High-pressure synthesis and crystal structure of the mixed-cation borate Ga₄In₄B₁₅O₃₃(OH)₃

2.1 Synthesis

In₂O₃ (99.9%, ChemPUR, Karlsruhe, Germany), Ga₂O₃ (99.998%, Strem Chemicals, Kehl, Germany), and H₃BO₃ (99.5%, Carl Roth, Karlsruhe, Germany) were weighed in a molar ratio of 1:3:8 and ground in an agate mortar. Then, the mixture was encapsulated in gold foil (99.99%, 0.025 mm, Sigma-Aldrich, USA) and transferred into a boron nitride crucible (α-BN, Henze Boron Nitride Products AG, Kempten, Germany). The high-pressure/high-temperature experiment was performed with a multianvil device based on a Walker-type module (Voggenreiter, Mainleus, Germany) that consisted of a 14/8 assembly surrounded by eight tungsten carbide cubes (ha-7% Co, Hawedia, Marklkofen, Germany). The detailed procedure is described in the literature [15], [16], [17]. Ga₄In₄B₁₅O₃₃(OH)₃ formed under conditions of 12.5 GPa and 1300°C. The maximum pressure was built up within 342 min and held for 42 min during the heating period. Within 7 min, the maximum temperature of 1300°C was reached, kept for 5 min and afterwards the temperature was decreased to 900°C within 30 min. The following careful pressure reduction to ambient conditions took 1013 min.

The reaction product appeared grey-white and its powder pattern did not match that of any known phase. A suitable single-crystal was isolated from the sample. The solution of the structure of Ga₄In₄B₁₅O₃₃(OH)₃ explains some of the reflections in the powder pattern of the reaction product (see Fig. 1). However, the majority of the reflections is still unknown up to now. All subsequent attempts to purposefully and stoichiometrically synthesize Ga₄In₄B₁₅O₃₃(OH)₃ failed up to now.

Fig. 1:

Experimental powder pattern of the reaction product (top) compared to a simulation of the theoretical powder pattern of Ga₄In₄B₁₅O₃₃(OH)₃ based on single-crystal data (bottom). Reflections marked with an asterisk do not result from the title compound.

2.2 X-ray structure determination

At first, the reaction product was analyzed with a Stoe Stadi P powder diffractometer equipped with a Mythen 1 K detector (Dectris, Switzerland). The measurement was carried out with Ge(111)-monochromatized MoKα₁ radiation (λ=0.7093 Å) in transmission geometry across a 2θ range of 2.0–60.5°. Figure 1 shows the experimental powder pattern. Most of the reflections could not be associated with any known phases. All reflections that do not match with the simulation of Ga₄In₄B₁₅O₃₃(OH)₃ derived from single-crystal data are marked with asterisks.

A colorless, prismatic single-crystal of Ga₄In₄B₁₅O₃₃(OH)₃ was isolated from the reaction product and measured on a Bruker D8 Quest diffractometer equipped with a Photon 100 CMOS detector. The data was collected at room temperature and a multiscan absorption correction was performed with Sadabs 2014/5 [18]. For the structure solution and parameter refinement, the Shelxs/l-2013 [19], [20] software implemented in the program Wingx-2013.3 [21] was used. The single-crystal was treated as a non-merohedral twin with a domain fraction of 0.165–0.835 and was refined with the twin matrix [010 100 001̅]. Aside from the proton, all atoms in Ga₄In₄B₁₅O₃₃(OH)₃ could be refined anisotropically with site occupancy factors of 1. The proton position was fixed with the DFIX command in a distance of 0.83 Å to O8, since this is the only oxygen atom with otherwise only two coordination partners. Due to charge neutrality reasons, the proton H8 was refined with a site occupancy factor of 0.75. In the end, the refinement of Ga₄In₄B₁₅O₃₃(OH)₃ led to satisfying results with total R (all data) values of R₁=0.0192 and wR₂=0.0422 as well as a Goodness-of-fit of 1.078.

CCDC 1895780 (Ga₄In₄B₁₅O₃₃(OH)₃) contains the supplementary crystallographic data for this paper. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.

3.1 Crystal structure

Ga₄In₄B₁₅O₃₃(OH)₃ crystallizes in the tetragonal space group I4₁/a (no. 88, origin choice 2) with four formula units (Z=4) and the lattice parameters a=11.382(2), c=15.244(2) Å, and V=1974.9(4) ų. Details of the crystal structure refinement can be found in Table 1. Figure 2 shows the asymmetric unit of Ga₄In₄B₁₅O₃₃(OH)₃. The structure consists of five different boron atom positions (corner-sharing BO₄ tetrahedra) and one octahedrally coordinated indium and gallium atom position. The proton, which forms a hydroxyl group with the donor atom O8, is the only atom site that shows a diminished occupancy factor of 0.75 for charge neutrality reasons. A complete list of occupancy factors, Wyckoff positions, fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters can be found in Table 2. All anisotropic displacement parameters are listed in Table 3. The structure of Ga₄In₄B₁₅O₃₃(OH)₃ will now be explained in detail starting with the In-O and Ga-O networks.

Fig. 2:

Asymmetric unit of Ga₄In₄B₁₅O₃₃(OH)₃ consisting of five different boron atom positions forming corner-sharing BO₄ tetrahedra, an InO₆ octahedron, a GaO₆ octahedron, and a hydroxyl group (atoms O8 and H8).

Both metal atoms are coordinated octahedrally by oxygen atoms. Due to the symmetry of the space group, indium forms edge-sharing octahedra in dinuclear units caused by an inversion axis, and gallium assembles dense units of four edge-sharing octahedra induced by the 4̅ axis. The whole unit cell contains four isolated tetranuclear Ga–O units and eight isolated dinuclear In-O units. Figure 3 shows both octahedral units and their arrangement in the unit cell of Ga₄In₄B₁₅O₃₃(OH)₃. All metal-oxygen distances are in good accordance with values reported in the literature (Ga–O=1.928(2)–2.126(2) Å, Ø=2.00 Å; In–O=2.121(2) –2.219(2) Å, Ø=2.17 Å) [1], [7], [9], [22], while the axial angles with average values of 168.6° for O–Ga–O and 170.3° for O–In–O are a bit smaller than the expected one of 180°. A complete list of distances and angles is given in Tables 4 and 5.

Fig. 3:

Isolated edge-sharing InO₆ octahedra (top left, yellow) and GaO₆ octahedra (bottom left, green) and their arrangement in the unit cell of Ga₄In₄B₁₅O₃₃(OH)₃ (middle and right).

The complex network of exclusively corner-sharing BO₄ tetrahedra interconnects the various isolated metal-oxygen units. In doing so, the different BO₄ tetrahedra form dreier and vierer rings [23]. B5O₄ is the only tetrahedron that does not contribute to a ring system but connects to the proton H8 via O8 instead (see Fig. 4). The complete structure is shown in Fig. 5. The hydrogen bonding situation is illustrated in Fig. 6, angles and distances of the hydrogen bonds are given in Table 6. All B–O bond lengths are within the range of 1.418(3)–1.556(3) Å (average 1.49 Å) and fit to reported distances in tetrahedral borate groups [24]. The average O–B–O angles of 109.4° (for B1) and 109.5° (for B2–5) indicate perfectly shaped polyhedra.

Fig. 4:

Visualization of the network of corner-sharing B-O tetrahedra in Ga₄In₄B₁₅O₃₃(OH)₃. Tetrahedra centered by the boron atoms B2, B3 and B4 build up vierer rings (turquoise) that are further connected to dreier rings via B1O₄ tetrahedra (blue). The ring-systems are interlinked by the B5O₄ tetrahedra containing the hydroxyl group (orange).

Fig. 5:

Complete unit cell of Ga₄In₄B₁₅O₃₃(OH)₃.

Fig. 6:

Hydrogen bonds in Ga₄In₄B₁₅O₃₃(OH)₃.

To evaluate the charge distribution, we performed bond-length/bond-strength calculations (see Table 7) [25], [26]. Solely O9 shows an unusually high charge which can be attributed to the number of bonding partners. While all other oxygen atoms in Ga₄In₄B₁₅O₃₃(OH)₃ coordinate three other atoms, O9 forms quite long but nonetheless definite bonds to three Ga atoms and one B atom.