Synthesis and structure refinement of the zinc hydroxide boracite: Zn₃B₇O₁₃(OH)

2.1 Synthesis

Zn₃B₇O₁₃OH has been synthesized from ZnO (48.8 mg, 0.6 mmol, Merck, Darmstadt, Germany, >99 %), B(OH)₃ (24.7 mg, 0.4 mmol, Roth, Karlsruhe, Germany, ≥99.8 %), and B₂O₃ (34.8 mg, 0.5 mmol, Alfa Aesar, Haverhill, USA, 99 %) at 2.5 GPa and 673 K in a Walker-type multi-anvil 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 together and ground in an agate mortar under air. The powder was filled 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 [21], [22], [23]. The pressure was increased to 2.5 GPa within 60 min and held constant during the heating programme. The temperature was raised in 10 min to 673 K and maintained constant for 5 min. Subsequently, the sample was cooled to 373 K within 120 min and then quenched to room temperature, followed by a pressure release to ambient conditions in 600 min. The octahedron was cracked, and the white crystalline sample was recovered from the surrounding parts with a spike.

2.2 X-ray powder diffraction

The sample was 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 [24]. 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 [25]. 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 [26,27]. The contribution of the diffractometer was adjusted by refining a LaB₆ standard. The background was corrected with Chebychev polynomials to the 16^th order. The graph was made with OriginPro [28].

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 was collected at T = 301 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 300 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 [29]. The structure was solved with ShelXT [30] using Intrinsic Phasing and refined with the ShelXL [31] refinement package using least squares minimisation. The programme Olex2 [32] was used as graphical interface and illustrations were made with Diamond [33]. The non-centrosymmetric space group R3c was verified with the ADDSYM [34] routine of the Platon [35] programme package. The oxygen atom OA was isotropically refined, due to its split position. Apart from that, all non-hydrogen atoms were refined anisotropically. The atoms O/OA are the only oxygen atoms coming into consideration to be bound to the hydrogen atoms, which fits to the isostructural compounds Fe₃B₇O₁₃(OH) [16,17] and Cd₃B₇O₁₃(OH) [11]. The estimated positions of the hydrogen atoms are shown as black spheres in the figures. However, the hydrogen atoms could not be refined in the single-crystal structure which was refined as inversion twin. 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. The fractional atomic coordinates fit those of the structurally related compound Zn₃B₇O₁₃Cl, which are also included in Table 2.

CSD 2301661 contains the supplementary crystallographic data for this paper. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures.

3.1 Crystal structure

Zn₃B₇O₁₃(OH) crystallizes trigonally in the space group R3c (no. 161) with the lattice parameters a = 849.76(3), c = 2099.8(2) pm, V = 1.3131(2) nm³, and six formula units per unit cell (Z = 6). Its structure consists of trigonal planar (BO₃), tetrahedral (BO₄), and distorted square pyramidal (ZnO₅) units. The zinc hydroxide boracite is isostructural to e.g. Fe₃B₇O₁₃(OH) [16,17], Cd₃B₇O₁₃(OH) [11], and Zn₃B₇O₁₃Cl [10]. Its structure is shown on the right side of Figure 2. Looking at the M ²⁺ and X ⁻ ions, the anion X ⁻ is displaced from the central position of the Zn₆ octahedra of the cubic modification (of e.g. Cu₃B₇O₁₃Br [13]) (Figure 3). In low-temperature modifications e.g. Zn₃B₇O₁₃Cl [10] or Zn₃B₇O₁₃(OH) (both crystallize in the trigonal space group R3c), the interatomic distances on one side of the octahedron decrease (Figure 3, lavender), while that to the opposite atom distance increases (Figure 3, orange). This fact leads to five-fold coordinated Zn²⁺ cations. In hydroxide boracites, the OH group replaces the halogenide. The oxygen atom is even more displaced to one side, because of the shorter distance M ²⁺–O²⁻ compared with M ²⁺–Cl⁻, M ²⁺–Br⁻, or M ²⁺–I⁻. This leaves enough space for the hydrogen atom, which is part of a hydroxyl group without an acceptor anion.

Figure 3:

X ⁻ coordination in zinc chloride boracite (left) and in Zn₃B₇O₁₃(OH). Lines of the same colour have the same length. All distances are given in pm.

Zn₃B₇O₁₃(OH) has disordered Zn/ZnA and O/OA atoms, with a site occupancy factor (s.o.f.) of 0.94/0.06. During cooling from the cubic high-temperature to the trigonal low temperature modification, the oxygen atom O is not always displaced toward the same side of the Zn₆ octahedra. If the atom O is displaced to the other side also the Zn atom is disordered, resulting in a nearly identical structure inverted by 180° (Figure 4).

Figure 4:

Disordered Zn/ZnA and O/OA atoms in Zn₃B₇O₁₃(OH).

All distances (Table 4) and angles (Table 5) in the zinc hydroxy boracite are within usual values.

3.2 Rietveld analysis

The Rietveld refinement is shown in Figure 5. The sample contains 49.9(3) % Zn₃B₇O₁₃OH, 41.0(3) % Zn₄O(BO₂)₆ and 9.1(5) % γ-HBO₂ (cP96). The refined cell parameters fit well to those from the single-crystal structure refinement (Table 1). We were not able to produce the zinc hydroxide boracite phase pure. Therefore, no further analytical investigations were carried out.

Figure 5:

X-ray powder diffraction pattern (MoK-L₃ radiation, λ = 70.93 pm) and Rietveld refinement of Zn₃B₇O₁₃OH. The reflection positions of Zn₃B₇O₁₃OH (R _Bragg = 1.84 %), Zn₄O(BO₂)₆ (R _Bragg = 1.29 %) and γ-HBO₂ (cP96) (R _Bragg = 2.00 %) are shown in orange, green, and lavender, respectively. (R _exp = 3.28 %, R _wp = 3.78 %, R _p = 2.96 %, GooF = 1.15).