High-pressure/high-temperature syntheses of the new borate NiB₆O₈(OH)₄

2.1 Synthesis

The starting materials NiO (99.9 %, Strem Chemicals, Bischheim, France) and H₃BO₃ (99.8 %, Carl Roth, Karlsruhe, Germany) in the form of powders were mixed in the molar ratio of 1:6 and transferred into a Pt capsule. The experiment was carried out via high-pressure/high-temperature methods using a Walker-type multianvil module within a 1,000 t hydraulic press (Max Voggenreiter GmbH, Mainleus, Germany) in an 18/11 assembly. Further details concerning this kind of setup have been described previously in the literature. ^{12 , 24 , 25}

The pressure of 9 GPa was built up within 240 min, followed by heating up to 1,200 °C within 15 min. The maximum temperature of the reaction mixture was held for 20 min, then reduced to 400 °C with a cooling rate of 2.5 K min⁻¹. Afterwards, the product was quenched to room temperature within a few minutes. Decompression to ambient conditions was performed within 920 min.

2.2 Phase identification and structure determination by single-crystal X-ray diffraction

The reaction product was analysed with an STOE Stadi P powder X-ray diffractometer (STOE & Cie GmbH, Darmstadt, Germany) equipped with a Mythen 1 K microstrip detector (Dectris, Baden-Daettwil, Switzerland). Measurements were performed in transmission geometry with Ge(111)-monochromatized MoK-L₃ radiation (λ = 0.7093 Å) within a range of 2θ = 2–50° and a step size of 0.015°.

A suitable single crystal was isolated under a Leica 125M polarization microscope and measured using a Bruker D8 Quest diffractometer equipped with a CMOS Photon 300 detector. A multiscan absorption correction was performed with Sadabs-2014/5. ²⁶ Structure solution and refinement were performed within the program Olex2 1.5 and ShelXL-2018/3 algorithms. ^{27 , 28 , 29 , 30}

2.3 Infrared spectroscopy

A single crystal of NiB₆O₈(OH)₄ was mounted on a BaF₂ plate and the spectrum was collected with a Bruker Vertex 70 FT-IR spectrometer. The spectrometer was equipped with a KBr beam splitter, a liquid nitrogen-cooled Mercury Cadmium Telluride (MCT) detector, and a Bruker Hyperion 3,000 microscope. A Globar (silicon carbide) rod was used as mid-IR source and the radiation was focused on the sample with a 15 × IR objective. For data collection and correction for atmospheric influences, the Opus 7.2 software ³¹ was employed.

3.1 Powder X-ray diffraction

The product consisted of a light green/colorless microcrystalline matrix, in which one could observe larger light-green crystals of NiB₆O₈(OH)₄. The powder pattern is shown in Figure 1 (top). The side phase remains unknown due to a lack of measurable crystals.

Figure 1:

Powder X-ray diffraction pattern of the sample as obtained from the synthesis (top) and theoretical pattern of the new borate modelled on the basis of the single-crystal data (bottom). Reflections of the unknown side product are marked with asterisks.

3.2 Crystal structure description

NiB₆O₈(OH)₄ crystallizes in the orthorhombic space group Fdd2 (no. 43) with the unit cell parameters a = 39.097(4) Å, b = 4.3880(4) Å, c = 7.5899(8) Å, V = 1,302.1(2) Å³, and eight formula units per cell. The structure and refinement data are shown in Table 1. Further details on the structure refinement such as atomic coordinates, anisotropic displacement parameters, selected interatomic distances, and bond angles are provided in Tables 2–5.

Further details of the crystal structure investigation may be obtained from Fachinformationszentrum Karlsruhe, 76, 344 Eggenstein-Leopoldshafen, Germany (fax: +49 7247 808 666; e-mail: crysdata@fiz-karlsruhe.de, https://www.ccdc.cam.ac.uk/structures/) on quoting the deposition number CSD-2475184.

NiB₆O₈(OH)₄ is isostructural to ScB₆O₉(OH)₃ ²³ and InB₆O₉(OH)₃ ²² (Table 6). Since Ni²⁺ is smaller than Sc³⁺ and In^{3+ 32} (see Table 6), the lattice parameters b and c – which are directly affected by the size of the MO₆ layers – are also the smallest in the group for the Ni-containing structure.

The crystal structure of NiB₆O₈(OH)₄ (Figure 2a) is built up of triple layers of corner-sharing BO₄ tetrahedra, separated by NiO₆ layers. Along the c axis, one can observe four blocks of these layers, each of these being offset by ¼ ¼ ¼ in relation to the previous one. The middle B2O₄ layer consists of two infinite corner-sharing chains (Figure 2b). Two other BO₄ layers are framing NiO₆ octahedra with tetrahedra oriented away from the layer of octahedra. Each of the BO₄ layers consists of alternating B1O₄ and B3O₄ tetrahedra, all oriented in the same direction within a layer forming sechser-rings (Figure 2c), a consequence of the non-centrosymmetric structure.

Figure 2:

(a) Crystal structure of NiB₆O₈(OH)₄ with a view along the b axis; (b) infinite chains of B2O₄ tetrahedra (orange) in the middle layer, (c) first and third infinite ring layers consisting of alternating B1O₄ and B3O₄ tetrahedra (blue), which are surrounding the NiO₆ octahedra (green). Non-disordered oxygen atomic sites are shown in blue. Due to the disorder on the sites O5A (shown in light pink) and O5B (shown in purple), the orange B2O₄ tetrahedra have two atoms in two of the corners. Hyrdogen atoms are shown in grey and the hydrogen bonds are shown in orange.

The isostructural Sc compound ²³ has a disorder within the middle layer in both boron and oxygen atomic sites, while the isostructural In phase does not show any disorder. ²² In comparison to them, the here presented Ni compound also exhibits a disordered oxygen site (O5A and O5B, respectively) within the middle layer, but no signs of a disorder of the boron atomic sites were observed.

As is also observed for the Sc and In compounds, the hydrogen atoms in NiB₆O₈(OH)₄ are located on the middle layer. In order to keep up the charge balance, for the Ni²⁺ cations, not one, but two protons are required. However, due to the disorder in the structure, their exact location on the basis of X-ray data was not possible. Therefore, BLBS and CHARDI calculations were performed (vide infra).

Average bond lengths in the BO₄ tetrahedra range from 1.480–1.485 Å (see Table 4), however, some bonds are elongated: B3–O4 (1.636(6) Å), B2–O4 (1.514(4) Å), and B2–O6 (1.514(4) Å). These bonds are all in the range of known values (1.373–1.699 Å), ³³ but their elongation can be seen as another evidence of disorder. A similar effect occurs for the O–B–O angles (see Table 5) in the BO₄ tetrahedra: average values are about 109.0(6)°, but several single values (for example, O5A–B2–O6 and O4–B2–O5B (118.3(7)°)) are on the top border (119.43°), ³³ while some other single values such as O4–B2–O6 (102.7(3)°) are close to the lower border (95.72°). ³³

Bond valence sums (Table 7) were calculated according to the bond-length/bond-strength (BLBS) ³⁴ and charge distribution (CHARDI) ³⁵ concepts.

It can be seen from both BLBS and CHARDI calculations that while the oxidation states of the cations Ni²⁺ and the O²⁻ anions O1–O3 are in a good agreement with the expected values, this is not the case for O4, O6, and O5A/B (bold-formatted values). Based on that observation, and considering elongated B–O bond lengths and the aspect of charge neutrality, it becomes evident that four hydrogen atoms per formula unit are needed. Those should be covalently bound to O4 and O6, establishing weak hydrogen bonds between the oxygen atoms O4 and O5A/B (O4–H4⋯O5A/B).

3.3 Single-grain IR spectroscopy

The infrared spectrum of NiB₆O₈(OH)₄ is shown in Figure 3. A strong signal in the range of ∼2,900–3,600 cm⁻¹ can be attributed to OH⁻ stretching modes, and therefore, supports the proposed presence of hydroxyl groups in the structure. ³⁶ Signals around 1,250–1,500 cm⁻¹ can be attributed to O–B–O stretching vibrations. ³⁷ The complicated continuous character of absorptions in the range between 700 and 1,000 cm⁻¹ can be explained as the sum of vibration modes of BO₄ and NiO₆ units, overlapping with bending vibrations of the BO₄ tetrahedra. ^{38 , 39}

Figure 3:

The single-crystal infrared spectrum of NiB₆O₈(OH)₄.