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Synthesis and Characterization of the High-Pressure Nickel Borate γ‑NiB4O7 Martin K. Schmitt,† Oliver Janka,$ Oliver Niehaus,$ Thomas Dresselhaus,Ω,Δ Rainer Pöttgen,$ Florian Pielnhofer,# Richard Weihrich,& Maria Krzhizhanovskaya,§ Stanislav Filatov,§ Rimma Bubnova,§,£ Lkhamsuren Bayarjargal,∥ Björn Winkler,∥ Robert Glaum,⊥ and Hubert Huppertz*,† †

Institut für Allgemeine, Anorganische und Theoretische Chemie, Universität Innsbruck, Innrain 80-82, 6020 Innsbruck, Austria Institut für Anorganische und Analytische Chemie, ΩOrganisch-Chemisches Institut, and ΔCenter for Multiscale Theory and Computation, Universität Münster, 48149 Münster, Germany # Institut für Anorganische Chemie, Universität Regensburg, Universitätsstraße 31, 93040 Regensburg, Germany & Institut für Materials Resource Management, Universität Augsburg, Universitätsstraße 1, 86135 Augsburg, Germany § Department of Crystallography, St. Petersburg State University, University Embankment 7/9, 199034 St. Petersburg, Russia £ Grebenshchkov Institute of Silicate Chemistry, Russian Academy of Sciences, Makarov Embankment 2, 199034 St. Petersburg, Russia ∥ Institut für Geowissenschaften, Universität Frankfurt, Altenhöferallee 1, 60438 Frankfurt/Main, Germany ⊥ Institut für Anorganische Chemie, Universität Bonn, Gerhard-Domagk-Straße 1, 53121 Bonn, Germany $

S Supporting Information *

ABSTRACT: γ-NiB4O7 was synthesized in a high-pressure/high-temperature experiment at 5 GPa and 900 °C. The single-crystal structure analysis yielded the following results: space group P6522 (No. 179), a = 425.6(2), c = 3490.5(2) pm, V = 0.5475(2) nm3, Z = 6, and Flack parameter x = −0.010(5). Second harmonic generation measurements confirmed the acentric crystal structure. Furthermore, γ-NiB4O7 was characterized via vibrational as well as single-crystal electronic absorption spectroscopy, magnetic measurements, high-temperature X-ray diffraction, differential scanning calorimetry, and thermogravimetry. Density functional theory-based calculations were performed to facilitate band assignments to vibrational modes and to evaluate the elastic properties and phase stability of γ-NiB4O7.

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INTRODUCTION

HP-NiB2O4), respectively. In view of the fact that the 3d transition metals Fe, Co, and Ni often form isotypic compounds but only two crystal structures are known to incorporate all of these elements (β/HP-MB2O4 and β-MB4O7 (M = Fe, Co, Ni), this motivated us to further investigate the system Ni−B−O. Within the ternary system Ni−B−O, four compounds are known: two phases that require high pressure for their synthesis, namely, HP-NiB2O4 and β-NiB4O7, and two compounds that can be obtained at ambient pressure, Ni3(BO3)212−15 and Ni2B2O5.16,17 No detailed crystallographic data have been published for the latter compound. It was only described regarding the composition and the peak positions of the X-ray powder diffractogram. In the following, we report about the synthesis and crystal structure of a fifth anhydrous nickel borate, γ-NiB4O7 (the

During the past decade, considerable effort was made to explore transition-metal borates at high-pressure/high-temperature conditions. These investigations led to the discovery of numerous compounds with new compositions or interesting structural motifs, for example, Co7B24O42(OH)2·2H2O,1 M6B22O39·H2O (M = Fe, Co, Cd),2,3 HP-MB2O4 (M = Co, Ni),4,5 and β-FeB2O4.6 The latter three compounds were the first discoveries of borates in which every BO4 tetrahedron shares one common edge with a second BO4 tetrahedron. Furthermore, Sohr et al. recently synthesized Cd(NH3)2[B3O5(NH3)]2,7 the first compound in the new substance class of ammine borates. When comparing the systems Fe−B−O(−H), Co−B− O(−H), and Ni−B−O(−H), it is remarkable that the number of known high-pressure borates in each system decreases from five (Fe3B7O13OH·1.5H2O,8 β-FeB4O7,9 and α-FeB2O410 in addition to those mentioned above) to four (β-CoB4O79 in addition to those mentioned above) to two (β-NiB4O711 and © XXXX American Chemical Society

Received: January 27, 2017

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DOI: 10.1021/acs.inorgchem.7b00243 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. (top) Experimental powder diffraction pattern of the reaction product. (bottom) Calculated powder diffraction pattern of γ-NiB4O7 based on single-crystal diffraction data. Reflections marked with an asterisk are either affected by or originate from metallic nickel and at least one unknown compound. intensity maximum at low 2θ angles in the powder diffractogram (Figure 1).

nomenclature will be discussed later). The compound was characterized via magnetic measurements, high-temperature Xray diffraction (HTXRD), differential scanning calorimetry (DSC), thermogravimetry (TG), second harmonic generation measurements, and vibrational as well as electronic absorption spectroscopy. Furthermore, density functional theory (DFT) calculations were performed to assign the experimentally obtained infrared (IR) and Raman bands to atomic vibrations and to evaluate the elastic properties and phase stability of γNiB4O7.

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5 GPa,900 ° C

NiO + 2B2O3 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ γ ‐NiB4 O7

(1)

To grow larger crystals, B2O3 was partially substituted by H3BO3 (Carl Roth GmbH + Co. KG, Karlsruhe, Germany, ≥99.8%). The largest crystals (up to ∼0.2 mm edge length) could be synthesized from a 2:3:2 molar mixture of NiO/B2O3/H3BO3 at similar reaction conditions as described above, except for the temperature being maintained at 900 °C for 420 min and lowered to 550 °C in 120 min. The compression and decompression processes lasted for 150 and 540 min, respectively. Apparently, the water that gets released by the boric acid upon heating affects the crystal growth positively. However, the crucible increasingly reacts with the starting mixture with growing water content. A complete substitution of B2O3 with H3BO3 resulted in the formation of metallic nickel and the ammonium borate HP(NH4)B3O521 instead of γ-NiB4O7. Additional high-pressure experiments were performed to estimate the location of the phase boundary between γ-NiB4O7 and β-NiB4O7. An experiment at 5 GPa/1150 °C yielded a reaction product consisting almost exclusively of γ-NiB4O7. Only some minor reflections could not be assigned to any compound listed in the ICDD database. Another experiment at 7.5 GPa/900 °C led to the formation of β-NiB4O7 and, as a side product, HP-NiB2O4. These findings will be discussed below. Single-Crystal Structure Analysis. The single-crystal intensity data were collected at ambient temperature using a Bruker D8 Quest Kappa diffractometer (Bruker, Billerica, USA) equipped with a Photon 100 detector. An Incoatec microfocus X-ray tube (Incoatec, Geesthacht, Germany) operated at 50 kV/1 mA power settings and a multilayer optic were used to generate monochromatized Mo Kα radiation (λ = 71.07 pm). To allow for a correct determination of the absolute structure, a complete data set including all Friedel pairs was collected and corrected for absorption effects employing SADABS.22 The structure was solved in the hexagonal, acentric, chiral space group P6522 (No. 179) with SHELXS23 (version 2013/1) and subsequently refined with

EXPERIMENTAL SECTION

Synthesis. γ-NiB4O7 was synthesized in a high-pressure/hightemperature reaction starting from a 1:2 molar mixture of NiO (Strem Chemicals, Newburyport, USA, 99.9+ %) and B2O3 (Strem Chemicals, Newburyport, USA, 99.9+ %; eq 1). Because of the hygroscopic nature of B2O3, the mixture was weighed and ground in an agate mortar under argon atmosphere using a glovebox. Afterward, the starting mixture was filled into a crucible, closed with a lid (both made of αBN; Henze Boron Nitride Products AG, Lauben, Germany), placed in an 18/11 assembly, and compressed by eight tungsten carbide cubes (Hawedia, Marklkofen, Germany) using a Walker-type module (Max Voggenreiter GmbH, Mainleus, Germany) and a hydraulic 1000 t press (also Max Voggenreiter GmbH). Details of the assembly and its preparation are described elsewhere.18−20 The reactant mixture was compressed to 5 GPa within 125 min and subsequently heated to 900 °C within 10 min. To allow sufficient time for the reaction to proceed to completion, the temperature was maintained for 120 min and then lowered to 550 °C in 10 min before the heating was switched off and the 14 h process of decompression started. A nearly phase-pure, microcrystalline, greenish product was obtained, which was covered by a thin layer of metallic nickel owing to the reductive nature of the BN crucible. This implies, however, that some B2O3 could not react according to eq 1. The excess B2O3 is presumably present as an amorphous phase accounting for the broad B


Article

Inorganic Chemistry SHELXL24 (version 2014/7) as implemented in the WINGX25 (version 2013/3) suite. In the refinement process (full-matrix leastsquares against F2), all atoms could be refined anisotropically, leading to values of 0.0183 and 0.0471 for R1 and wR2 (all data), respectively. 207 intensity quotients were used to determine the Flack parameter26 (x = −0.010(5)) according to Parsons et al.27 The small standard uncertainty (u = 0.005) of the Flack parameter implies a strong inversion-distinguishing power of the collected data.28,29 The value of x = −0.010(5) indicates that space group P6522 (No. 179) is the correct choice out of the pair of enantiomorphic space groups P6122 (No. 178) and P6522 (No. 179). However, this finding is valid only for the measured crystal and allows no inference regarding the enantiopurity of the bulk compound. The structure data were standardized employing STRUCTURE TIDY30 as implemented in PLATON31 (version 201115). Of a pair of enantiomorphic space groups, STRUCTURE TIDY chooses the space group with the smallest index for the relevant screw axis, which would be P6122 (No. 178) in this case, and adapts the atomic coordinates correspondingly.32 However, since the refinement process clearly yielded P6522 (No. 179) as the correct space group of the measured crystal, the signs of the standardized atomic coordinates had to be inverted to allow for a correct refinement of the structure in P6522. The positional parameters, anisotropic displacement parameters, selected interatomic distances, and bond angles are listed in Tables S1−S4, respectively (see Supporting Information). X-ray Powder Diffraction. The reaction product was mechanically separated from the thin layer of metallic nickel as good as possible. A flat powder sample (thickness ≈ 130 μm) was analyzed by means of a Stoe Stadi P powder diffractometer (Stoe & Cie GmbH, Darmstadt, Germany) in transmission geometry. The measurement was performed with Ge(111)-monochromatized Mo Kα1 radiation (λ = 70.93 pm) in the 2θ range of 2−60.76° with a step size of 0.01°. Vibrational Spectroscopy. A Bruker Alpha-P spectrometer (Bruker, Billerica, MA) was used to obtain the Fourier transform infrared attenuated total reflection (FTIR-ATR) spectrum of a powder sample. The spectrometer was equipped with a 2 × 2 mm diamond ATR crystal and a deuterated triglycine sulfate (DTGS) detector. 320 scans of the sample were recorded in the spectral range of 400−4000 cm−1 (spectral resolution 4 cm−1) and corrected for atmospheric influences employing OPUS 7.2.33 A Raman spectrum of an arbitrarily oriented single crystal was acquired with a Horiba Jobin Yvon Labram HR 800 spectrometer (Horiba, Tulln, Austria) using the 532 nm emission line of a 100 mW frequency-doubled Nd:YAG laser for excitation. The spectrum was recorded in the range of 100−4000 cm−1 through a 50× objective. The scattered light was dispersed by means of an optical grating with 1800 lines mm−1 and detected by an open-electrode CCD detector with 1024 × 256 pixels. Density Functional Theory Calculations. Quantum chemical calculations were performed in the framework of density functional theory (DFT) using a linear combination of Gaussian-type functions (LCGTF) scheme as implemented in CRYSTAL14.34,35 The total energy calculations including full structural optimizations were performed with the GGA (PBE)36 and LDA (VWN) xc-functional.37 Because of a better reproduction of experimental lattice constants, the van der Waals correction developed by Grimme was applied to PBE calculations.38 The convergence criterion considering the energy was set to 1 × 10−7 a.u. with a k-mesh sampling of 6 × 6 × 2. A proper description of the atoms was given by optimized all-electron basis sets.39,40 The relative stabilities of β-NiB4O7 and γ-NiB4O7 were evaluated with equation of state calculations using a third-order Birch− Murnaghan fit. Vibrational frequencies calculations with IR intensities were run on fully optimized structural models so that no imaginary frequency was obtained. The GGA (PBE) functional leads to the best results, due to well-reproduced atomic distances. The IR spectrum was simulated with the J-ICE application,41 and the plot was fitted to Gaussian functions. For the Raman spectrum, only the band positions were calculated.

Additional DFT-based calculations were performed to study the relative stabilities and the compressibility of β-NiB4O7 and γ-NiB4O7 using a plane-wave basis set in conjunction with pseudopotentials. For these calculations, the CASTEP package42 was employed. A generalized gradient approximation in the Wu-Cohen formulation43 and the “on the fly” pseudopotentials from the CASTEP database were employed throughout. Population analysis in CASTEP is performed using a projection of the plane-wave states onto a localized basis using a technique described by Sanchez-Portal et al.44 The population analysis of the resulting projected states is then performed using the Mulliken formalism.45,46 The kinetic cutoff energy for the spinpolarized calculations was 610 eV. A Hubbard U of 2.5 eV was employed for the Ni d electrons. Distances between k-points for Brillouin zone sampling were less than 0.00029 pm−1. Full geometry calculations were performed. Equations of state were fitted using a third-order Birch−Murnaghan equation of state using the program EOSFIT.47 Magnetic Properties. γ-NiB4O7 was used as a polycrystalline powder, packed in polyethylene (PE) capsules and attached to the sample holder rod of a Vibrating Sample Magnetometer (VSM) unit for measuring the magnetization M(T, H) in a Quantum Design Physical Property Measurement System (PPMS). In total, three batches of the same compound were investigated in the temperature range of 3−350 K with a magnetic flux density of 10 kOe. High-Temperature Powder Diffraction and Differential Scanning Calorimetry. The thermal behavior of γ-NiB4O7 upon heating in air was studied by high-temperature X-ray diffraction using a Rigaku Ultima IV (Cu Kα radiation, 40 kV/30 mA, Bragg−Brentano geometry, PSD Detector D-Tex Ultra) with a high-temperature camera in the temperature range of 25−1000 °C with steps of 20 °C. A powder sample (∼10 mg) was fixed on a Pt sample holder (20 × 12 × 1.5 mm) from a heptane suspension. The ambient pressure phase Ni3(BO3)2,13 obtained during the HTXRD experiment as a decomposition product, was cooled and then also investigated applying the same temperature program. Calculations of the unit cell parameters at different temperatures and eigenvalues of the thermal expansion tensors were performed using the TTT program package, the algorithm of which is shortly described in Bubnova et al.48 The high-temperature behavior of γ-NiB4O7 was additionally studied by differential scanning calorimetry and thermogravimetry on a Netzsch STA 449 F3 calorimeter with a heating rate of 5 K/min. A powder sample (∼10 mg) was placed in a platinum crucible for the investigations. Calcinated corundum served as a reference sample. Second Harmonic Generation Measurements. The second harmonic generation (SHG) signal is only allowed in acentric media.49 A detailed description of the experimental setup for the powder SHG measurements is given by Bayarjargal et al.50 A Q-switched Nd:YLF laser system (Falcon 217D, Quantronix), operating at 1054 nm and a pulse width of 130 ns, provided the fundamental wave. The sample was placed on an adhesive tape, which did not generate an SHG signal on its own. The generated SHG signal (527 nm) was collected at different positions on the sample at ambient conditions. At each position, 15 measurements were performed, and the intensities of five different positions were averaged. Quartz (α-SiO2) and Al2O3 powders were used as reference materials. Electronic Absorption Spectra. Single-crystal electronic spectra of arbitrary faces of γ-NiB4O7 crystals were measured at ambient temperature using a strongly modified CARY 17 microcrystal spectral photometer (Spectra Services, ANU Canberra, Australia).51,52 The spectrometer allows the measurement of polarized spectra of very small single crystals with diameters down to 0.1 mm. Details on the spectrometer have already been described in literature.44,45

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RESULTS AND DISCUSSION Crystal Structure. γ-NiB4O7 crystallizes in the hexagonal, acentric, chiral space group P6522 (No. 179) with the lattice parameters a = 425.6(2), c = 3490.5(2) pm, and V = 0.5475(2) nm3. The asymmetric unit comprises four oxygen, two boron, C


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Inorganic Chemistry and one nickel position. All atoms are located at general positions (12c) except for O4 and Ni1, which lie at special positions (6b). Hence, the unit cell contains 72 atoms corresponding to Z = 6 formula units. Relevant details of the structure refinement are listed in Table 1. Table 1. Crystal Dataa and Structure Refinement of γNiB4O7 empirical formula molar mass, g mol−1 crystal system space group powder data powder diffractometer radiation a, pm c, pm V, nm3 single-crystal data single-crystal diffractometer radiation a, pm c, pm V, nm3 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 independent reflections with I ≥ 2σ(I) completeness, % data/ref. parameters Rint Rσ absorption correction final R1/wR2 (I ≥ 2σ(I)) final R1/wR2 (all data) goodness-of-fit on Fi2 Flack parameter x largest diff. peak/hole, e Å−3 a

NiB4O7 213.95 hexagonal P6522 (No. 179) Stoe Stadi P Mo Kα1 (λ = 70.93 pm) 425.79(2) 3489.8(2) 0.54792(4)

Figure 2. Corrugated chains built of corner-linked B1-tetrahedra (blue). These chains are connected along c via B2O7 groups composed of two B2-tetrahedra (turquoise). Parallel running chains (not shown) are linked through B2-tetrahedra via the atoms O1, O2, and O3. The tricoordinated oxygen atoms O1 are shown in yellow. All atoms drawn with 90% displacement ellipsoids.

Bruker D8 Quest Kappa Mo Kα (λ = 71.07 pm) 425.6(2) 3490.5(2) 0.5475(2) 6 3.89 0.03 × 0.05 × 0.09 293(2) 55 0.4°/frame, 16 s/frame 5.3 624 3.5−35.0 ±6; ±6; ±56 27 860/801 790 100 801/56 0.0317 0.0080 multiscan 0.0180/0.0470 0.0183/0.0471 1.219 −0.010(5) 0.35/−0.78

Figure 3. View along [001]. The nickel atoms (green) are helically arranged around the 65-screw axis running along the junction of the four unit cells. The corner-linked chains of BO4 tetrahedra are shifted by c/6 and rotated counterclockwise by 60° relative to the preceding chain. For the sake of clarity, only every second chain is shown. Boron: red; twofold coordinated oxygen: blue; threefold coordinated oxygen: yellow. All atoms drawn with 90% displacement ellipsoids.

Standard deviations in parentheses.

The anionic framework of the compound is built of two crystallographically distinguishable BO4 tetrahedra with either B1 (B1-tetrahedra; ligands: 2 × O1, O2, O3) or B2 (B2tetrahedra; ligands: O1, O2, O3, O4) as central atom. The bond lengths within these tetrahedra vary rather widely between 142.6(3) pm and 154.9(4) pm. This is due to O1 being coordinated by three boron atoms, which will be discussed below. However, despite this distortion, the bond lengths lie within the common range for B−O distances of 137.3−169.9 pm.53 The distortion is also reflected in the bond angles (Table S4) varying between 101.8(2) and 115.3(2)° (literature:53 95.72−119.43°). Each B1-tetrahedron is connected via two common corners (2 × O1) with two neighboring B1-tetrahedra. Thus,

corrugated chains running perpendicular to c are formed (Figure 2). Because of the 65-screw axis running parallel to c, each chain is shifted by c/6 and rotated counterclockwise by 60° with regard to the preceding chain (Figure 3). B2O7 groups, composed of two B2-tetrahedra sharing a common oxygen atom (O4), interconnect these chains along c (Figure 2 and Figure 4). The linking oxygen atoms are O1 and O2. O1 is therefore coordinated by three boron atoms (2 × B1 and B2), which is reflected in significantly larger bond lengths (B−O1 ≥ 152.0(3) pm compared to B−O2,3,4 ≤ 149.1(2) pm; see also Table S3). In addition to the connection along c, the B2tetrahedra also provide cross-linkage between parallel chains via D


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Inorganic Chemistry

Figure 4. Two unit cells viewed along [01̅0]. The crystal structure of γ-NiB4O7 is built of corrugated chains of BO4 tetrahedra (blue), which are interconnected through B2O7 groups (turquoise). The octahedrally coordinated nickel atoms (green) are arranged in the form of left-handed helices parallel to [001]. Boron: red; twofold coordinated oxygen: blue; threefold coordinated oxygen: yellow. All atoms drawn with 90% displacement ellipsoids. The thermal expansion behavior of γ-NiB4O7 is represented by the large green ellipsoid.

nomenclature) between the title compound and the transition-metal borates α-MB4O7 (M = Mn, Co, Zn, Cd, Hg)59−63 and β-MB4O7 (M = Mn, Fe, Co, Ni, Cu, Zn).9,11,64 X-ray Powder Diffraction. Figure 1 shows a comparison of the measured powder pattern and the theoretical powder pattern calculated from the single-crystal data. As can be seen clearly, the reaction product consists primarily of γ-NiB4O7 with minor impurities of metallic nickel and at least one unknown compound. The amorphous halo at low 2θ angles originates presumably from excess B2O3 (see above). The lattice parameters obtained from the powder pattern via the Pawley method65 using the program TOPAS66 deviate only slightly from the corresponding values derived from the singlecrystal experiment (Table 1). Vibrational Spectroscopy. The results of the structural optimizations performed with CRYSTAL14 are given in Table S7. A comparison of measured and calculated vibrational frequencies is shown in Figure 5. At 1158 cm−1, a doubly degenerate B−O vibration consists of symmetric and asymmetric stretching contributions in c-direction, whereas at 1095 cm−1 the deviation of B and O atoms predominantly occurs in the a−b plane. Breathing-modes of six-membered rings (B2−O2−B2−O1−B2−O1) can be assigned to a doubly degenerate vibration at 1056 cm−1, which also contains B−O stretching in a−b and along the cell diagonal. The mode at 843 cm−1 is singly degenerate and shows mainly asymmetric B−O stretching along c. Highest intensities below 800 cm−1 (763 and 744 cm−1) are assigned to stretching in a−b and smaller contributions of breathing modes of the above-mentioned sixmembered rings. Additionally, bending contributions increase below 800 cm−1. Ni−O vibrations are present below 378 cm−1. The calculated Raman active modes fit very well to the measured data (Figure 5, bottom). The section between 1218 and 893 cm−1 is dominated by B−O stretching modes. Similar to IR, the amount of bending modes increases below 800 cm−1, and Ni−O vibrations occur below 350 cm−1. The origin of a small peak at ∼3200 cm−1 (not displayed in Figure 5) could not be determined. Usually, bands in this wavenumber region can be attributed to water. However, neither the structure refinement, the bond-valence sums (s. Table S5), nor crystal chemical considerations point toward the presence of water in the crystal structure, which we therefore rule out.

the atoms O1, O2, and O3. This leads to a highly condensed anionic framework built solely of Q4 tetrahedra.54 Looking at the structure along [001], the anionic framework reveals narrow channels. These channels contain the nickel atoms arranged in the form of a left-handed helix surrounding the 65-screw axis (Figure 3). The nickel atoms are coordinated octahedrally (2 × O2, 2 × O3, 2 × O4) with bond lengths between 201.4(2) and 217.2(2) pm (Table S3). These values are similar to the corresponding bond lengths for octahedrally coordinated nickel in other borates, for example, Ni3(BO3)212−15 (202.3(8)−212.5(8) pm) or HP-NiB2O45 (203.4(1)−217.7(1) pm). Additional crystallographic information is available in the Supporting Information. As an additional check on the reliability of the determined structure, the bond valences were calculated according to both the bond-length/bond-strength (∑V)55,56 and the CHARDI (∑Q)57 concept (Table S5). The obtained values fit well to the expected formal ionic charges, with the largest discrepancy at the O1 atom (∑Q = −1.73), which is due to the uncommon coordination by three boron atoms. To the best of our knowledge, there is no compound with an atomic structure similar to that of γ-NiB4O7, which therefore constitutes its own structure type. Even when comparing γNiB4O7 with its high-pressure modification58 β-NiB4O7, one will hardly find any structural similarities apart from the existence of tricoordinated oxygen atoms and the fact that both structures are exclusively built from Q4 tetrahedra. As expected for a low-pressure modification, the calculated density of γNiB4O7 (3.89 g cm−3) is slightly lower than that of β-NiB4O7 (3.96 g cm−3). The assignment of a suitable designation of γ-NiB4O7 posed a problem. The two NiB4O7 polymorphs were synthesized at 7.5 GPa/1150 °C (β-NiB4O7) and 5 GPa/900 °C (γ-NiB4O7), respectively. Hence, γ-NiB4O7 constitutes the low-pressure modification of β-NiB4O7. At first sight, this nomenclature may seem counterintuitive, as consecutive Greek letters are frequently used to denote polymorphs with increasing pressure. This is, however, an intrinsic drawback of this nomenclature, since low-pressure polymorphs might be discovered later than the corresponding high-pressure polymorphs, as is the case here. We still decided to use the prefix “γ” to indicate the structural differences (without using an entirely different E


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Inorganic Chemistry

Figure 5. (top) Calculated (red) and measured (black) IR spectrum of a powder sample of γ-NiB4O7. (bottom) Experimental single-crystal Raman spectrum of γ-NiB4O7 (black). Calculated band positions are displayed as red lines.

Figure 6. Magnetic properties of γ-NiB4O7: (top) temperature dependence of the magnetic susceptibility χ and its inverse χ−1 measured with a magnetic field strength of 10 kOe; (bottom) temperature dependence of the magnetic susceptibility χ measured in ZFC/FC mode with magnetic field strengths of 500 and 1000 Oe.

Magnetic Properties. The temperature dependence of the magnetic susceptibility χ of γ-NiB4O7 was measured at 10 kOe showing an anti-ferromagnetic-like behavior with a broad maximum centered around 60 K (Figure 6, top). Zero-fieldcooled/field-cooled (ZFC-FC) measurements performed at 500 and 1000 Oe confirm the anti-ferromagnetic ground state of the material along with the broad maximum observed in the high-field measurements (Figure 6, bottom). As described in the Crystal Structure Section, an octahedral coordination environment of the Ni atoms is found; hence, d-electron magnetism is expected. A closer look reveals that the [NiO6] octahedra form slightly undulating chains by condensation over the trans-standing corners (Figure 7). Linking of the strands, however, is realized only by the borate fragments. Therefore, no Ni−O−Ni contacts other than the ones within the chain are found. Superexchange causes the system to order antiferromagnetically but only within one dimension. Consequently, γ-NiB4O7 is considered to be a so-called onedimensional (1D) Heisenberg anti-ferromagnet.67 Within the field of coordination polymers and transition-metal coordination chemistry, several examples of these magnetically 1D systems containing octahedrally coordinated high-spin Ni2+ ions (S = 1) are known. The susceptibility data of these compounds can be fitted using an analytical expression introduced by Kahn et al.,68 which was modified by us with the χ0 term to account for diamagnetic impurities, for example, B2O3 or boron nitride (BN) from the crucible:

Figure 7. 1D Linkage of undulating, trans-corner sharing [NiO6] octahedra.

with

X=

|J | kBT

with N being Avogadro’s constant, μB being the Bohr magneton, μ0 being the conversion between SI and CGS system, gfit being the fitted electron g-factor, kB being the Boltzmann constant, T being the respective temperature, and finally J being the nearest-neighbor exchange coupling constant. The susceptibility data above 25 K of three independently synthesized batches was fitted yielding coupling constants of J ≈ −32 cm−1 and different diamagnetic contributions χ0 ranging from 4.4−9.3 × 10−4 emu mol−1 (Figure 8). By fitting the g value (gfit = 2.11−2.18) to account for small contributions to

⎞ NμB2 μ0 g fit 2 ⎛ 2 + 0.0194X + 0.777X2 χ= ⎜ 2 3⎟ kBT ⎝ 3 + 4.346X + 3.232X + 5.834X ⎠ + χ0 F


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Inorganic Chemistry

mass losses at 280 °C. The corresponding phase could be removed by the drying procedure. Several thermal transformations were observed during the heating of the sample (Figure 10, dotted lines). γ-NiB4O7 decomposes above 700 °C (second line from bottom) with the formation of the ambient pressure phase Ni3(BO3)2. The latter one then slowly decomposes above 900 °C (third line) into NiO, the reflections of which gradually emerge above this temperature. Both steps are for stoichiometric reasons associated with a release of boron, possibly in the form of molten B2O3 in view of the fact that the temperatures greatly exceed the melting point (450 °C) of B2O3.72 As already mentioned, the initial sample contained some amount of metallic Ni. At ∼570 °C, the 2θ values of the Ni peaks increase sharply, as the cell parameter decreases by ∼1 pm (Figure 11b). Note that the full width at half-maximum of these peaks decreases by a factor of 2 in the temperature range of 400−570 °C. This might be related to recrystallization processes of small Ni particles. The thermal behavior of Ni3(BO3)2 obtained from γ-NiB4O7 by 12 h of heating at 670 °C in the HTXRD camera was also studied in an additional HTXRD measurement in the range of 25−1000 °C. It fully decomposes at 890 °C; only the maxima of NiO and the Pt sample holder were registered at 900 °C. The unit cell parameters of γ-NiB4O7, Ni3(BO3)2, and Ni depending on the temperature are shown in Figure 11, and the calculated thermal expansion coefficients of γ-NiB4O7 and Ni3(BO3)2 using the temperature dependences are in Table 2. Compared to alkali borates, both studied Ni borates demonstrate quite low (⟨αV⟩ = 24 × 10−6 K−1) and nearly isotropic thermal expansion.73 For coordination number six, the size of Ni2+ (r = 83 pm) is similar to that of Li+ (r = 90 pm).74 However, the average thermal expansion of Li-borates (⟨αV⟩ = 47 × 10−6 K−1)73 is 2 times greater than that of the Ni borates. This can be explained by the fact that the thermal expansion of borates, like in other compounds, is reduced when the chemical bonds become stronger: in this case due to the increasing valence of the cation from +I (Li) to +II (Ni).73 The ratio αmax/αmin is not more than 1.8 for both compounds over the whole temperature range (Table 2). In the case of Ni3(BO3)2 the anisotropy is slightly greater, especially at ambient temperature. The direction of maximal expansion in this structure is practically perpendicular to the BO3 triangle planes, which is typical for borate structures composed of triangles only (Figure S1).75,76 Second Harmonic Generation Measurements. Semiquantitative SHG measurements were performed on an ungraded powder sample employing the method described by Kurtz and Perry.49 The SHG intensity of γ-NiB4O7 was ∼151(66) counts and thus weaker than that of the quartz sample (360(94) counts). However, the intensity is much stronger than the intensity (4(10) counts) of a centrosymmetric Al2O3 reference sample, which confirms the acentric structure of γ-NiB4O7. Furthermore, we performed SHG measurements in reflection geometry on a pure Ni sample as reference to estimate the influence of the Ni particles on the SHG signal. The SHG intensity of pure Ni was ∼19(3) counts. The SHG intensity is proportional to the concentration. Therefore, the tiny amount of Ni has only a negligibly small influence on the result. The possibility that the sample contained a minor amount of an unknown, acentric phase cannot be ruled out. However, the size of our laser spot is ∼60 μm. Hence we measure the SHG

Figure 8. Temperature dependence of the magnetic susceptibility χ of three batches of γ-NiB4O7, fitted according to the model of a 1D Heisenberg anti-ferromagnet (HAF).

spin−orbit coupling, the ratio gfit/g0 can be determined, which can be used to calculate the effective magnetic moment. For the calculated effective magnetic moment μeff = gfit/g0 × μ(Ni2+) = 2.98−3.08(1) μB is found, which is higher compared to the expected magnetic moment for a spin-only high-spin Ni2+ ion (μ(Ni2+) = 2.83 μB) but in line with the magnetic moment of μ = 3.16 μB that has been predicted by the angular overlap modeling of γ-NiB4O7 (see below). Figure 8 depicts the experimental ZFC data along with the respective fits and the extracted parameters. The coupling constants J are within the range of what has been found for different Ni-containing coordination polymers ([Ni(dmpn)2NO2]PF6·H2O: J = −6 cm−1, Ni(ox) (en)·2H2O: J = −30 cm−1, [Ni(232-tet)2N3]ClO4·H2O: J = −30 cm−1, [Ni(en)2(NO2)]ClO4: J = −33 cm−1, [Ni(323-tet)2N3]ClO4· H2O: J = −63 cm−1, [Ni(333-tet)2N3]ClO4·H2O: J = −81 cm−1).69−71 High-Temperature Powder Diffraction and Differential Scanning Calorimetry. The DSC curve of γ-NiB4O7 clearly demonstrates three endothermic maxima (Figure 9).

Figure 9. TG (upper) and DSC (lower) curve of γ-NiB4O7.

The first and the second, accompanied by some mass losses, can presumably be attributed to the decomposition of water/ OH-containing admixtures. The third effect at ∼700 °C obviously corresponds to the decomposition of γ-NiB4O7. Between 500 and 600 °C, the DSC curve shows a weak kink, which could reflect the unusual thermal behavior of the elemental Ni (see below). The initial sample contained γ-NiB4O7, elemental Ni, and traces of at least one unknown phase, which is presumably hydrous. It was preliminarily dried at 300 °C for 3 h prior to the HTXRD measurement, because the DSC curve showed some G


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Inorganic Chemistry

Figure 10. 3D plot of the HTXRD data. The first line from bottom indicates the sharp increase of the 2θ values of the Ni peaks. γ-NiB4O7 decomposes above 700 °C (second line from bottom) with the formation of the ambient pressure phase Ni3(BO3)2, which in turn is stable up to ∼900 °C. Above this temperature, a slow decomposition into NiO can be observed.

Table 2. Thermal Expansion Coefficients α (×106 K−1) of γNiB4O7 and Ni3(BO3)2 formula

T, °C

α11 = αa

α22 = αb

α33 = αc

αV

αmax/αmin

γ-NiB4O7

25 200 400 600 25 200 400 600 800

5.1(1) 6.1(8) 7.2(5) 8.3(1) 11.7(2) 12.4(1) 13.1(1) 13.7(1) 14.3(2)

5.1(1) 6.1(8) 7.2(5) 8.3(1) 6.6(2) 7.6(1) 8.6(1) 9.7(1) 10.7(2)

4.2(2) 4.6(1) 5.1(7) 5.6(2) 7.1(3) 8.1(2) 9.1(8) 9.9(1) 10.9(2)

14 17 19 22 25 28 30 33 36

1.2 1.3 1.4 1.5 1.8 1.6 1.5 1.4 1.3

Ni3(BO3)2

we believe it is very unlikely that the SHG signal is due to an impurity. The point group 622 has only one nonzero-independent SHG coefficient (d14). The intensity ratio between γ-NiB4O7 and quartz (d11 = 0.30 pm/V) allows us to estimate the SHG coefficient d14 ≈ 0.13 pm/V for γ-NiB4O7. According to the Kleinman symmetry conditions,77 the SHG coefficient should vanish for crystals belonging to point group 622. Since γNiB4O7 clearly shows second harmonic generation, the Kleinman conditions do obviously not apply for this compound. Similar cases were described by Singh et al., Chemla and Jerphagnon, and Ohkoshi et al., who reported on the observation of SHG in paratellurite (TeO2) and an ironoctacyanoniobate, respectively.78−80 These compounds crystallize in the tetragonal point group 422 and should therefore not exhibit SHG according to the Kleinman symmetry conditions. Electronic Absorption Spectra. Crystals of γ-NiB4O7 are green. Absorption bands at ν̃1 = 7100 cm−1, ν̃2a = 11 160 cm−1, ν̃2b = 14 770 cm−1, and ν̃3 = 24 610 cm−1 are found (Figure 12). The band around 24 610 cm−1 shows a shoulder at 25 200 cm−1, but it is still rather narrow. At ∼20 350 and 22 310 cm−1, weak bands, possibly originating from spin-forbidden electronic transitions, are observed. On the basis of the barycenters of bands ν̃2 and ν̃3 at 13 400 and 24 600 cm−1, an evaluation according to Tanabe and Sugano81 leads for the chromophore [NiIIO6] to Δo = 8060 cm−1 and B = 926 cm−1 (β = B/Bfree ion = 0.89). Angular Overlap Modeling of γ-NiB4O7. For the chromophore [NiIIO6] strong radial and angular distortions

Figure 11. Thermal evolution of the unit cell parameters obtained from the HTXRD data: (a) γ-NiB4O7, (b) Ni admixture, (c) Ni3(BO3)2 obtained by γ-NiB4O7 decomposition at 700 °C.

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