High-pressure synthesis and crystal structure of Ga₅Na_1–x B₁₂O_26–x (OH) _x (x = 0.12)

2.1 Synthesis

The synthesis of the title compound was performed under high-pressure/high-temperature conditions in a multianvil apparatus. A mixture of Ga₂O₃ (99.99 %, Tradium GmbH, Frankfurt am Main, Germany), H₃BO₃ (≥99.8 %, Carl Roth GmbH & Co KG, Karlsruhe, Germany), and NaCN (>98 %, Fluka Chemie GmbH, Buchs, Switzerland) in a molar ratio of 5:24:3 (stoichiometric ratio apart from NaCN, which was used abundantly to improve the incorporation of Na⁺ compared to a previous experiment that was performed with less Na⁺) was ground in an agate mortar and encapsulated in a platinum foil (0.027 mm, 99.9 %, Chem-PUR, Karlsruhe, Germany). The capsule was placed in a crucible with a lid, both made of α-BN (HeBoSint, P100, Henze Boron Nitride Products AG, Kempten, Germany) and centered in a “14/8” assembly, which was equipped with a graphite furnace (FE 254, Schunk-group GmbH, Vienna, Austria) for resistance heating. The octahedral assembly was arranged in the center of eight beveled tungsten carbide cubes (HA-7% Co, Hawedia, Marklkofen, Germany). The synthesis was performed in a modified Walker-type multianvil device (mavo press LPR 1000-400/50, Max Voggenreiter GmbH, Mainleus, Germany). More detailed information on the experimental setup is available in the literature. ^{7 , 8 , 9} After compression of the sample to 12.4 GPa, the sample was heated to 1,280 °C within 7 min, kept constant for 5 min, then cooled to 900 °C within 30 min, before the heating was switched off. After decompression, a in most of its parts colorless sample containing crystals of Ga₅Na_1–x B₁₂O_26–x (OH) _x with some additional black impurities was isolated.

2.2 X-ray structure determination

The reaction product was analyzed with a STOE Stadi P powder X-ray diffractometer (STOE & Cie GmbH, Darmstadt, Germany) equipped with a Mythen 1 K microstrip detector (Dectris, Baden-Daettwil, Switzerland). The measurement was performed with Ge(111)-monochromatized Mo-Kα ₁ radiation (λ = 0.7093 Å) in transmission geometry across a 2θ range of 2.0–60.5°. Figure 1 shows the experimental powder pattern in comparison to a calculated pattern derived from the single-crystal structure data. While the great majority of reflections could be explained by the title compound, a few minor reflections are caused by an unidentified by-product.

Figure 1:

Experimental powder pattern (black) of Ga₅Na_1–x B₁₂O_26–x (OH) _x compared to a calculated powder pattern based on single-crystal structure data of Ga₅Na_1–x B₁₂O_26–x (OH) _x (x = 0.12) (colored blue, inverted). The asterisk-marked reflections refer to an unidentified byproduct.

A colorless single-crystal of Ga₅Na_1–x B₁₂O_26–x (OH) _x (x = 0.12) was measured with a Bruker D8 Quest Photon III C14 diffractometer (Bruker Corporation, Billerica, Massachusetts, U.S.). For data collection and processing, the programs Saint (v8.40 B) ¹⁰ and Apex4 (v2021.4.0) ¹¹ were used and a multi-scan absorption correction was performed by the program Sadabs (2016/2). ¹² For structure solution and parameter refinement, the software tools Shelxt (2018/2) ¹³ and Shelxl (2018/3) ¹⁴ implemented in the program Olex2-1.5 ¹⁵ were applied. All atoms could be refined anisotropically, except the hydrogen atom, which could not be located.

Details of the data collection are specified in the synoptical Table 1. Tables 2 and 3 show the positional and displacement parameters, the Wyckoff positions, and the site occupancy factors (S.O.F.).

CSD-2376207 contains the supplementary crystallographic data of Ga₅Na_1–x B₁₂O_26–x (OH) _x (x = 0.12) for this paper. The data can be obtained free of charge from The Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/data_request/cif.

2.3 Infrared spectroscopy

The transmission FT-IR spectrum of a single-crystal of Ga₅Na_1–x B₁₂O_26–x (OH) _x (x = 0.12) was measured with a Vertex 70 FT-IR spectrometer (spectral resolution 4 cm⁻¹, 15 × IR objective as focus), which was equipped with a KBr beam splitter, a liquid nitrogen cooled mercury cadmium telluride detector and attached to a Hyperion 3,000 microscope (Bruker Corporation, Billerica, Massachusetts, U.S.A.). As a mid-infrared source, a Globar (silicon carbide) rod was applied. During the measurement, the sample was placed on a BaF₂ window. 320 scans were recorded in a spectral range of 600–4,000 cm⁻¹. Atmospheric influences were corrected with the software Opus 7.2. ¹⁶

2.4 Energy-dispersive X-ray spectroscopy

The chemical composition of the title compound was further investigated with a field emission gun scanning electron microscope (FEG-SEM) Clara Ultra High Resolution (UHR) from TESCAN equipped with an energy-dispersive X-ray spectroscopy (EDX) detector Ultim Max, 65 mm² from OXFORD. Grinded sample material was prepared on a carbon sticker on an aluminum stub. Imaging and EDX measurements were performed in analysis mode at an acceleration voltage of 20 keV and a beam current of 3 nA at a working distance of 9 mm. For EDX, preferably flat and horizontally aligned surfaces of single crystals or crystalline aggregates were selected.

3.1 Crystal structure

Ga₅Na_1–x B₁₂O_26–x (OH) _x (x = 0.12) crystallizes in the tetragonal space group I4₁/acd (no. 142) with eight formula units (Z = 8) and the lattice parameters a = 11.0884(6), c = 21.730(2) Å, and V = 2,671.8(4) Å³. The structure is homeotypic to M ₅B₁₂O₂₅(OH) (M = Al, Cr, Ga, Ga/In, V), ^{2 , 3 , 4} In₅B₁₂O₂₅(OH):Eu³⁺, ² Ti₅B₁₂O₂₆, ¹ Mn₅MnB₁₂O₂₂(OH)₄, ⁴ Mn₅Mn_0.83B₁₂O₂₆, ⁴ Fe₅Fe_0.14B₁₂O_24.3(OH)_1.7 ⁴ and V_1.36Co_3.64Co_0.53[B₁₂O₂₄(OH)₂]. ⁵ Details of the single-crystal data and structure refinement are given in Table 1, the positional and displacement parameters and site occupancy factors in Table 2 and 3. Ga₅B₁₂O₂₅(OH) ² seemed to be most suited for a comparison, which contains the same elements, apart from the vacant cuboctahedral void and a slightly shifted Ga2, which leads to a different site of Ga2.

Ga₅Na_1–x B₁₂O_26–x (OH) _x (x = 0.12) has two different sites for Ga in octahedral, one for Na in cuboctahedral and three different sites for B in tetrahedral coordination (see Table 2). Furthermore, there is a hydrogen site required for charge neutrality, but this could not be exactly assigned in the structure refinement and the situation discussed below. As compared in Table 2, all atoms have the same Wyckoff position as in Ga₅B₁₂O₂₅(OH), ² apart from Na1, which is not present in Ga₅B₁₂O₂₅(OH), ² hydrogen, which could not be refined in Ga₅Na_1–x B₁₂O_26–x (OH) _x (x = 0.12), and Ga2, which is directly located on a special site in the title compound and has therefore a higher site symmetry. In Ga₅B₁₂O₂₅(OH), ² this positional shift at Ga2 is presumably caused in a distortion of the borate network due to a strong hydrogen bond to O3 (see Figure 4). ² Since there is only a small amount of hydrogen in the title compound, the influence of this hydrogen bond is inferior and did not lead to a shift of the position of Ga2. Figure 2 depicts the arrangement of the metal cation polyhedra in the unit cell. The green and orange edge-sharing units are crystallographically identical, but are differently colored in order to distinguish paired units. Thereby, two [Ga1O₆] octahedra are linked with a common edge, forming a [Ga1₂O₁₀] unit. Along the crystallographic c axis, every second unit is either rotated by 90° or shifted along the b axis relative to the adjacent unit. The Ga2 atoms are surrounded by six O atoms forming single [Ga2O₆] octahedra and are located in the holes along the 4 ¯ inversion axis (colored cyan in Figure 2). The [Na1O₁₂] cuboctahedra are alternating (checkered) with the [Ga2O₆] octahedra. However, Na1 has a site occupancy factor (S.O.F.) of 0.876(4), and so not all theoretically possible sites are occupied. Therefore, we considered hydrogen atoms for charge neutrality reasons and this could also be confirmed by infrared spectroscopy (see below). Since a site for H could not be assigned in the structure refinement, we compared the title compound with Ga₅B₁₂O₂₅(OH). ² As depicted in Figure 4, Vitzthum et al. could locate hydrogen bonds formed by O4 as the donor atom and O3, O7, and O6 as the acceptor atoms in their structure refinement. ² To account for charge neutrality, and considering the obvious similarities of the title compound to Ga₅B₁₂O₂₅(OH), ² in particular the same distortion of the Ga2 octahedra, we assume that the H atom is also connected to O4.

Figure 2:

Visualization of the arrangement of [GaO₆] octahedra (orange, green or cyan) and [NaO₁₂] cuboctahedra (purple) in Ga₅Na_1–x B₁₂O_26–x (OH) _x (x = 0.12) in viewing direction [100]. To better distinguish the crystallographically identical [Ga₂O₁₀] units, they were colored orange and green.

Figure 3:

Formation of the network built of [BO₄] tetrahedra in Ga₅Na_1–x B₁₂O_26–x (OH) _x (x = 0.12) in viewing direction [100]. Four tetrahedrally arranged [B₃O₉] units form a [Na1O₁₂] cuboctahedron, which is repeated eight times per unit cell.

Figure 4:

Visualization of the hydrogen bonds in the homeotypic structure Ga₅B₁₂O₂₅(OH), ² which may be similarly arranged in the title compound. O4 represents the donor atom and O3, O6, and O7 the acceptor atoms. Statistically only about 12 % of the four hydrogen bonds and one of the two Ga2 atoms depicted are present at the same time. The same color code as in Figure 2 is used.

The B–O network has a complex arrangement of 96 corner-sharing [BO₄] tetrahedra. Four tetrahedrally arranged [B₃O₉] units form a [B₁₂O₃₀] unit and a cave is formed within three- and six-membered rings containing the Na⁺ cation in its centre, the assembly thus featuring a [Na1O₁₂] cuboctahedron (see Figure 3).

The Ga1–O distances of the edge-sharing double-octahedra are in the range between 1.9163(5) and 2.0326(6) Å (see Figure 5(a)) with an average value of 1.96 Å, which is in accordance with typical Ga–O distances in borates. ^{2 , 17 , 18 , 19} However, the Ga2–O bond lengths, which are ranging from 1.9908(8) to 2.1066(7) Å (see Figure 5(b)) with an average value of 2.07 Å, are peculiarly long. Nevertheless, extraordinarily long M2–O distances have been observed in all the other known homeotypic phases ^{1 , 2 , 3 , 4 , 5} with the exception of Ga₄InB₁₂O₂₅(OH), ³ where the larger In atom shows common In–O bond lengths. Furthermore, in Table 4 the comparison of the distances to the values of the previously published homeotypic structure Ga₅B₁₂O₂₅(OH) ² shows that they are in good agreement. The majority of the values as well as the lattice parameters of Ga₅Na_1–x B₁₂O_26–x (OH) _x (x = 0.12) are slightly shorter than those of Ga₅B₁₂O₂₅(OH), ² which may be due to the lower measurement temperature (see Table 1). The Na–O distances are in the range between 2.5292(5) and 2.5711(6) Å (see Figure 5(c)) with a mean value of 2.55 Å, which corresponds to values in the literature of 2.65 Å for NaMn₇O₁₂ ²⁰ and 2.94 Å for NaTi₈O₁₃, ²¹ if one considers that the size of the cuboctahedral void predominantly depends on the surrounding framework. The effective ionic radius for Na⁺ in 12-fold coordination was estimated at 1.53 Å by Shannon in 1976. ²² Compared to the smallest diameter of the cuboctahedral void of the title compound of about 5.1 Å between two O atoms, there is sufficient space for Na⁺, giving rise to only in a minimal distortion. The B–O distances are in the range between 1.4624(9) and 1.5099(9) Å with average values of 1.48 Å for B1–O and B2–O, and 1.49 Å for B3–O. These values are in accordance with the mean value of 1.476(35) Å for B–O distances in BO₄ tetrahedra ²³ and with values commonly found in borates. ^{24 , 25 , 26}

Figure 5:

Oak Ridge Thermal Ellipsoid Plot Diagram (Ortep)-type representation of [Ga1₂O₁₀] (a) and [Ga2O₆] octahedra (b) and [Na1O₁₂] cuboctahedra (c) in Ga₅Na_1–x B₁₂O_26–x (OH) _x (x = 0.12) with displacement ellipsoids at the 99.99 % probability level. Bond lengths are depicted in Å. The values are shown here without standard deviation, but can be completely found in Table 4.

The angles can be found in Table 5, where they are compared to those in Ga₅B₁₂O₂₅(OH), ² where very similar values have been observed. Furthermore, the mean values for the octahedra show only a minimal deviation from the regular values of 90°, 180°, and those for the Na1 cuboctahedron with 60° and 120°. Individual angle values deviate within acceptable limits.

To further confirm the structure refinement, calculations of the bond-length/bond-strength values ²⁷ (BLBS, see Table 6), and the charge distribution ²⁸ (CHARDI, see Table 6) were performed. For this purpose, a full occupation of Na1 had to be set, because at a specific position inside the structure there is only the possibility of a full Na atom or none. According to the CHARDI-concept, ²⁸ only minimal deviations (<±1.7 %) of the calculated values compared to the regular values of +3 for Ga and B, +1 for Na, and −2 for O were observed. According to the BLBS-concept, ²⁷ most values also have only a slight deviation from the regular values (≤±8 %), but Ga2 and Na1 do not fit well, because of comparatively long bond lengths. However, compared to the homeotypic compounds, the long Ga2–O distances are indeed to be expected to be characteristic for this structure type ^{1 , 2 , 3 , 4 , 5} and the Na1–O distances may depend on the surrounding framework. Therefore, the very good values according to the CHARDI ²⁸ concept may be more crucial, due to its consideration of the spatial arrangement of atoms and their electron configuration.

3.2 IR spectroscopy

Figure 6 shows a FT-IR spectrum of a single crystal of Ga₅Na_1–x B₁₂O_26–x (OH) _x (x = 0.12) in comparison to published data of the homeotypic phase Ga₅B₁₂O₂₅(OH) ² in the spectral region of 600–4,000 cm⁻¹. In the fingerprint region up to ∼1,400 cm⁻¹, the peaks can be explained by vibrations of M–O, B–O or various overlapping combinations. Specifically, the stretching modes of [BO₄] tetrahedra are typically observed between 850 and 1,100 cm⁻¹, ²⁹ the characteristic frequencies of [AlO₆] octahedra between 750 and 400 cm⁻¹, ³⁰ which are both present in the experimental spectrum. The position of an O–H stretching vibration was observed in the homeotypic structure Ga₅B₁₂O₂₅(OH) ² at around 3,000 cm⁻¹. If we consider the small amount of O–H in Ga₅Na_1–x B₁₂O_26–x (OH) _x (x = 0.12) based on the single-crystal data, only a weak peak is expected, which seems to be present in the FT-IR spectrum.

Figure 6:

Single-crystal IR spectrum of Ga₅Na_1–x B₁₂O_26–x (OH) _x (x = 0.12) (black) compared to published data of the homeotypic structure Ga₅B₁₂O₂₅(OH) ² (blue) in the spectral region of 600–4,000 cm⁻¹ (both experimental data).

3.3 Energy-dispersive X-ray spectroscopy

The results of a semi-quantitative energy-dispersive X-ray (EDX) spectroscopy study confirmed the presence and approximate quantity (within 3 standard deviations or less) of all the elements identified by single-crystal X-ray diffraction with the exception of H, which cannot be detected with this method. Especially the measured sodium content of 1.9(3) at% (averaged from 14 point measurements) is in good agreement with the expected value of 2.0 at%. Figure 7 shows a representative crystal of the title compound, which was obtained by a scanning electron microscope (SEM).

Figure 7:

Scanning electron microscope (SEM) image of a representative crystal of Ga₅Na_1–x B₁₂O_26–x (OH) _x .