Hydrothermal synthesis of a new lead(II) borate (Pb₄O)Pb₂B₆O₁₄-II

2.1 Synthesis

The compound (Pb₄O)Pb₂B₆O₁₄-II was prepared via a hydrothermal synthesis. In detail, Pb(BO₂)₂·H₂O [344 mg, 1.1 mmol, p.a., Alfa Aesar (Karlsruhe, Germany)], H₂O (3 mL), and a few drops of a 2 m aqueous KOH solution (pH value ~10) were sealed in a Teflon-lined stainless steel autoclave (8 mL), heated to and kept at 483 K for 4 days and afterwards slowly cooled down to 323°K (2 Kh⁻¹). After removing the autoclave from the circulating air oven and cooling down to room temperature, transparent crystals in a clear liquid solution were obtained and placed in the drying oven over night. The powder was analysed by X-ray powder diffraction. Furthermore, single colorless needles of the modification (Pb₄O)Pb₂B₆O₁₄-II were isolated and measured by single-crystal X-ray diffraction.

Notably, the use of alkali nitrates like potassium nitrate or sodium nitrate in combination with lead borate as starting materials leads to the crystallization of the compound Pb₆B₁₂O₂₁(OH)₆. However, if no alkali nitrate as starting material was used, a good crystal growth of the compound (Pb₄O)Pb₂B₆O₁₄-II was observed, but no measurable crystals of the compound Pb₆B₁₂O₂₁(OH)₆ could be obtained. This means that the presence of an alkali nitrate in the starting materials favors the crystal growth of the compound Pb₆B₁₂O₂₁(OH)₆.

For the preparation of the NCS compound (Pb₄O)Pb₂B₆O₁₄-I [25], PbO, H₃BO₃, and KOH were mixed and sealed in a FEP Teflon pouch (“Teflon pouch method”). Afterwards, the pouch was put in a 100 mL Teflon-lined autoclave and backfilled with 30 mL of distilled water. The autoclave was sealed and heated at 210°C for 4 days. Afterwards, the reaction product was slowly cooled down to room temperature leading to colorless (Pb₄O)Pb₂B₆O₁₄-I platelets [25].

2.2 X-ray structure determination

The reaction product was measured by X-ray powder diffraction in transmission geometry with MoK_α1 (λ=70.93 pm) radiation applying a focusing Ge(111) primary beam monochromator and a Mythen 2 DCS4 detector (Dectris^®, Baden, Switzerland). Figure 1 (center) shows the experimental powder pattern of the reaction product in comparison with the theoretical pattern simulated from the single-crystal data of (Pb₄O)Pb₂B₆O₁₄-II [Fig. 1 (top)]. The reflections marked with red asterisks result from the compound Pb₆B₁₂O₂₁(OH)₆ [21] representing the main phase of the reaction product. For comparison, the theoretical pattern stemming from the single-crystal data of the structure Pb₆B₁₂O₂₁(OH)₆ is shown in Fig. 1 (bottom). The corresponding reflections of the phase (Pb₄O)Pb₂B₆O₁₄-II were indexed and refined [29]. The lattice parameters fit well with the parameters obtained from the single-crystal data (see Table 2). The structural refinement of the Rietveld analysis (Fig. 2) was performed with the program Topas 5.2 [30] by using the powder pattern and crystal structure parameters of Pb₆B₁₂O₂₁(OH)₆ and (Pb₄O)Pb₂B₆O₁₄-II. The Rietveld analysis led to the result that the new modification (Pb₄O)Pb₂B₆O₁₄-II was obtained as a side phase with a fraction of 23% surrounded by the lead borate Pb₆B₁₂O₂₁(OH)₆ as the major phase with an amount of 77%. The characterization and synthesis of the lead(II) borate Pb₆B₁₂O₂₁(OH)₆ was described in detail in ref. [21]. The crystallographic data obtained from the Rietveld refinement are listed in Table 2.

Fig. 1:

Top: theoretical powder pattern (MoK_α radiation) obtained from single-crystal data of the compound (Pb₄O)Pb₂B₆O₁₄-II. Centre: experimental powder pattern of the product of the hydrothermal synthesis of (Pb₄O)Pb₂B₆O₁₄-II. The reflections marked with red asterisks stem from the compound Pb₆B₁₂O₂₁(OH)₆ [21]. Bottom: theoretical powder pattern obtained from single-crystal data of the compound Pb₆B₁₂O₂₁(OH)₆.

Fig. 2:

Experimental (black) and calculated patterns of (Pb₄O)Pb₂B₆O₁₄-II (red) and Pb₆B₁₂O₂₁(OH)₆ [21] (green) including the difference curve (blue).

For the single-crystal structure analysis, colorless needles of (Pb₄O)Pb₂B₆O₁₄-II were isolated through polarization contrast microscopy and analyzed via single-crystal X-ray diffraction. The intensity data were collected at room temperature with a Bruker D8 Quest diffractometer (Photon 100) equipped with an Incoatec Microfocus source generator (MoK_α radiation, λ=71.073 pm, monochromatized by multi-layered optics). Multi-scan absorption corrections were applied with the program Sadabs-2014/5 [31]. In addition, a numerical absorption correction was applied to the intensity data (Habitus [32]). As higher symmetry was not observed, the triclinic space groups P1 (no. 1) and P1̅ (no. 2) were derived. After the structure solution and parameter refinement with anisotropic displacement parameters for all atoms using the Shelxs/l-97 software suite [33], [34], the space group P1̅ was found to be correct. All relevant details of the data collections and evaluations are given in Table 3. The atomic coordinates and isotropic equivalent displacement parameters, interatomic distances, and angles are listed in the Tables 4–7.

Further details of the crystal structure investigations may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: crysdata@fiz-karlsruhe.de, https://icsd.fiz-karlsruhe.de/search/basic.xhtml) on quoting the deposition number CSD-431518.

2.3 Vibrational spectroscopy

The transmission FT-IR spectrum of a single crystal of the compound (Pb₄O)Pb₂B₆O₁₄-II was measured in the spectral range of 400–4000 cm⁻¹ with a Bruker Vertex 70 FT-IR spectrometer (spectral resolution 4 cm⁻¹) equipped with a KBr beam splitter and attached to a Hyperion 3000 microscope. A LN-MCT (Mercury Cadmium Telluride) detector was used in absorption mode to obtain the spectrum of a single crystal. As mid-infrared source, a Globar (silicon carbide) rod and a 15×IR objective as focus were used, whereby 320 scans of the single crystal placed on a BaF₂ sample holder were acquired. A correction of atmospheric influences was performed with the software Opus 6.5 [35].

3.1 Crystal structure

In contrast to the modification (Pb₄O)Pb₂B₆O₁₄-I of Zhang et al. [25] crystallizing in the NCS space group P1 (no. 1), the new polymorph (Pb₄O)Pb₂B₆O₁₄-II crystallizes in the centrosymmetric space group P1̅ (no. 2). Therefore, both structures differ significantly in their cell parameters. Due to the centric space group, the c axis in (Pb₄O)Pb₂B₆O₁₄-II is twice as long as the corresponding b axis in the NCS structure of Zhang et al. To confirm the centrosymmetric space group with a doubled length of the c axis, the 0kl and 1kl layers, which were simulated from the single-crystal data, are shown in Fig. 3. Table 8 shows a comparison of the lattice parameters and the crystal data of both modifications.

Fig. 3:

(a) Illustration of the 0kl layer and in (b) of the 1kl layer. The reflections along the c* axis can be seen, which lead to doubling of the c axis in real space and to the halving of the c* axis in reciprocal space.

The crystal structure of (Pb₄O)Pb₂B₆O₁₄-II is built up from B₆O₁₄ groups which consist of two crystallographically different B₃O₈ rings. The fundamental building block (FBB) consists of two BO₄ tetrahedra and one BO₃ triangle described as Δ2□:〈Δ2□〉 [36], whereby two BO₄ tetrahedra and one BO₃ triangle are connected to form a B₃O₈ ring (Fig. 4). A rotation by 180° of this ring leads to the second crystallographically different B₃O₈ ring as shown in Fig. 4. The two crystallographically different rings are linked by the oxygen atom O6 to form a B₆O₁₄ group. Further connection of this B₆O₁₄ groups to each other by sharing the oxygen atoms O10 and O2 leads to infinite chains of linked B₃O₈ rings along the b axis (Fig. 4). It should be briefly mentioned that the FBB in the publication of Zhang et al. is given by the descriptor 2Δ4□:〈Δ2□〉〈Δ2□〉 (two rings) [25]. Due to the fact that the basis is still a ring consisting of two BO₄ tetrahedra and one trigonal BO₃ group, which is duplicated, rotated, and linked to the first one, the descriptor can be simplified to Δ2□:〈Δ2□〉. The Pb²⁺ cations are arranged around the anionic borate chains. The oxygen atom O15 is an exception because it is surrounded by four lead atoms Pb3–Pb6 to form a cationic distorted OPb₄ tetrahedron with Pb–O bond lengths in the range of 222.2(4)–240.7(4) pm (Fig. 5). The Pb–Pb distances in this distorted OPb₄ tetrahedron range from 345.5(3) to 367.9(4) pm. The structure of (Pb₄O)Pb₂B₆O₁₄-II can be described as the centrosymmetric modification of (Pb₄O)Pb₂B₆O₁₄-I of Zhang et al., forming infinite anti-parallel B₆O₁₄ chains connected via OPb₄ tetrahedra and Pb²⁺ cations [25].

Fig. 4:

Single anionic borate chain along the b axis of the new centrosymmetric modification (Pb₄O)Pb₂B₆O₁₄-II with the two crystallographically different rings [37]. The ring forming the Fundamental Building Block (FBB) Δ2□:〈Δ2□〉 is encircled in red.

Fig. 5:

Coordination sphere of the oxygen atom O15 displayed with interatomic Pb–O distances.

The boron positions B1–B4 are coordinated by four oxygen anions forming corner sharing BO₄ tetrahedra. The B–O distances in the tetrahedra are in the range 145.4(8)–150(7) pm with an average value of 148.5 pm, fitting well to the known average value of 147.6 pm which is generally found in borates [36], [38]. The bond angle values range from 104.0(4) to 113.0(5)° with an average value of 108.5°, which is close to the ideal tetrahedron angle of 109.5° [38].

The positions B5 and B6 are threefold coordinated by oxygen atoms forming BO₃ groups. The bond angle values range from 118.3(5) to 122.4(5)° and the B–O distances from 135.5(7) to 138.(7) pm, with an average of 136.9 pm. This value agrees very well with the literature known B–O distance of 137 pm [36].

There are six crystallographically unique Pb atoms, which possess different coordination environments (Fig. 6). Pb6 is enclosed by five oxygen atoms in the range of 222.1(4)–289.3(4) pm (average value: 248.8 pm) and Pb2 is enclosed by four oxygen atoms in the range of 224.0(4)–255.8(4) pm (average value: 233.1 pm). Pb1 and Pb3–Pb5 are coordinated by six oxygen atoms in the range of 227.8(4)–299.2(4) pm, with an average of 257.6 pm. The average values fit very well with the mean Pb–O distance of 257.4 pm found in the literature, where irregular coordination polyhedra around lead cations are described [13], [16]. From Fig. 6 it is obvious that all Pb²⁺ cations exhibit a stereochemically active 6s² lone pair of electrons directed into the open flank of each coordination environment.

Fig. 6:

Coordination spheres of the lead cations in (Pb₄O)Pb₂B₆O₁₄-II. The oxygen atom O15, which is coordinated to four lead atoms forming an OPb₄ tetrahedron, is displayed in pink.

Using the bond-length/bond-strength (BLBS) and the CHARDI concept, the bonding valences of oxygen, boron, and lead atoms were calculated to be in the ranges from −1.67 up to −2.35, +2.96 to +3.04, and +1.72 to +2.19 (Table 9), respectively. All calculated values are consistent within the limits of both concepts. The bold values in Table 9 represent the lead-coordinated oxygen atoms O15 in (Pb₄O)Pb₂B₆O₁₄-II and O14 in the modification of Zhang et al. [25]. With values of −2.35 (BLBS) and −2.20 (CHARDI) for the tetrahedrally coordinated oxygen atom O15, the bond valence sums are significantly higher in comparison with the remaining oxygen atoms as shown in Table 9 (bold values). Comparing those two values with Zhang’s data from the BLBS and CHARDI concept, it is noteworthy that our BLBS value of −2.35 is slightly higher than Zhang’s value of −1.96. However, the calculated values based on the CHARDI concept show nearly identical values of −2.20 for (Pb₄O)Pb₂B₆O₁₄-II and −2.21 for (Pb₄O)Pb₂B₆O₁₄-I as shown in Table 9.

Both crystal structures consist of the same B₆O₁₄ double rings, which are joined together to form infinite chains. Figure 7 shows the unit cells of both modifications. In the new compound (Pb₄O)Pb₂B₆O₁₄-II, the infinite B₆O₁₄ chains shown in Fig. 8 top (left side, chain 1 and 2) are anti-parallel to each other. Therefore, the chains are crystallographically not identical. Also the OPb₄ tetrahedra are oriented differently, one is directed with its tip to the right and the other one to the left as shown in Fig. 8 (top, left side). In contrast, all chains in (Pb₄O)Pb₂B₆O₁₄-I (Fig. 8 (top, right side), chain 1 and 2) are oriented parallel, and consequently all oxygen-centered OPb₄ tetrahedra are oriented in the same direction. Figure 8 (top, left side) shows the borate chain (encircled in red), which is identical to the chains in the structure of Zhang et al. [25]. To obtain a direct comparison between (Pb₄O)Pb₂B₆O₁₄-I and (Pb₄O)Pb₂B₆O₁₄-II, the 2×2 unit cells of the new modification and of the compound (Pb₄O)Pb₂B₆O₁₄-I are shown in Fig. 8a and b (bottom).

Fig. 7:

(a) Unit cell of the centrosymmetric compound (Pb₄O)Pb₂B₆O₁₄-II with two anti-parallel borate chains and two oxygen-centered OPb₄ tetrahedra, which are aligned in different directions, as viewed along the crystallographic a axis; (b) the unit cell of the (Pb₄O)Pb₂B₆O₁₄-I modification of Zhang et al. [25] with the parallel borate chains and one oxygen-centered OPb₄ tetrahedron viewed along the b axis.

Fig. 8:

On the left side (top): Two anti-parallel borate chains 1 and 2 of the modification (Pb₄O)Pb₂B₆O₁₄-II viewed along the crystallographic a axis. a) Bottom: 2×2 unit cells of (Pb₄O)Pb₂B₆O₁₄-II along the a axis. On the right side (top): The two parallel borate chains 1 and 2 of the (Pb₄O)Pb₂B₆O₁₄-I [25] modification along the a axis. b) Bottom: 2×2 unit cells of the (Pb₄O)Pb₂B₆O₁₄-I compound of Zhang et al. along the a axis. The red encircled borate chain is identical to the chains in the structure of Zhang. O²⁻: small blue spheres at the corners of the tetrahedra and of the trigonal-planar groups; B³⁺: small red spheres; Pb²⁺: green spheres.

3.2 FT-IR spectroscopy

Figure 9 shows the IR spectrum of (Pb₄O)Pb₂B₆O₁₄-II in the range of 400–4000 cm⁻¹. The existence of triangular BO₃ and tetrahedral BO₄ groups in the structure was confirmed. The strong bands between 1250 and 1400 cm⁻¹ can be attributed to the asymmetric stretching of BO₃ groups and the absorption peak at 1000 cm⁻¹ can be attributed to the symmetric stretching of BO₃ groups. Furthermore, the absorption peaks of the asymmetric and symmetric stretching of BO₄ groups are located in the range between 1025 and 900 cm⁻¹. In the range between 500 and 700 cm⁻¹, the bending vibrations of BO₃ and BO₄ groups are found. The weak band at 3500 cm⁻¹, which is normally assigned to O–H stretching vibrations, could come from small amounts of absorbed water on the surface of the crystal [25].

Fig. 9:

FT-IR absorbance spectrum of a single-crystal of (Pb₄O)Pb₂B₆O₁₄-II in the range of 400–4000 cm⁻¹.