Ni₆B₂₂O₃₉·H₂O – extending the field of nickel borates

2.1 Crystal structure

Ni₆B₂₂O₃₉·H₂O crystallizes in the orthorhombic, acentric space group Pmn2₁ (no. 31) with the lattice parameters a=7.664(2), b=8.121(2) and c=17.402(2) Å (Table 1). It is isotypic to the compounds M₆B₂₂O₃₉·H₂O (M=Fe, Co) [6] and structurally related to Cd₆B₂₂O₃₉·H₂O [7]. The latter phase shows a similar crystal structure but crystallizes in the higher symmetric space group Pnma (no. 62).

A comparison of the lattice parameters and cell volumes of the isotypic phases is given in Table 2. All parameters decrease from the iron via the cobalt to the nickel compound. This finding is consistent with the given ionic radii only, if a high-spin configuration is assumed for iron and cobalt. If a low-spin configuration was present, the lattice parameters should evolve conversely.

The crystal structure of Ni₆B₂₂O₃₉·H₂O consists of vertex-sharing [BO₄] tetrahedra and non-planar, pyramidal [BO₃] groups forming corrugated layers parallel to (001). These layers are interconnected along [001] through [BO₄] tetrahedra centered by the boron atom B11 (Fig. 1; light blue tetrahedra). The nickel atoms are located in the channels between the borate layers.

Fig. 1:

Left: crystal structure of Ni₆B₂₂O₃₉·H₂O. The corrugated layers consisting of [BO₄] tetrahedra and non-planar [BO₃] groups are interconnected along [001] through [BO₄] tetrahedra (light blue). Right: crystal structure of Co₆B₂₂O₃₉·H₂O. Here, the corrugated layers are interconnected via non-planar, pyramidal [BO₃] groups. The orientation of both crystal structures is different as the atomic coordinates of Ni₆B₂₂O₃₉·H₂O had to be inverted during the refinement (see Experimental Section for more details).

Similar to M₆B₂₂O₃₉·H₂O (M=Fe, Co), Ni₆B₂₂O₃₉·H₂O shows a structural feature that can be considered as an intermediate state between a planar [BO₃] and a tetrahedral [BO₄] group (Fig. 2a). In the nickel compound, however, the non-planar [BO₃] group involves the atom B8, whereas the corresponding arrangement is observed for B11 in M₆B₂₂O₃₉·H₂O (M=Fe, Co) (Fig. 2). Interestingly, the boron site B6 in Cd₆B₂₂O₃₉·H₂O that corresponds to the B11 site (there are less independent atom sites due to the higher symmetry of the cadmium phase, hence the atom names differ) shows a nearly trigonal planar configuration. Apparently, the coordination environments of B8 and B11 change depending on the radius of the transition metal. An increasing radius promotes a shift towards a (more regular) tetrahedral coordination of B8 and, at the same time, a trigonal planar coordination of B11 (Fig. 2 and Table 3).

Fig. 2:

Comparison of the coordination environments of B8 and B11 (and their analogs B5 and B6 in the cadmium phase) in M₆B₂₂O₃₉·H₂O. (a) M=Ni; (b) M=Co; (c) M=Fe; (d) M=Cd. Distances in (Å) (see also Table 3).

The individual B–O bond lengths of the tetrahedrally coordinated boron sites (B1–B7 and B9–B11) range from 1.402(4) to 1.611(5) Å (Table 4) with an average value of 1.479 Å, which is in accordance with the literature value (1.476(35) Å) [16]. The corresponding B–O distances in the non-planar [BO₃] group (B8) are 1.416(5)–1.454(5) Å (average: 1.430 Å). Similar to M₆B₂₂O₃₉·H₂O (M=Fe, Co), the mean value is significantly larger compared to planar [BO₃] groups (1.370(19) Å) [17].

There are six crystallographically distinguishable nickel sites in Ni₆B₂₂O₃₉·H₂O. Four are coordinated octahedrally [coordination number (CN) 6; Ni1, Ni3–5], Ni2 shows a monocapped tetrahedral coordination (CN 4+1), and Ni6 is coordinated in the form of a trigonal bipyramid (CN 5) (Fig. 3). These coordination environments match those observed in M₆B₂₂O₃₉·H₂O (M=Fe, Co) except for Ni6. In the latter compounds, the corresponding M6 position shows a 4+1 coordination like M2. The reason for this structural deviation is the difference between the two longest M6–O bonds, which is much larger in the iron and cobalt phases compared to the nickel compound (Table 5). To our knowledge, a trigonal bipyramidal coordination of nickel through oxygen was observed only rarely in solid state chemistry. The compound (Mg_1.55Ni_0.45)(OH)(AsO₄), for example, exhibits a trigonal bipyramidally coordinated magnesium site, which is also partially (~4%) occupied by nickel.

Fig. 3:

Coordination environments of the six crystallographically different nickel atoms. All atoms are drawn as displacement ellipsoids at the 90% probability level.

Table 6 shows the Ni–O bond lengths in Ni₆B₂₂O₃₉·H₂O. The bond lengths of the octahedrally coordinated nickel atoms vary between 1.995(3) and 2.390(4) Å, which is a larger range compared to other high-pressure nickel borates (e.g. Ni₃B₁₈O₂₈(OH)₄·H₂O: 2.064(2)–2.184(3) Å [13] or NiB₃O₅(OH): 1.998(2)–2.171(2) Å [14]), but similar to the distances observed in Na₂Ni₂B₁₂O₂₁ (1.97(1)–2.33(1) Å) [18]. The Ni–O distances of the sites Ni2 (CN 4+1) and Ni6 (CN 5) lie between 1.925(4) and 2.356(4) Å. Compared to Ni–O bond lengths in [NiO₄] tetrahedra (NiCr₂O₄: 1.969 Å [19]) or [NiO₅] square pyramids (β-NiB₄O₇: 2.001(2)–2.060(4) Å [11]), the observed values spread over a larger range. The displacement ellipsoids of Ni2 and Ni6 show a more anisotropic shape than those of the remaining nickel atoms (Fig. 3 and Table 7), which can be related to the different coordination geometries of these nickel sites. The positions of the ligands and the corresponding bond lengths are relatively homogenously distributed for the octahedrally coordinated nickel sites. As Ni2 and Ni6 exhibit a less balanced coordination sphere, a stronger anisotropic displacement of these atoms can be observed whereby the strongest displacement occurs in directions where existing bonds have to be stretched or compressed as little as possible. This effect is more pronounced for Ni6, as the bond lengths (and hence the bond strengths) towards the equatorial and apical oxygen atoms differ significantly (Table 6).

The water molecule in Ni₆B₂₂O₃₉·H₂O is built up by the atoms O25 and H1. A strong hydrogen bond is formed towards O15, which connects the non-planar [BO₃] group and the adjacent [BO₄] tetrahedron, and a weaker one towards O2 (Fig. 4 and Table 8). Atom O25 does not coordinate to any boron atom. Apart from the hydrogen atoms, it only interacts with Ni1 and Ni5.

Fig. 4:

Hydrogen bonds in Ni₆B₂₂O₃₉·H₂O. D–H bonds are drawn as solid lines, H···A bonds as dashed lines.

Bond valence sums were calculated according to the bond-length/bond-strength (BLBS) concept (Table 9) [20], [21]. Within the limits of this concept, the calculated values correspond to the expected formal ionic charges. The “undersaturation” of the transition metal sites M1–M3 and M6 were also observed in M₆B₂₂O₃₉·H₂O (M=Fe, Co) [6], albeit to a lesser extent. The positional parameters of Ni₆B₂₂O₃₉·H₂O are given in Table 10.

Further details of the crystal structure investigation may be obtained from FIZ Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: crysdata@fiz-karlsruhe.de) on quoting the deposition number CSD-433578 for Ni₆B₂₂O₃₉·H₂O.

2.2 X-ray powder diffraction

Figure 5 displays the result of the Rietveld refinement. Apart from an amorphous phase, which is indicated by the halo at low 2θ angles, the reaction product consisted primarily of Ni₆B₂₂O₃₉·H₂O (~52 wt%) and NiB₃O₅(OH) [14] (~48 wt%). Only few minor peaks could not be ascribed to any known phase. More details concerning the refinement are given in Table 1.

Fig. 5:

Rietveld refinement plot.

2.3 Vibrational spectroscopy

The IR and Raman spectra of a single-crystal are shown in Fig. 6. Between approximately 3600 and 2900 cm⁻¹, the stretching vibrations of the water molecule can be seen in the IR spectrum [22]. The origin of the broad peak at 2000 cm⁻¹ is uncertain, whereas the band at ~1450 cm⁻¹ can presumably be ascribed to stretching vibrations of the (non-planar) [BO₃] group [23]. Below ~1300 cm⁻¹, the IR spectrum is poorly resolved and allows no band assignment.

Fig. 6:

Single-crystal IR (red) and Raman (black) spectrum of Ni₆B₂₂O₃₉·H₂O.

As the Raman spectrum exhibited strong fluorescence effects above ~1500 cm⁻¹, only the lower part of the spectrum is shown. Stretching vibrations of [BO₄] tetrahedra can be observed between 1200 and 900 cm⁻¹, while bending and complex vibrations of both [BO₃] and [BO₄] groups give rise to bands in the wavenumber region ~900–200 cm⁻¹ [24], [25]. Lattice vibrations which involve the nickel atoms occur below approximately 350 cm⁻¹ [12].

4.1 Synthesis

The high-pressure/high-temperature synthesis at 5 GPa/900°C was carried out in a Walker-type multianvil apparatus in combination with a 1000 t press (both Max Voggenreiter GmbH, Germany). A 1:2 molar mixture of NiO (Avocado Research Chemicals, UK, 99.5%) and B₂O₃ (Strem Chemicals, USA, 99.9+%) was used as starting material. The reactants were weighed and ground in an agate mortar under ambient conditions, which led to a partial hydrolysis of B₂O₃. The starting mixture was then encapsulated in molybdenum foil before it was placed in a crucible, which was closed with a lid (both made of α-BN; Henze Boron Nitride Products AG, Germany). The synthesis was performed using an 18/11 assembly, which was compressed via a two-step mechanism. A more detailed description of the experimental setup can be found in the literature [26], [27], [28].

The high-pressure synthesis comprised several steps: first, the sample was compressed to 5 GPa over 125 min. It was then heated to 900°C within 10 min and exposed to this temperature for 100 min. Subsequently, the temperature was reduced to 550°C over 50 min before the heating was switched off to quench the sample to room temperature. Finally, the pressure was released within 12 h. The reaction product consisted of light green (NiB₃O₅(OH) [14]) and dichroic (purple↔dark green; Ni₆B₂₂O₃₉·H₂O) crystals. A similar dichroism (red↔bluish) was also observed for Co₆B₂₂O₃₉·H₂O [6].

Ni₆B₂₂O₃₉·H₂O also occurred in a reaction at 6 GPa/1000°C using a reagent mixture in the correct stoichiometry. However, besides Ni₆B₂₂O₃₉·H₂O the reaction product contained also γ-NiB₄O₇ [12] and at least one unknown compound. Further attempts to synthesize a phase pure sample of Ni₆B₂₂O₃₉·H₂O failed.

4.2 Single-crystal structure analysis

The single-crystal diffraction data were obtained using a Nonius Kappa-CCD diffractometer with graphite-monochromatized MoK_α radiation (λ=0.7107 Å). The data set was corrected for absorption effects employing a semi-empirical approach based on equivalent and redundant intensities (Scalepack [29]). The crystal structure was refined with Shelxl [30] (version 2016/6) as implemented in Wingx [31] (version 2013/3) starting from the structural model of the isotypic compound Co₆B₂₂O₃₉·H₂O [6]. As this model corresponded to the wrong twin domain, all atomic coordinates were inverted which led to a Flack parameter of x=0.01(1) (calculated according to Parsons et al. [32] based on 1923 intensity quotients). All non-hydrogen atoms were refined using anisotropic displacement parameters. A restraint was applied for the distance O25–H1. More details on the data collection and structure refinement are presented in Table 1.

4.3 X-ray powder diffraction

A powdered sample of the reaction product was analyzed using a Stoe Stadi P powder diffractometer equipped with a Dectris Mythen 1K detector. The measurement was carried out in transmission geometry with Ge(111)-monochromatized MoK_α1 radiation (λ=0.7093 Å). Topas 4.2 [33] was used for the Rietveld refinement. More details are provided in Table 1.

4.4 Vibrational spectroscopy

A Bruker Vertex 70 FT-IR spectrometer (spectral resolution 4 cm⁻¹) equipped with a Globar mid-IR source and a KBr beam splitter was used to acquire the single-crystal absorption spectrum in the wavenumber range 600–4000 cm⁻¹. The spectrometer was attached to a Bruker Hyperion 3000 microscope and equipped with a mercury cadmium telluride (MCT) detector. Three hundred and twenty scans of the sample were recorded in transmittance mode using a BaF₂ window and a 15×IR objective lens. Data processing was accomplished employing Opus [34].

The Raman spectrum was measured on an arbitrarily oriented single-crystal using a Horiba Jobin Yvon Labram-HR-800 spectrometer. A 532 nm frequency-doubled Nd:YAG laser, a 100× objective lens, a 100 μm slit, a 1000 μm pinhole and an optical grating with 1800 lines mm⁻¹ were used for the measurement. The excited area had a diameter of ~5 μm. An Andor CCD detector was employed to collect the scattered radiation in the wavenumber range 100–3700 cm⁻¹. Two spectra with an acquisition time of 150 s each were averaged using Labspec [35] (version 5.93.20). The baseline of the spectrum was fitted manually. Owing to strong fluorescence effects above 1500 cm⁻¹, the corresponding part of the Raman spectrum is not displayed in Fig. 6.