Abstract
A new centrosymmetric modification of the lead borate (Pb4O)Pb2B6O14 has been obtained as a side phase through a facile hydrothermal reaction from Pb(BO2)2·H2O, H2O, and KOH as starting materials. The compound (Pb4O)Pb2B6O14-II crystallizes in the space group P1̅ (no. 2) with the lattice parameters a=695.9(3), b=778.0(3), c=1408.3(3) pm, α=97.35(1)°, β=100.39(1)°, and γ=103.02(1)°. The structure consists of anti-parallel arranged B6O14 chains and isolated oxygen-centered OPb4 tetrahedra. The compound Pb6B12O21(OH)6 constitutes the major phase of the synthesis, as verified through a Rietveld analysis. The characterization of (Pb4O)Pb2B6O14-II is based on a Rietveld analysis, single-crystal X-ray diffraction data, and FT-IR spectroscopy.
1 Introduction
Borates play a strategic role in the design of new nonlinear optical (NLO) materials. Especially those borates, which are active in the UV (ultraviolet) and deep UV region, have become increasingly important, because they are of great interest in photochemical synthesis and scientific instruments, e.g. deep-UV photoemission electron microscopy (DUV-PEEM), ultrahigh-resolution, and angle-resolved photoemission spectrometry (URPEA and ARPES). Li2B4O7 [1], LiB3O5 [2], and the low temperature modification β-BaB2O4 (β-BBO) [2], [3], [4] were the first materials showing high quality UV-NLO properties. Especially, deep-UV NLO materials, which can produce coherent light of wavelengths below 200 nm, play an important and unique part in laser science and technology. In the field of deep-UV generating materials, KBe2BO3F2 (KBBF) [5], [6], [7], γ-KBe2B3O7 [8], Na2CsBe6B5O15 [9], Na2Be4B4O11 [10], and LiNa5Be12O33 [10] are the sole NLO crystals which can generate deep-UV coherent light by direct second-harmonic generation (SHG) processes [10], [11]. Therefore, new borates with excellent NLO properties attracted more and more attention in recent years [11]. Especially non-centrosymmetric (NCS) building units like BO3 groups, BO4 tetrahedra, combinations of these, and additional structural distortions originated through stereochemically active lone pairs of certain metal cations enable the generation of nonlinear optical effects [12].
One class of compounds which fulfill these requirements are lead borates, with several representatives described in the literature as, e.g. Pb5(B3O8(OH)3)H2O [13], Pb2B3O5.5(OH)2 [14], Pb6B11O18(OH)9 [15], PbB4O7 [16], Pb6B10O21 [17], Pb2[B5O9](OH)·H2O [18], Pb[B6O10(OH)·B2O(OH)3] [19], Pb6B12O24·H2O [20], and Pb6B12O21(OH)6 [21]. In this publication, our main focus lies on lead borates including OPb4 tetrahedra as special units. Table 1 gives a brief overview of the already existing borate compounds containing OPb4 tetrahedra.
Species | Space group | Synthesis | Structure characteristic | Reference |
---|---|---|---|---|
Pb4O(BO3)2 | Aba2 | Solid-state reaction (300–430°C for 4 h) | Pb4O10 tetramers are connected to each other by sharing oxygen atoms to form a network structure | [22] |
Pb2O[BO2(OH)] | C2/m | Hydrothermal (180°C for 24 h) | [Pb2O]2+ chains and edge-sharing OPb4 tetrahedra, which are connected through orthoborate ligands | [23] |
Pb6B4O11(OH)2 | Pnma | Hydrothermal (170°C for 6 days) | Infinite [Pb6O4] chains connected through B4O9 linkers | [24] |
Pb4O(Pb2B6O14)-I | P1 | Hydrothermal (210°C for 4 days) | Paralleled spiral B6O14 chains along the c axis, connected by Pb2+ cations and isolated Pb4O tetrahedra, which are all oriented in the same direction | [25] |
Pb4O(Pb2B6O14)-II | P1̅ | Hydrothermal (210°C for 4 days) | Infinite B6O14 chains along the b axis connected by Pb2+ cations and isolated Pb4O tetrahedra, which are oriented differently | this work |
Obviously, the system PbO–B2O3 has been widely investigated because of its special features, e.g. high asymmetric bonding, sufficient mechanical strength, and transparency in the deep UV region [22]. From the structure point of view, it is well known that the boron atoms in the system PbO-B2O3 can coordinate to three or four oxygen atoms to form BO3 triangles or BO4 tetrahedra. These fundamental units can be linked to more complicated BxOy building blocks. Furthermore, it also should be mentioned that the Pb2+ cations can constitute a special form of anion-centered OPb4 tetrahedra [25]. Several minerals and synthetic inorganic compounds feature anion-centred polyhedra [XA4], wherein X is O2− or sometimes N3−, and A are, in particular, the lone-pair cations Pb2+ and Bi3+, the rare earth cations RE3+ and Cu2+, but also even Be2+, Zn2+, Ca2+, and Sr2+ [26]. On the basis of the relatively long A–X distances, the repulsive forces between the X anions are relatively weak. One consequence is that anion-centred [XAm] polyhedra cannot only share corners but also edges and faces. Interestingly, these features lead to a markedly enhanced structural diversity of borates and silicates [23], [26], [27], [28]. Compounds built up from anion-centered tetrahedra, and especially oxo-centered lead tetrahedra, possess interesting physical properties like ion-conducting or nonlinear optical (NLO) properties. The ion-conducting properties are sometimes related to a certain cationic disorder inside of the oxo-centered frameworks of these compounds [23].
The lead borate (Pb4O)Pb2B6O14 was recently published by Zhang et al. [25]. To distinguish between (Pb4O)Pb2B6O14 of Zhang et al. and the here presented new modification with the identical composition (Pb4O)Pb2B6O14, we designate the original compound as (Pb4O)Pb2B6O14-I and the new modification as (Pb4O)Pb2B6O14-II. The original compound (Pb4O)Pb2B6O14-I crystallizes in the NCS polar space group P1 with the lattice parameters a=695.36(13), b=720.26(13), and c=780.03(14) pm, α=76.248(11)°, β=76.694(11)°, and γ=73.982(12)°. Structurally, this compound is built up from parallel B6O14 chains, which are connected by Pb2+ cations and isolated oxygen-centered OPb4 tetrahedra [25].
The new modification (Pb4O)Pb2B6O14-II, prepared under moderate hydrothermal conditions, is built up from anti-parallel B6O14 chains and isolated OPb4 tetrahedra representing the centrosymmetric version of the lead borate (Pb4O)Pb2B6O14-I.
2 Experimental section
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 MoKα1 (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.
Empirical formula | Pb6B12O21(OH)6 | (Pb4O)Pb2B6O14-II |
Radiation; wavelength, pm | MoKα1λ=70.93 | |
Crystal system | Trigonal | Triclinic |
Space group | P32 (no. 145) | P1̅ (no. 2) |
a, pm | 1176.87(2) | 696.46(7) |
b, pm | 778.73(1) | |
c, pm | 1333.92(3) | 1409.28(2) |
α, deg | 97.34(2) | |
β, deg | 100.38(1) | |
γ, deg | 103.02(2) | |
Calculated density, g cm−3 | 5.62 | 7.13 |
Z | 3 | 2 |
Wt% – Rietveld | 77 | 23 |
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 (MoKα 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.
Empirical formula | Pb6B6O15 |
Molar mass, g mol−1 | 1548.00 |
Crystal system | Triclinic |
Space group | P1̅ (no. 2) |
Single-crystal diffractometer | Bruker D8 QUEST PHOTON 100 |
Radiation; wavelength, pm | MoKα; λ=71.07 |
a, pm | 695.91(3) |
b, pm | 778.00(3) |
c, pm | 1408.34(5) |
α, deg | 97.35(1) |
β, deg | 100.39(1) |
γ, deg | 103.02(1) |
V, nm3 | 0.7194(5) |
Formula units per cell, Z | 2 |
Calculated density, g cm−3 | 7.15 |
Crystal size, mm3 | 0.010×0.035×0.080 |
Temperature, K | 293(2) |
F(000), e | 1284 |
Absorption coefficient, mm−1 | 70.0 |
2θ range, deg | 5.5–70.0 |
Range in hkl | ±11, ±12, ±22 |
Total no. of reflections | 33716 |
Independent reflections/Rint | 6348/0.0543 |
Reflections with I>2 σ(I) | 5212 |
Absorption correction | multi-scan (Bruker Sadabs 2014/5) |
Data/ref. parameters | 6348/244 |
Goodness-of-fit on Fi2 | 1.042 |
Final R1/wR2 [I>2 σ(I)] | 0.0270/0.0471 |
R1/wR2 (all data) | 0.0424/0.0498 |
Largest diff. peak/hole, e Å−3 | 2.59/−2.46 |
Atom | X | y | z | Ueq |
---|---|---|---|---|
Pb1 | 0.0630(3) | 0.7564(3) | 0.8961(2) | 0.0081(4) |
Pb2 | 0.6020(3) | 0.8966(3) | 0.8748(2) | 0.0090(4) |
Pb3 | 0.5534(3) | 0.3852(3) | 0.8329(2) | 0.0106(4) |
Pb4 | 0.7141(3) | 0.5721(3) | 0.6421(2) | 0.0107(4) |
Pb5 | 0.1553(3) | 0.2261(3) | 0.5939(2) | 0.0090(4) |
Pb6 | 0.6297(3) | 0.0727(3) | 0.6298(2) | 0.0089(4) |
O1 | 0.1229(6) | 0.4274(5) | 0.8945(3) | 0.0099(8) |
O2 | 0.8637(6) | 0.8951(5) | 0.8011(3) | 0.0083(7) |
O3 | 0.7190(6) | 0.5776(5) | 0.6096(3) | 0.0073(7) |
O4 | 0.8658(6) | 0.5030(5) | 0.7865(3) | 0.0058(7) |
O5 | 0.8929(6) | 0.1973(5) | 0.7682(3) | 0.0057(7) |
O6 | 0.8517(6) | 0.4233(5) | 0.7272(3) | 0.0074(7) |
O7 | 0.1483(6) | 0.7441(5) | 0.7414(3) | 0.0081(7) |
O8 | 0.4559(6) | 0.9979(5) | 0.7427(3) | 0.0093(8) |
O9 | 0.4789(6) | 0.6292(5) | 0.7763(3) | 0.0100(8) |
O10 | 0.8867(6) | 0.9679(5) | 0.2850(3) | 0.0099(8) |
O11 | 0.2449(6) | 0.3301(5) | 0.0420(3) | 0.0106(8) |
O12 | 0.1458(6) | 0.1287(5) | 0.8908(3) | 0.0101(8) |
O13 | 0.7839(6) | 0.1388(5) | 0.4076(3) | 0.0097(8) |
O14 | 0.1864(7) | 0.6530(6) | 0.4509(3) | 0.0127(8) |
O15 | 0.5021(6) | 0.3069(5) | 0.6576(3) | 0.0100(8) |
B1 | 0.2751(9) | 0.6001(8) | 0.7165(4) | 0.0059(1) |
B2 | 0.0038(1) | 0.0606(8) | 0.7941(5) | 0.0081(1) |
B3 | 0.2439(1) | 0.9106(8) | 0.7011(5) | 0.0080(1) |
B4 | 0.0053(9) | 0.3862(8) | 0.7921(5) | 0.0082(1) |
B5 | 0.1738(9) | 0.2959(9) | 0.9430(5) | 0.0083(1) |
B6 | 0.2255(9) | 0.6940(8) | 0.5501(5) | 0.0067(1) |
Ueq is defined as one third of the trace of the orthogonalized Uij tensor (standard deviations in parentheses). All atoms are positioned on the same Wyckoff position 2i (x, y, z).
Atom | U11 | U22 | U33 | U23 | U13 | U12 |
---|---|---|---|---|---|---|
Pb1 | 0.011(2) | 0.011(2) | 0.025(2) | 0.002(2) | 0.003(2) | 0.004(2) |
Pb2 | 0.016(2) | 0.012(2) | 0.017(2) | −0.000(2) | 0.001(2) | 0.008(2) |
Pb3 | 0.011(2) | 0.013(2) | 0.016(2) | 0.000(2) | 0.001(2) | 0.004(2) |
Pb4 | 0.011(2) | 0.017(2) | 0.014(2) | 0.002(2) | 0.000(2) | 0.007(2) |
Pb5 | 0.014(2) | 0.018(2) | 0.016(2) | −0.004(2) | −0.003(2) | 0.011(2) |
Pb6 | 0.012(2) | 0.015(2) | 0.015(2) | 0.001(2) | 0.001(2) | 0.008(2) |
O1 | 0.009(3) | 0.004(2) | 0.018(3) | 0.004(2) | −0.001(2) | −0.002(2) |
O2 | 0.011(3) | 0.008(2) | 0.012(2) | 0.000(2) | −0.002(2) | 0.001(2) |
O3 | 0.015(3) | 0.006(2) | 0.022(3) | −0.003(2) | −0.006(2) | 0.005(2) |
O4 | 0.010(3) | 0.012(3) | 0.009(2) | 0.001(2) | −0.000(2) | 0.006(2) |
O5 | 0.007(2) | 0.017(3) | 0.015(3) | −0.006(2) | −0.003(2) | 0.006(2) |
O6 | 0.008(3) | 0.016(3) | 0.009(2) | −0.001(2) | 0.000(2) | 0.005(2) |
O7 | 0.017(3) | 0.007(3) | 0.029(4) | 0.002(2) | 0.005(3) | 0.003(3) |
O8 | 0.010(3) | 0.014(3) | 0.019(3) | −0.001(2) | −0.005(2) | 0.005(2) |
O9 | 0.014(3) | 0.010(3) | 0.031(4) | 0.005(3) | 0.007(3) | 0.004(3) |
O10 | 0.014(3) | 0.014(3) | 0.013(3) | −0.001(2) | −0.004(2) | 0.009(2) |
O11 | 0.023(3) | 0.014(3) | 0.010(2) | −0.001(2) | −0.003(2) | 0.016(3) |
O12 | 0.012(3) | 0.011(3) | 0.013(3) | 0.000(2) | −0.005(2) | 0.005(2) |
O13 | 0.009(2) | 0.009(3) | 0.010(2) | −0.002(2) | −0.002(2) | 0.004(2) |
O14 | 0.010(3) | 0.016(3) | 0.011(3) | 0.002(2) | 0.002(2) | 0.010(2) |
O15 | 0.016(3) | 0.018(3) | 0.010(2) | 0.002(2) | 0.001(2) | 0.013(3) |
B1 | 0.011(4) | 0.009(4) | 0.009(4) | 0.000(3) | 0.001(3) | 0.004(3) |
B2 | 0.008(3) | 0.005(3) | 0.009(3) | 0.001(3) | −0.001(3) | 0.002(3) |
B3 | 0.009(4) | 0.007(3) | 0.009(3) | −0.001(3) | 0.000(3) | −0.001(3) |
B4 | 0.009(4) | 0.010(3) | 0.009(3) | −0.001(3) | −0.001(3) | 0.006(3) |
B5 | 0.015(4) | 0.005(3) | 0.008(3) | −0.003(2) | −0.002(3) | 0.004(3) |
B6 | 0.011(4) | 0.015(4) | 0.006(3) | 0.000(3) | 0.001(3) | 0.008(3) |
Pb1–O2 | 229.2(4) | Pb4–O4 | 228.3(4) | B1–O3 | 150.3(7) | B4–O4 | 147.1(7) |
Pb1–O41 | 232.1(4) | Pb4–O15 | 231.1(4) | B1–O6 | 149.9(7) | B4–O5 | 146.4(7) |
Pb1–O72 | 244.5(4) | Pb4–O146 | 235.4(4) | B1–O7 | 146.3(7) | B4–O14 | 148.0(8) |
Pb1–O113 | 245.5(4) | Pb4–O9 | 277.0(4) | B1–O94 | 146.5(7) | B4–O612 | 147.5(7) |
Pb1–O1 | 268.2(4) | Pb4–O3 | 297.7(4) | ∅ =148.3 | ∅=147.3 | ||
Pb1–O12 | 283.7(4) | Pb4–O2 | 299.2(4) | ||||
∅=250.5 | ∅=261.5 | B2–O2 | 145.6(7) | B5–O1 | 137.6(7) | ||
B2–O5 | 149.3(7) | B5–O11 | 136.3(7) | ||||
Pb2–O9 | 224.0(4) | Pb5–O6 | 228.3(4) | B2–O1011 | 147.9(7) | B5–O121 | 136.4(8) |
Pb2–O2 | 225.3(4) | Pb5–O15 | 232.8(4) | B2–O12 | 148.6(8) | ∅=136.8 | |
Pb2–O8 | 227.4(4) | Pb5–O107 | 243.2(4) | ∅=147.9 | |||
Pb2–O113 | 255.8(4) | Pb5–O3 | 264.4(4) | B6–O3 | 138.2(7) | ||
∅=233.1 | Pb5–O148 | 274.9(4) | B3–O8 | 145.4(8) | B6–O136 | 138.0(7) | |
Pb5–O13 | 296.1(4) | B3–O71 | 149.0(7) | B6–O14 | 135.5(7) | ||
Pb3–O94 | 227.8(4) | ∅=256.6 | B3–O107 | 147.0(7) | ∅=137.2 | ||
Pb3–O4 | 240.0(4) | B3–O137 | 149.6(7) | ||||
Pb3–O15 | 240.7(4) | Pb6–O15 | 222.1(4) | ∅=147.8 | |||
Pb3–O115 | 258.7(4) | Pb6–O8 | 222.3(4) | ||||
Pb3–O8 | 299.0(4) | Pb6–O5 | 235.1(4) | ||||
Pb3–O6 | 303.8(4) | Pb6–O146 | 272.0(4) | ||||
∅=261.7 | Pb6–O146 | 289.3(4) | |||||
∅=248.2 |
aSymmetry operations: 1+x, –1+y, +z; 21+x, −1+y, +z; 3−x, 1−y, 2−z; 4+x, 1+y, +z; 5−x, 2−y, 2−z; 6−1−x, 3−y, 1−z; 7−1−x, 2−y, 1−z; 8−2−x, 3−y, 1−z; 9−1+x, +y, +z; 10−1+x, 1+y, +z; 11−x, 2−y, 1–z; 121+x, +y, +z.
O2–Pb1–O41 | 82.2(1) | O4–Pb4–O146 | 91.4(1) | O71–B3–O138 | 108.5(5) |
O2–Pb1–O72 | 75.6(1) | O4–Pb4–O15 | 75.2(1) | O8–B3–O71 | 110.7(5) |
O2–Pb1–O113 | 81.2(1) | O15–Pb4–O146 | 75.4(1) | O8–B3–O108 | 112.1(5) |
O41–Pb1–O72 | 70.7(1) | O8–B3–O138 | 108.7(5) | ||
O41–Pb1–O113 | 75.0(1) | O5–Pb6–O146 | 84.8(1) | O108–B3–O71 | 111.5(4) |
O72–Pb1–O113 | 85.1(1) | O8–Pb6–O5 | 82.6(1) | O108–B3–O138 | 105.1(5) |
O15–Pb6–O5 | 89.7(1) | ∅=109.4 | |||
O2–Pb2–O8 | 87.0(1) | O15–Pb6–O8 | 78.1(1) | ||
O2–Pb2–O113 | 79.5(1) | O4–B4–O15 | 106.4(5) | ||
O9–Pb2–O2 | 81.2(1) | O6–B1–O3 | 104.0(4) | O4–B4–O612 | 111.9(5) |
O9–Pb2–O8 | 83.3(1) | O7–B1–O3 | 109.9(4) | O5–B4–O15 | 109.7(5) |
O9–Pb2–O113 | 72.4(1) | O7–B1–O6 | 110.6(4) | O5–B4–O4 | 110.6(4) |
O7–B1–O95 | 113.0(5) | O5–B4–O612 | 110.1(5) | ||
O4–Pb3–O114 | 71.1(1) | O95–B1–O3 | 110.0(4) | O612–B4–O15 | 108.0(5) |
O4–Pb3–O15 | 71.7(1) | O95–B1–O6 | 108.9(4) | ∅=109.5 | |
O95–Pb3–O4 | 81.4(1) | ∅=109.4 | |||
O95–Pb3–O114 | 71.2(1) | O11–B5–O1 | 120.7(5) | ||
O95–Pb3–O15 | 75.9(1) | O2–B2–O5 | 110.7(5) | O11–B5–O121 | 120.4(5) |
O2–B2–O1011 | 111.4(5) | O121–B5–O1 | 118.9(5) | ||
O3–Pb5–O147 | 73.9(1) | O2–B2–O12 | 108.0(5) | ∅=120.0 | |
O6–Pb5–O108 | 78.3(1) | O1011–B2–O5 | 105.8(4) | ||
O6–Pb5–O147 | 72.3(1) | O1011–B2–O12 | 111.3(5) | O136–B6–O3 | 119.2(5) |
O6–Pb5–O15 | 83.9(1) | O12–B2–O5 | 109.7(4) | O14–B6–O3 | 122.4(5) |
O15–Pb5–O3 | 72.5(1) | ∅=109.5 | O14–B6–O136 | 118.3(5) | |
O15–Pb5–O108 | 89.2(1) | ∅=120.0 |
aSymmetry operations: 1+x, −1+y, +z; 21+x, −1+y, +z; 3−x, 1−y, 2−z; 4+x, 1+y, +z; 5−x, 2−y, 2−z; 6−1−x, 3−y, 1−z; 7−1−x, 2−y, 1−z; 8−2−x, 3−y, 1−z; 9−1+x, +y, +z; 10−1+x, 1+y, +z; 11−x, 2−y, 1−z; 121+x, +y, +z.
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 Results and discussion
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.
Empirical formula | (Pb4O)Pb2B6O14-II | (Pb4O)Pb2B6O14-I |
Formula weight | 1548.00 | 1548.00 |
Crystal system | Triclinic | Triclinic |
Space group | P1̅ (no. 2) | P1 (no. 1) |
a, pm | 696.46(7) | 695.36(13) |
b, pm | 778.00(3) | 720.26(13) |
c, pm | 1408.34(5) | 780.03(14) |
α, deg | 97.4(1) | 76.248(11) |
β, deg | 100.4(1) | 76.694(11) |
γ, deg | 103.0(1) | 73.982(12) |
Volume, Å3 | 721.09(1) | 359.03(11) |
Z | 2 | 1 |
Final R1/wR2 indices [I>2 σ(I)] | 0.0270/0.0471 | 0.03820/0.0932 |
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].
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.
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.
(Pb4O)Pb2B6O14-II | (Pb4O)Pb2B6O14-I [25] | ||||
---|---|---|---|---|---|
Atom | ΣV | ΣQ | Atom | ΣV | ΣQ |
Pb1 | +2.33 | +1.96 | Pb1 | 2.23 | +2.16 |
Pb2 | +2.32 | +2.14 | Pb2 | 2.13 | +1.85 |
Pb3 | +1.99 | +1.65 | Pb3 | 2.06 | +1.98 |
Pb4 | +2.08 | +1.66 | Pb4 | 1.98 | +1.93 |
Pb5 | +2.11 | +1.93 | Pb5 | 2.16 | +1.79 |
Pb6 | +2.31 | +2.00 | Pb6 | 1.93 | +1.56 |
B1 | +2.96 | +2.88 | B1 | 2.99 | +2.97 |
B2 | +2.99 | +3.00 | B2 | 2.98 | +3.15 |
B3 | +3.00 | +3.10 | B3 | 3.03 | +3.07 |
B4 | +3.04 | +2.86 | B4 | 2.96 | +3.35 |
B5 | +3.03 | +3.41 | B5 | 2.99 | +2.95 |
B6 | +2.99 | +3.28 | B6 | 2.92 | +3.23 |
O1 | −1.98 | −1.91 | O1 | −2.13 | −2.23 |
O2 | −2.18 | −1.97 | O2 | −2.00 | −2.04 |
O3 | −2.00 | −1.90 | O3 | −2.12 | −1.95 |
O4 | −2.38 | −2.23 | O4 | −2.04 | −1.91 |
O5 | −2.02 | −2.04 | O5 | −2.03 | −1.87 |
O6 | −2.09 | −2.09 | O6 | −2.16 | −2.03 |
O7 | −2.03 | −2.03 | O7 | −1.98 | −1.95 |
O8 | −2.27 | −1.95 | O8 | −2.03 | −1.96 |
O9 | −2.28 | −2.13 | O9 | −2.08 | −2.04 |
O10 | −1.93 | −2.00 | O10 | −2.04 | −1.98 |
O11 | −2.00 | −1.92 | O11 | −1.96 | −1.89 |
O12 | −1.94 | −1.86 | O12 | −2.00 | −2.05 |
O13 | −1.91 | −1.83 | O13 | −1.81 | −1.96 |
O14 | −1.94 | −1.94 | O14 | −1.96 | −2.21 |
O15 | −2.35 | −2.20 | O15 | −2.02 | −1.94 |
The bold values represent the lead-coordinated oxygen atoms inside of the OPb4 tetrahedron.
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).
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].
4 Conclusions
Through a facile hydrothermal synthesis it was possible to synthesize a new modification of a lead(II) borate designated as (Pb4O)Pb2B6O14-II. The structure contains infinite anti-parallel B6O14 chains running along the crystallographic b axis and isolated oxygen-centered Pb4 tetrahedra. It should be noted that this is a new centrosymmetric modification of the lead borate (Pb4O)Pb2B6O14-I. The main differences of the two modifications are the different cell parameters, the arrangement of the chains in the cell, and the orientation of the OPb4 tetrahedra.
Acknowledgments
We especially thank Mag. D. Vitzthum for collecting the FT-IR data as well as Univ.-Prof. Dr. Roland Stalder (Institute for Mineralogy and Petrography, University of Innsbruck) for the access to the FTIR microscope and Daniel Schildhammer for the Rietveld refinement.
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