High-pressure synthesis and characterization of the non-centrosymmetric scandium borate ScB₆O₉(OH)₃

2.1 Crystal structure

ScB₆O₉(OH)₃ crystallizes with eight formula units (Z = 8) in the non-centrosymmetric orthorhombic space group Fdd2 (no. 43) with the lattice parameters a = 38.935(4), b = 4.4136(4), and c = 7.6342(6) Å and a volume of V = 1311.9(2) ų. All relevant crystallographic and refinement data can be found in Table 1.

Like the already known isotypic compound InB₆O₉(OH)₃ [13], the crystal structure is built up of triple layers of corner-sharing BO₄ tetrahedra, separated by scandium layers (Figure 1, middle). The BO₄ tetrahedra in the first and the third layer of these triples, with the alternating central boron atoms B1 and B2, both form six-membered rings, where all the tetrahedra point towards the same direction (Figure 1, bottom left and right). In the first layer, this direction is +a, whereas in the third layer the orientation is changed to −a. The third layer is shifted by 1/4 in the direction of b and 1/4 in the direction of c, in relation to the first layer. Very similar six-membered rings can be found in other high-pressure borates, for example in CdB₂O₄ [14] and α-FeB₂O₄ [15]. In contrast to ScB₆O₉(OH)₃ presented here, where all BO₄ tetrahedra show the same orientation in one layer – downwards (D) for the first and upwards (U) for the third layer – the sequences UDUDUD (for one-fourth of the six-membered rings) and UUUDDD are developed for the cadmium-compound, and UUDUDD for the iron-compound. In ScB₆O₉(OH)₃, the layers of six-membered rings are connected via an intermediate layer, consisting of corrugated bands of BO₄ tetrahedra. In this middle layer, there are two half-occupied boron and the accompanying oxygen positions, designated as B3/B3a and O6/O6a, respectively, where only one is occupied at a time (Figure 1, bottom middle), leading to layers consisting exclusively either of B3O₄ tetrahedra (light blue) or of B3aO₄ tetrahedra (orange). The bands run in direction of [01¯1¯] or [011¯] for the tetrahedra with the central cation B3 and in direction of [011] or [01¯1] for the tetrahedra with the central cation B3a (Figure 1, top). In contrast, the isotypic indium compound InB₆O₉(OH)₃ [13] features only one fully occupied boron position in this layer, which is identical to the boron position B3 here. It is also within this middle layer, where we presume two hydrogen atoms to be bound to O6 and O6a. Due to the only half occupied positions, they could not be assigned in the refinement process, but this assumption coincides with the result for the isotypic InB₆O₉(OH)₃ [13], where this position could be refined. This leads to two hydrogen atoms in the overall formula MB₆O₉(OH)₃ (M = Sc, In), where the third one is postulated based on charge neutrality reasons.

Figure 1:

Top: Different orientations of the disordered middle layer. Middle: Crystal structure of ScB₆O₉(OH)₃ along [010]. Bottom (from left to right): First layer of six-membered rings pointing towards +a; second layer of either the light blue or the orange corrugated BO₄ chains (half occupied B3/B3a position); third layer of six-membered rings pointing towards −a.

Looking at the B–O bond lengths in this structure, a distinction can be made for the boron atoms B1 and B2, where the B–O distances lie within a much narrower range than for the only half-occupied atoms B3 and B3a. For the first two boron atoms, the average B–O distance is 1.471 Å, which is in good agreement with the literature (1.476(35) Å) [16]. In contrast, the overall average value of 1.466 Å is markedly shorter. A similar situation is found for the O–B–O angles. While the angles for the B1O₄ and B2O₄ tetrahedra vary only slightly, the angles for the tetrahedra with the half occupied central boron positions lie within a wider range of 104.8(7) to 113.6(8)°. Nevertheless, the average values of 109.4 and 109.5° match well with the expected tetrahedral angle. All the bond lengths and angles are listed in Tables 2 and 3, the atomic coordinates and isotropic displacement parameters are found in Table 4.

The scandium cations separate the triple layers and are positioned between the centres of two six-membered rings (Figure 2). Here, the upper layer is rotated about 180° compared to the lower layer, which is the reason for the very long a axis of 38.935(4) Å. Because of this rotation of the triple layers against each other, four triple layers are needed to return to the initial position instead of two without the rotation. The Sc³⁺ cations between the anionic triple layers are sixfold coordinated by oxygen atoms in an octahedral alignment. The Sc–O distances are in a narrow range of 2.130(2) to 2.139(2) Å, with an average distance of 2.136 Å. This distance is significantly shorter than in the two other known high-pressure scandium borates Sc₃B₅O₁₂ (Ø Sc–O: 2.246 Å) [8] and Sc_1.76B₃O₇ (Ø Sc–O: 2.244 Å) [9]. In these already known compounds, the scandium is coordinated by eight oxygen atoms in the form of distorted square antiprisms in contrast to the octahedral coordination in ScB₆O₉(OH)₃. This sixfold coordination is related to the octahedral coordination of scandium in the ambient pressure compound ScBO₃. Moreover, the Sc–O distance in this compound is 2.1200(4) Å, which is not far from the value derived from the crystal structure of ScB₆O₉(OH)₃.

Figure 2:

Octahedrally coordinated scandium position between the borate layers and in the middle of two six-membered rings. Sc–O distances in Å.

Corresponding to the increased In³⁺ cation size compared to Sc³⁺, the cell parameters of the indium compound are slightly larger and also the B–O (Ø = 1.489 Å) and In–O distances (Ø = 2.177 Å) are longer than in the presented scandium compound [13]. Nevertheless, these values are in good agreement with distances found in other high-pressure indium borates [17], [18].

The calculation of bond valence sums according to the bond length/bond strength (ΣV) concept [19], [20] reveals a more positive formal charge for O6 (ΣV = −1.55) and O6a (ΣV = −1.66), respectively, indicating a missing coordination, which agrees with the assumption that these two atoms are the bonding partners for two hydrogen atoms. The resulting formal ionic charges for the other atoms correspond well with the expected values within the limits of this concept and are shown in Table 5.

2.2 X-ray powder diffraction

The comparison between the experimental powder diffraction data and the pattern derived from the single-crystal refinement are depicted in Figure 3. Although ScB₆O₉(OH)₃ is clearly the main reaction product, a certain amount of Sc₃B₅O₁₂ can be seen in the powder pattern, refined to a fraction of 10(1)%. Additionally, the difference curve suggests that at least one other unknown compound can be present. More details on the Rietveld refinement can be found in Table 1.

Figure 3:

Rietveld refinement of a ScB₆O₉(OH)₃ sample, containing the side phase Sc₃B₅O₁₂.

2.3 Vibrational spectroscopy

Figure 4 shows the IR spectrum of a ScB₆O₉(OH)₃ powder sample in the range from 400 to 3800 cm⁻¹. The peak at around 3200 cm⁻¹ can be attributed to stretching vibrations associated with the hydroxyl groups [21], thus confirming their existence in the structure. Even though the sample used for this measurement also contained a small amount of Sc₃B₅O₁₂, any contribution to the peak at the upper range of the spectrum can be ruled out, because this by-product does not contain any hydrogen atoms. The tetrahedral BO₄ groups typically lead to the absorption bands in the range of 800–1150 cm⁻¹ [22], [23], [24]. The broad absorption between ∼1300 and 1500 cm⁻¹ and the peaks below 800 cm⁻¹ most likely originate from complex O–B–O, B–O–B, B–O, and Sc–B–O vibrations, which was confirmed by quantum mechanical calculations on β-ZnB₄O₇ and β-CaB₄O₇, two high pressure compounds with complex borate frameworks consisting of crystallographically independent BO₄ tetrahedra and O₃B groups [25].

Figure 4:

IR spectrum of a ScB₆O₉(OH)₃ powder sample, also containing a certain amount of Sc₃B₅O₁₂.

4.1 Synthesis

The title compound ScB₆O₉(OH)₃ was first obtained through a high-pressure/high-temperature experiment at 6 GPa and 1473 K. The synthesis was performed in an 18/11 assembly in a hydraulic 1000 t press with a modified Walker-type module (both Max Voggenreiter GmbH, Germany). Sc(NO₃)₃·5H₂O (99.9%) and B₂O₃ (both Strem Chemicals, Inc., Newburyport, Massachusetts, USA, >99.9%) were grinded together in a stoichiometric ratio under ambient conditions and filled into a Pt capsule, which was placed in a crucible made of α-BN and closed with a lid of the same material (both Henze Boron Nitride Products AG, Germany). For a more detailed description of the experimental setup, the reader is referred to the literature [26], [27], [28].

The 18/11 assembly was compressed to 6 GPa within 150 min and heated to T = 1473 K in the following 10 min at a constant pressure. After keeping this temperature for the next 30 min, the sample was cooled to T = 773 K within another 180 min. Then, the sample was quenched to room temperature by switching off the heating and a 450 min decompression process was started. The recovered pressure medium was broken apart and the product carefully separated from the surrounding crucible and Pt capsule. A mixture of ScB₆O₉(OH)₃ and Sc₃B₅O₁₂ appeared as colourless platelets.

4.2 Powder X-ray diffraction

A flat sample of the reaction product, containing ScB₆O₉(OH)₃ and Sc₃B₅O₁₂, was characterized by powder diffraction analysis in transmission geometry on an STOE Stadi P powder diffractometer (STOE & Cie GmbH, Darmstadt, Germany) equipped with a Mythen 1K microstrip detector (Dectris). The measurement was carried out with Ge(111)-monochromatized MoKα₁ radiation (λ = 70.93 pm) in the 2θ range of 2–52° with a step size of 0.015°. For a comparison with the values derived from the single-crystal structure analysis, a Rietveld refinement was performed employing the Topas 4.2 software [29].

4.3 Single-crystal X-ray analysis

The single-crystal structure determination was performed on a Bruker D8 Quest Kappa diffractometer with monochromatized MoKα radiation (λ = 71.07 pm), equipped with an Incoatec microfocus X-ray tube and a Photon 100 CMOS detector. The structure solution and parameter refinement were performed with Direct Methods using Shelxs [30], [31] and the refinement against |F_o|² was carried out with Shelxl, both implemented in the WinGX software [32]. Despite measuring the crystal at T = 183(2) K, it was not possible to unambiguously identify the position of the hydrogen atoms, also attributed to the half-occupied layer in which these atoms are expected to be found. All non-hydrogen atoms were refined with anisotropic displacement parameters.

Further details of the crystal structure investigation of ScB₆O₉(OH)₃ may be obtained from The Cambridge Crystallographic Data Centre CCDC/FIZ Karlsruhe deposition service via www.ccdc.cam.ac.uk/structures on quoting the deposition number CCDC 2000250.

4.4 Vibrational spectroscopy

For further characterization and to proof the existence of the postulated hydroxy groups in this compound, an FTIR-ATR (Attenuated Total Reflection) spectrum of a powder sample of ScB₆O₉(OH)₃ was recorded. A Bruker ALPHA Platinum-ATR spectrometer (Bruker, Billerica, USA) with a 2 × 2 mm diamond ATR-crystal and a DTGS detector were used to collect the spectrum in the spectral range of 400–3800 cm⁻¹. A total of 320 scans of the powder sample were obtained and corrected for atmospheric influences afterwards, employing the Opus 7.2 software [33].