correction of the intensity data was performed with SADABS 2014/ 5.19 To solve and refine the structure, the SHELXS/L-201320,21 software implemented in the program WINGX-2013.322 was used. First, the data set was integrated considering translational symmetry only and scaled for a primitive structure solution. During the structure solution process, a clear indication of the presence of F- or C-centering was discernible. Refinement attempts in the monoclinic space groups C2, C2/m, and Cm as well as in the orthorhombic space groups Fdd2 and C2221 led us to the conclusion that the most likely space group was Fdd2, for which then a new scaling was performed. A Flack x parameter of nearly 1 suggested an inversion of the structure, which was then moved by x = 0.25 − x, y = 0.25 − y, and z = 1 − z, as Fdd2 is one of seven nonenantiomorphic space groups, where the standard inversion movement would not lead to the inverted absolute structure.23,24 The final refinement of InB6O9(OH)3 showed no disorder or nonpositive definite atom displacement parameters, and all In, B, and O atoms could be refined anisotropically. Concerning the protons, one could be located with the DFIX command near O3, thus providing two protons in the sum formula of InB6O9(OH)3. The last missing proton can either be positioned near O4 or O6 with DFIX, but no charge density in the Fourier map indicated or favored any of these positions. An occupation of 0.25 on both positions did not allow a stable refinement. Thus, we decided not to refine the other proton position at all. Further crystallographic details of the refinement of InB6O9(OH)3 can be found in the synoptical Table 1. Vibrational Spectroscopy. The transmission infrared spectrum of an InB6O9(OH)3 single-crystal was measured with a Vertex 70 FT-IR spectrometer in a spectral range of 600−4000 cm−1 and a resolution of 4 cm−1. The spectrometer has a KBr beam splitter, a liquid nitrogen cooled mercury cadmium telluride (MCT) detector, and a Hyperion 3000 microscope (Bruker). For the measurement, the single-crystal was positioned on a BaF2 window and focused with a 15× IR objective. A Globar (silicon carbide) rod was used as mid-IR source, and 160 scans of the sample were recorded. Atmospheric influences were corrected with the software OPUS 6.5. The Raman spectrum of a randomly oriented InB6O9(OH)3 singlecrystal was recorded with a Labram-HR 800 Raman microspectrometer (Horiba Jobin Yvon) in the spectral range of 100− 4000 cm−1. The sample was irradiated with the 532 nm emission line of a frequency-doubled 100 mW Nd:YAG laser producing a spot of ∼1 μm in diameter on the surface. The scattered light was dispersed by an optical grating with 1800 lines mm−1, and an Olympus 50× objective was used. The spectrum was recorded at ambient conditions employing a 1024 × 256 open-electrode CCD detector. For the background correction, a fourth-order function was applied, and one point was adjusted manually. Second Harmonic Generation Measurements. SHG measurements were performed on a powder sample of InB6O9(OH)3. A detailed description of the experimental setup for powder SHG measurements can be found in the literature.25 The fundamental wave

Table 1. Crystal Data and Structure Refinement of Orthorhombic InB6O9(OH)3 empirical formula molar mass, g mol−1 crystal system space group powder data powder diffractometer radiation a, Å b, Å c, Å V, Å3 range, deg (2θ) Rexp Rwp Rp goodness-of-fit single-crystal data single-crystal diffractometer radiation a, Å b, Å c, Å V, Å3 formula units per cell Z calculated density, g cm−3 crystal size, mm3 temperature, K detector distance/mm exposure time absorption coefficient, mm−1 F(000), e θ range, deg range in hkl reflections total/independent Rint reflections with I > 2σ(I) Rσ data/restraints/parameters absorption correction final R1/wR2 [I > 2σ(I)] final R1/wR2 (all data) goodness-of-fit on Fi2 largest diff. peak/hole, e Å−3 Flack parameter

InB6O9(OH)3 374.70 orthorhombic Fdd2 (No. 43) Stoe Stadi P Mo Kα1 (λ = 70.93 pm) 38.959(2) 4.4648(2) 7.7154(3) 1342.1(1) 2.0−45.0 0.0446 0.0620 0.0436 1.39 Bruker D8 Quest Kappa Mo Kα (λ = 71.073 pm) 39.011(8) 4.4820(9) 7.740(2) 1353.3(5) 8 3.678 0.60 × 0.050 × 0.015 293(2) 40 0.5°/frame, 25 s/frame 3.583 1424 4.2−37.9 ±66; ±7; ±13 12 214/1840 0.0351 1774 0.0222 1840/2/91 multiscan 0.0222/0.0503 0.0238/0.0507 1.204 0.96/−1.89 0.03(2)

was provided by a Q-switched Nd:YLF laser system (Falcon 217D, Quantronix), which operates at 1054 nm and a pulse width of 130 ns. Thus, the generated SHG signal of 527 nm was collected on five different areas of the sample to check its homogeneity. On each position, 15 measurements were performed and averaged. The measured intensities were background-corrected by signals collected between the laser pulses. As reference materials Al2O3, quartz, KDP (KH2PO4), and BaTiO3 were used. Two samples of InB6O9(OH)3 were measured. In the first sample, the grains had an average size (estimated optically) of ∼7 μm, where the largest grains had diameters of 14 μm. In the second sample, the average grain size was ∼15 μm, and the largest grains were ∼25 μm. The reference quartz sample had an average grain size of ∼20 μm, where the maximum grains had diameters of 40 μm. For theoretical considerations,26 an increase in the grain size should lead to an increase in the SHG intensity. This is the case here, where the smaller grains of the InB6O9(OH)3 sample gave an intensity of 285(69) counts, while the larger grains yielded B

DOI: 10.1021/acs.inorgchem.8b00518 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry 941(265) counts. For comparison, the quartz sample gave 420(54) counts.

agreement with those derived from the single-crystal data (see Table 1). Crystal Structure. InB6O9(OH)3 crystallizes in the orthorhombic space group Fdd2 (No. 43) with eight formula units (Z = 8) and lattice parameters a = 39.011(8), b = 4.4820(9), c = 7.740(2) Å, and V = 1353.3(5) Å3. Details of the data collection and refinement are listed in Table 1. All atoms of the asymmetric unit including their Wyckoff positions, atomic coordinates, and displacement parameters can be found in Tables S2 and S3 (Supporting Information). InB6O9(OH)3 features one indium position that forms regularly shaped isolated octahedra with the oxygen atoms O1, O2, and O5. The bond lengths average out to an often observed value of 2.177 Å,15,29,30 and the octahedral angles show reasonable mean values of exactly 90.0 and 176.3°. All three crystallographically different boron positions each build up BO4 tetrahedra. The alternately arranged B1O4 and B2O4 tetrahedra form planar sechser rings,31 which are aligned to an infinite network along the bc plane (Figure 2). Two sechser rings each enclose one InO6 octahedron forming a BO4−InO6−BO4 sandwich layer of infinite length in the bc plane. The unusually elongated unit cell of InB6O9(OH)3 contains a series of four differently shifted sandwich layers as shown in Figure 2. Referring to the unit cell, each of the layers is offset by 1/4 1/4 1/4 relative to the previous one. The BO4 tetrahedra centered on the third boron position B3 interconnect these BO4−InO6−BO4 sandwich layers forming mixed dreier and sechser rings. The B3O4 tetrahedra themselves form infinite chains via corner sharing, which are, like the sandwich layers, staggered in four different ways (see Figure 2). Examined along a, every other chain runs in the same direction of either [01̅1̅] or [011̅] but at a different height. All B−O bond lengths are within the range of 1.457(7)−1.551(5) Å (average 1.489 Å) and fit to reported distances in tetrahedral borate groups.32 All three crystallographically different BO4 tetrahedra show perfect average tetrahedral angles of 109.5°. A complete list of all relevant bond lengths and angles can be found in the Supporting Information (Tables S4 and S5).

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RESULTS AND DISCUSSION X-ray Powder Diffraction. In Figure 1, a comparison of the experimental powder diffraction pattern and a calculated

Figure 1. Experimental pattern (black), calculated pattern (red), and difference curve (blue) of InB6O9(OH)3. Vertical ticks (green) indicate the position of the reflections.

pattern for InB6O9(OH)3 obtained from a Rietveld refinement27 with the program TOPAS 4.228 is shown. Clearly, the reaction product consists mainly of InB6O9(OH)3. The broad diffuse strong scattering observed in the diffractogram at low 2θ angles presumably originates from amorphous material such as most likely B2O3. The final refinement resulted in values of 4.46% for Rexp and 6.20% for Rwp, and the lattice parameters obtained from the powder diffraction data are in good

Figure 2. Unit cell of InB6O9(OH)3 and its different structural units. Alternately arranged B1O4 and B2O4 tetrahedra form infinite ring layers (blue) centering InO6 octahedra (yellow). The B3O4 tetrahedra (turquoise) form chains of different heights and orientations, which cross-link those layers. For reasons of clarity, the hydrogen atoms were not plotted. C

DOI: 10.1021/acs.inorgchem.8b00518 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Vibrational Spectroscopy. Figure 4 shows the singlecrystal IR spectrum of InB6O9(OH)3 in the range of 600−4000

One hydrogen position could be refined in InB6O9(OH)3. This proton is located near O3 and therefore is labeled H3. The hydrogen bonds between O3−H3 and O4 or O6, respectively, connect three corners of the sechser rings that consist of three crystallographically different boron sites (see Figure 3). The

Figure 4. IR spectrum of an InB6O9(OH)3 single-crystal.

cm−1. Because of experimental limitations, a characterization below 600 cm−1 was not possible. At the lowest wavenumbers up to ∼800 cm−1, bending and stretching vibrations of InO6 octahedra occur.15,37 As is generally known, BO4 vibrations lead to bands in the region of 800−1150 cm−1.38−40 However, our experimental IR spectrum also shows additional absorptions between 1300 and 1500 cm−1, which typically originate from BO3 group vibrations. As there are no BO3 groups in InB6O9(OH)3, there must be another explanation for these bands. Schmitt et al. also found this specific feature in the IR spectrum of their recently published nickel borate Ni3B18O28 (OH)4·H2O,41 which, like InB6O9(OH)3, solely consists of BO4 tetrahedra. Kaindl et al.42 described such high wavenumber modes, which they also found in β-ZnB4O7, corresponding to several bending and stretching vibrations of M−B−O (M = metal ion), O−B−O, B−O−B, and B−O units referring to their complex structure consisting of two crystallographically independent BO4 tetrahedra and OB3 groups. Thus, we believe that the bands at ∼1300−1500 cm−1 in InB6O9(OH)3 could result from complex IR active vibrations of the three crystallographically different boron positions (CN = 4) that form diverse entities like, for example, rings of different sizes. The peaks at ∼2800 and 3100 cm−1 result from the different types of hydrogen bonds, which could only partly be located. In general, one can say that stronger (and therefore shorter) hydrogen bonds appear at lower wavenumbers.43 The single-crystal Raman spectrum of InB6O9(OH)3, measured in the range of 100−4000 cm−1, can be seen in Figure 5. Density functional theory (DFT) calculations for Raman bands in In3B5O12 show that the peaks at the lowest wavenumbers can be attributed to bending and stretching vibrations of In−O bonds.44 According to DFT calculations for Ga2B3O7(OH), bands in the region of 800−1150 cm−1

Figure 3. Hydrogen bonds in InB6O9(OH)3. All non-hydrogen atoms are drawn with displacement ellipsoids (80% probability). The pink atom labels mark oxygen atoms that form hydrogen bonds.

corresponding angles and bond lengths of the hydrogen bonds can be found in Table S6. At this point, it should be mentioned that O−H bond lengths are generally underestimated in X-ray diffraction experiments that actually identify the electron density distribution of the bond and not the actual H atom position. The refinement of H3 led to two protons in the sum formula InB6O9(OH)3; the third proton was postulated based on charge neutrality reasons. As visible in Figure 3, there would still be space for another hydrogen bond in the sechser rings that could be formed by either O4 or O6 as donor atom. To evaluate the charge distribution of all and especially these two atoms, we performed bond-length/bond-strength33,34 and CHARDI (= charge distribution)35,36 calculations. As expected, both values for O4 (ΣV = −1.59, ΣQ = −1.66) and O6 (ΣV = −1.57, ΣQ = −1.63), respectively, show a missing charge contribution indicating an absent bonding partner like the nonlocalized hydrogen atom. The bond-length/bond-strength calculations led to reasonable values for all other atoms in InB6O9(OH)3. The CHARDI values for the cations are slightly higher than one would expect due to the missing proton, which is known to distort the calculation. The results of both concepts are given in Table 2. Additional crystallographic information is available in the Supporting Information.

Table 2. Charge Distribution in InB6O9(OH)3, Calculated with the Bond-Length/Bond-Strength (ΣV) and the CHARDI (ΣQ) Concept ΣV ΣQ

In1

B1

B2

B3

O1

O2

O3

O4

O5

O6

H3

2.85 3.03

2.97 3.20

2.98 3.18

3.00 3.22

−1.93 −1.97

−1.97 −2.02

−2.07 −2.27

−1.59 −1.66

−1.94 −1.96

−1.57 −1.63

1.07 0.88

D

DOI: 10.1021/acs.inorgchem.8b00518 Inorg. Chem. XXXX, XXX, XXX−XXX

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between. IR and Raman spectra confirmed the presence of hydroxyl groups in the title compound. SHG measurements of InB6O9(OH)3 yielded signals more than twice as intense as those from a reference quartz sample. Our experiments indicated that studies of high-pressure indium borates will likely produce further interesting compounds yet to be discovered.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00518. Further information on InB6O9(OH)3, alternative syntheses parameters, Wyckoff positions, atomic coordinates, isotropic and anisotropic displacement parameters, interatomic angles and distances, and details of the hydrogen bonds (PDF) Further details of the crystal structure investigation may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49−7247−808− 666; e-mail: crysdata@fiz-karlsruhe.de) on quoting the deposition number CSD−434138 for InB6O9(OH)3.

Figure 5. Raman spectrum of a randomly oriented InB6O9(OH)3 single-crystal.

presumably result from BO4 stretching vibrations.45 Similar to the IR spectrum, the highest wavenumber modes can be assigned to different types of O−H vibrations. According to group theory, the irreducible representations of InB6O9(OH)3 with the space group Fdd2 at the Γ-point are 31A1 + 31A2 + 32B1 + 32B2. The three acoustic modes have the irreducible representations A1 + B1 + B2. After subtraction of the three acoustic phonons, the irreducible representations of the optical vibrations are Γoptic = 30A1 + 31A2 + 31B1 + 31B2. While 30A1 + 31B1 + 31B2 modes are Raman and infrared active, 31A2 modes are only Raman active. The hydrogen atom, which could not be located, was neglected for these calculations. SHG measurements. The SHG property of an InB6O9(OH)3 powder sample was investigated with the powder SHG method developed by Kurtz and Perry.46 This method is commonly used as a first step to estimate the nonlinear optical properties of new materials. As InB6O9(OH)3 crystallizes in the acentric space group Fdd2, it could show an SHG signal, which is basically only allowed in systems without inversion symmetry.46 The SHG intensity of this indium borate could be measured and yielded 941(265) counts, which is ∼2.2 times higher than the corresponding SHG intensity of quartz (420(54) counts). This strong SHG signal strongly indicates that the sample crystallizes in a noncentrosymmetric space group. The point group mm2 has five independent SHG coefficients, namely, d31, d15, d32, d24, and d33.47 If the Kleinman symmetry relations48 are valid, d31 = d15 and d24 = d32, because all three subscripts of the SHG tensor can be permutated by neglecting dispersion. The effective SHG coefficient derived from the independent SHG coefficients contributes to the SHG intensity of the powder sample.46 The possibility of phasematching or not phase-matching was not determined in this work.

Accession Codes

CCDC 1827080 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +43 (512) 50757099. ORCID

Hubert Huppertz: 0000-0002-2098-6087 Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. K. Wurst for collecting the single-crystal data and giving us advice during the refinement process. Furthermore, we thank Dr. B. Joachim for the Raman measurements and Dr. R. Stalder for giving us access to the single-crystal IR spectrometer. Special thanks goes to Dr. M. Schmitt for the help with the Rietveld refinement.

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REFERENCES

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CONCLUSIONS This work describes the synthesis and characterization of the first acentric indium borate hydroxide InB6O9(OH)3 that was synthesized under high-pressure conditions. Including the title compound, there now exist three borates in the system In−B− O−H. Orthorhombic InB6O9(OH)3 is mainly built of cornersharing BO4 tetrahedra rings and isolated InO6 octahedra E

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.8b00518 Inorg. Chem. XXXX, XXX, XXX−XXX