Synthesis and crystal structure of the zinc borate Zn₆B₂₂O₃₉·H₂O

2.1 Synthesis

Zn₆B₂₂O₃₉·H₂O was synthesized from ZnO (32.5 mg, 0.4 mmol, Merck, Darmstadt, Germany, >99 %) and partially hydrolysed B₂O₃ (69.5 mg, 1.0 mmol, Alfa Aesar, Haverhill, USA, 99 %) at 7 GPa and 1523 K in a Walker-type multianvil high-pressure setup (1000 t downforce press (mavo press LPR 1000-400/50), Walker-type module (Max Voggenreiter GmbH, Mainleus, Germany)). The reactants were mixed and ground in an agate mortar under air. The powder was filled into a platinum capsule (99.95 %, Ögussa, Vienna, Austria) and transferred into a hexagonal BN (hP4) (Henze Boron Nitride Products AG, Lauben, Germany) crucible. The crucible was closed with a lid of the same material and placed in an 18/11 assembly. More information about the high-pressure apparatus can be found in the literature [10], [11], [12]. The pressure was increased to 7 GPa within 180 min and kept constant during the heating programme. The temperature was then raised within 10 minutes to 1523 K. Subsequently, the temperature was maintained constant for 20 min and then cooled to 673 K within 300 min. Afterwards, the sample was quenched to room temperature, followed by a pressure release to ambient conditions within 800 min. The octahedron was cracked, and the colourless crystalline needles with a few black areas were recovered from the surrounding parts with a spike. The black spots appeared in many experiments and up to now we were not able to identify their composition. Syntheses with the right stoichiometric ratio resulted in β-ZnB₄O₇ (oC48) as product. However, with a ratio of Zn to B = 1:5 we often received Zn₆B₂₂O₃₉·H₂O as side or main phase.

2.2 X-ray powder diffraction

The colourless needles were separated by hand from the black spots, ground in an agate mortar, and fixed between two thin polyacetate foils with vacuum grease. Subsequently, the sample was put in a flat sample holder and measured at room temperature in transmission geometry on an STOE Stadi P diffractometer (STOE & Cie GmbH, Darmstadt, Germany) with a Mythen 2 DCS4 detector, using the WinXPOW software package [13]. Ge(111)-monochromatized MoK-L₃ (λ = 0.7093 Å) radiation was applied to the sample in a 2θ range of 2–80° with a step size of 0.015° and 20 s exposure time. A Rietveld refinement was done with the Diffrac ^plus -Topas 4.2 software [14]. The single-crystal structure solution (below) was used as starting point and the peak shapes were fitted using modified Thompson-Cox-Hastings pseudo-Voigt profiles [15, 16]. The contribution of the diffractometer was adjusted by refining a LaB₆ standard. The background was corrected with Chebychev polynomials to the 18th order. The graph was made with OriginPro [17].

2.3 Single-crystal X-ray diffraction

A single crystal was picked from the oil-coated sample using a polarization microscope and mounted on a loop (MircoMounts™, MiTeGen, LLC, Ithaca, NY, USA). The data were collected at 123 K on a Bruker D8 Quest diffractometer (Bruker, Billerica, USA) with an Incoatec microfocus MoK-L_2,3 (λ = 0.71073 Å) X-ray source (Incoatec, Geesthacht, Germany) and a Photon III C14 detector system. The data collection routine, cell refinement, and data reduction were performed with the Apex3 programme package, as well as a multi-scan absorption correction based on spherical harmonics [18]. The structure was solved with ShelXT [19] using Intrinsic Phasing and refined with the ShelXL refinement package using least-squares minimisation. The programme Olex2 [20] was used as graphical interface and illustrations were made with the program Diamond [21]. The centrosymmetric space group Pnma was verified with the ADDSYM [22] routine of the Platon [23] programme package. Due to the split position of B6/B7, the boron atoms B6 and B7 were refined isotropically. Apart from that, all non-hydrogen atoms were refined anisotropically. The positions of the hydrogen atoms could not be detected and will be discussed in the chapter 3.1 (Crystal structure). The crystal data, data collection, and structure refinement results are shown in Table 1. The fractional atomic coordinates, Wyckoff positions, and displacement parameters are listed in Tables 2 and 3.

CSD 2301615 contains the supplementary crystallographic data for this paper. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/structures.

3.1 Crystal structure

Zn₆B₂₂O₃₉·H₂O crystallizes in the space group Pnma (no. 62) with the lattice parameters a = 818.77(4), b = 768.42(3), c = 1744.82(9) pm, V = 1.09777(9) nm³, and two formula units per cell (Z = 2). Its structure contains (BO₃), (BO₄), (ZnO₄), and (ZnO₆) units and is closely related to the structures of M ₆B₂₂O₃₉·H₂O (M = Fe, Co, Ni, Cd) [8, 9, 24]. In the M ₆B₂₂O₃₉·H₂O structures, the borate network is formed by corner-connected (BO₄) tetrahedra, forming corrugated layers (Figure 1).

Figure 1:

Corrugated layers in Zn₆B₂₂O₃₉·H₂O (top) and the Fundamental Building Block (FBB) (bottom, centre). The planar (B6O₃) and non-planar (B7O₃) units are shown (bottom, left and bottom, right).

These layers are corner-connected by planar or non-planar (BO₃) units. In case of Fe, Co, and Ni, they are connected by non-planar (BO₃) units, which show intermediate states between (BO₃) triangles and (BO₄) tetrahedra. The cadmium compound exhibits exclusively planar (BO₃) units and the new zinc borate features a mixture of both, planar (B6O₃) and non-planar (B7O₃) units. The B7 atom is displaced out of the O₃-triangle plane, while the B6 atom is not. This is evident from the distances B6–O5 (212(3) pm) and B7–O5 (183(2) pm) (Table 4). In the Fe, Co, and Ni compounds, these B–O distances are 188.3, 169.5, and 172.1 pm. B6 and B7 are present with site occupancy factors of 27 and 23 %, respectively. Furthermore, B6 and B7 form diborate units with O13, being separated by a water molecule (O14). This kind of disorder has two possibilities in the structure, which are shown in Figure 2 (atom B7 is not shown for clarity).

Figure 2:

Two possibilities for the disorder of the (B6O₃) units (forming ((B6)₂O₅) diborate units) and O13/O14 oxygen atoms. Atom B7 is not shown for clarity.

The hydrogen atoms must be bonded to the oxygen atom O14, otherwise O14 would only be coordinating to Zn1 and Zn3 (Figure 3). The hydrogen atoms were also described at that location in the structures of the compounds M ₆B₂₂O₃₉·H₂O (M = Fe, Co, Ni, Cd). However, we were not able to locate the hydrogen atom via single-crystal structure analysis in the structure presented here. The hydrogen atom environment and possible hydrogen bonds to O12 are proposed as red dashed lines in Figure 3.

Figure 3:

Hydrogen environment and possible hydrogen bonds (red dashed lines). Atom B7 is not shown for clarity.

The Zn–O distances in Zn₆B₂₂O₃₉·H₂O (Figure 4 and Table 4) fit those reported by Gagné and Hawthorne (coordination number (CN) 4: 184.7–207.6 pm, av. 195.2 pm; CN 6: 188.6–269.6 pm, av. 211.0 pm) [25]. Even the longest Zn–O distance of 266.0 pm (Zn1–O13) in Zn₆B₂₂O₃₉·H₂O fits the reported range.

Figure 4:

Coordination spheres of Zn1 (top, left), Zn2 (top, right), Zn3 (bottom, left), and O14 (bottom right) in Zn₆B₂₂O₃₉·H₂O. Distances are given in pm.

The B–O distances and O–B–O angles (Table 5) for B1, B2, B3, B4, and B5 fit the values for (BO₄) tetrahedra reported by Zobetz (B–O distances from 137.3 to 169.9 pm, av. 147.6 pm, O–B–O angles vary between 95.7 and 119.4°, av. 109.4°) [26].

The B6–O and B7–O distances are longer than expected for (BO₃) units. They are on average 146.0 and 149.6 pm for trigonal planar (B6O₃) and trigonal non-planar (B7O₃), respectively. These averages are closer to the average of (BO₄) tetrahedra (147.6 pm) [26] than to trigonal planar (BO₃) units (137.0 pm) [27]. This supports the thesis of a transition state to edge-sharing (BO₄) tetrahedra. A similar arrangement exists in the structures of M ₆B₂₂O₃₉·H₂O (M = Fe, Co, Ni) [8, 9].

3.2 Rietveld analysis

The Rietveld refinement of Zn₆B₂₂O₃₉·H₂O is depicted in Figure 5. The main phase is Zn₆B₂₂O₃₉·H₂O with 74(4)% and (NH₄)B₃O₅ formed as side phase with 26(4)%. The unit cell parameters found in the Rietveld refinement fit well to those obtained by single-crystal X-ray diffraction (Table 1). The high background at low 2θ values is a result of a small sample size due to mechanical separation of the black side product from the colourless needles. The reflections marked with an asterisk originate from an unknown side phase.

Figure 5:

X-ray powder diffraction pattern (MoK-L₃ radiation, λ = 70.93 pm) and Rietveld refinement of Zn₆B₂₂O₃₉·H₂O. The positions of the reflections of Zn₆B₂₂O₃₉·H₂O (R _Bragg = 0.86 %) are shown in green, those of NH₄B₃O₅ (R _Bragg = 0.87 %) in blue. (R _exp = 2.02 %, R _wp = 2.27 %, R _p = 1.70 %, GooF = 1.12).