Pb4B6O13: A Polar Lead Oxyborate with Uncommon ∞(B6O12)6

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Pb4B6O13: A Polar Lead Oxyborate with Uncommon ∞(B6O12)6− Layers Exhibiting a Large Second Harmonic Generation Response Lingyun Dong,†,‡ Ying Wang,*,† Bingbing Zhang,† Zhihua Yang,§ Xusheng Ge,‡ Kai Liu,†,‡ and Shigang Shen*,†

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†

Key Laboratory of Analytical Science and Technology of Hebei Province, College of Chemistry and Environmental Science, Hebei University, 180 East Wusi Road, Baoding 071002, China ‡ College of Biochemistry and Environmental Engineering, Baoding University, 3027 East Qiyi Road, Baoding 071002, China § Xinjiang Key Laboratory of Electronic Information Materials and Devices, CAS Key Laboratory of Functional Materials and Devices for Special Environments, Xinjiang Technical Institute of Physics & Chemistry of CAS, 40-1 South Beijing Road, Urumqi 830011, China S Supporting Information *

The lead borate system is a representative one for the exploration of new NLO materials because of the strong possibility of obtaining new crystals containing both πconjugated triangular BO3 groups and Pb2+ cations with SCALPs. For example, PbB 4 O 7 , 2 7 Pb 4 O(BO 3 ) 2 , 8 Pb1.5(BO2.25)2,28 and (Pb4O)Pb2B6O1429 have been reported with good SHG responses in the Pb−B−O system. Note that the anion-centered OPb4 tetrahedron is also well-known in inorganic lead oxysalts.30 Because the centered “additional” O atoms connect with only four neighboring lead atoms, the Pb− O bonds in the OPb4 tetrahedron are shorter and stronger than other typical Pb−O bonds. In addition, the OPb4 tetrahedra could further generate complex polyions with various structure types and dimensionality and eventually affect certain physical properties.30 A survey of lead borates containing OPb4 tetrahedra is presented in Table S1, demonstrating the diversity of anion-centered polyions. Recently, it is reported that the OPb4 tetrahedra play special roles in the SHG response of the NLO crystal.29 Here, via combination of the BO3 triangles and Pb2+ cations as well as the special OPb4 tetrahedra, Pb4B6O13, a new polar lead oxyborate, is reported for the first time. Interestingly, Pb4B6O13 features uncommon 6− layers and ∞1[OPb2] chains and exhibits an SHG ∞(B6O12) response that is ∼3 times that of KDP. Single crystals of Pb4B6O13 (Figure S1) were obtained by the flux method in the PbF2−PbO−B2O3 ternary phase system (experimental details are presented in the Supporting Information). Pb4B6O13 belongs to the NCS and polar space group Cc with four crystallographically distinct Pb atoms, six B atoms, and 13 O atoms in its asymmetric unit (Tables S2−S4). The crystal structure of Pb4B6O13 can be described as a layered B−O anionic structure with interspaces filled by Pb cations. In the structure, the B1, B4, and B5 atoms exhibit a trigonalplanar coordination environment with surrounding O atoms. The B−O bond lengths of these BO3 triangles range from 1.31(6) to 1.49(6) Å, and the O−B−O bond angles vary from 111(3)° to 130(4)°. The other three B atoms (B2, B3, and B6) possess a tetrahedral coordination geometry with the B−O

ABSTRACT: A new lead oxyborate, Pb4B6O13, has been successfully synthesized by introducing stereochemically active Pb2+ cations and distorted OPb4 tetrahedra into asymmetric borates. Pb4B6O13 exhibits an unprecedented two-dimensional ∞(B6O12)6− layer structure with a large second harmonic generation (SHG) response that is 3 times that of KH2PO4. In addition, theoretical work, including dipole moment calculations, electronic structure, and SHG coefficients combined with SHG density analysis, is reported. The results suggest that the enhanced SHG of Pb4B6O13 is attributed to the synergy effect of three functional units.

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ith the development and applications of laser technology, finding new nonlinear optical (NLO) crystal materials with better optical performance has been a research hot spot for nearly half a century.1,2 Although many commercial NLO materials, such as KH2PO4 (KDP), βBaB2O4 (BBO), LiB3O5 (LBO), and AgGaS2, have been widely used in many different fields, these materials cannot meet the strict requirements of applications in the specific wavelength ranges.3 Through continuous exploration, many new NLO materials with superior properties have been reported.4−7 As one of the basic credentials for NLO materials, a noncentrosymmetric (NCS) crystal with a large second harmonic generation (SHG) response is desired. It is evident that NLO materials with large a SHG response can be found in the compounds containing π-conjugated groups,8−10 d0 or d10 cation-centered octahedra,11,12 NCS chalcogenide units,13,14 or cations with stereochemically active lone pairs (SCALPs).15,16 Also, many compounds with one or more types of these NLO-active structural units, such as CsPbCO3F,17 Cd4BiO(BO3)3,18 Bi3TeBO9,19 and Pb2(BO3)(NO3),20 exhibit enhanced SHG responses due to the synergetic effect. In addition, the rational design of new NLO-active structural units, e.g., (BO4−xFx)5−x, (PO4−xFx)3−x, and (O2)2−, would also greatly promote the development of NLO materials.21−26 © XXXX American Chemical Society

Received: December 4, 2018

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DOI: 10.1021/acs.inorgchem.8b03356 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

generate ∞1[OPb2] chains extending along the c axis (Figure 1d,e). The one-dimensional ∞1[OPb2] chains are common in minerals and can also be found in other lead borates, such as Pb2(O4Pb8)(BO3)3Br334 and Pb2O[BO2(OH)].35 Note that the Pb3 and Pb4 atoms do not form OPb4 tetrahedra and fill the void of the 3D crystal structure to keep the charge balance. As suggested by Smith et al.,36 the crystal−chemical formula of Pb4B6O13 can also be written as (OPb2)Pb2B6O12. To check the structural rationale, a bond valence sum (BVS) calculation37 was performed. The BVS values are found to be reasonable except for the high BVS value (2.4) of the central O1 atom in OPb4 tetrahedra (Table S3). This unusual phenomenon may be caused by the lone pair effect of the surrounding Pb2+ cations.30 For characterization, polycrystalline samples of Pb4B6O13 were obtained by grinding as-prepared single crystals. The purity is checked by a powder X-ray study (Figure S4). The infrared spectrum of Pb4B6O13 is presented in Figure S5. According to the literature,8,28,38 the absorption peaks are mainly attributed to the stretching and bending vibrations of B−O bonds in BO3 and BO4 groups (Table S5). The ultraviolet−visible−near-infrared diffuse reflectance spectrum indicates that Pb4B6O13 has no obvious absorption from 290 to 2600 nm with a band gap of 3.21 eV (Figure S6), which is in agreement with the colorless feature of Pb4B6O13 (Figure S1). The thermal behavior of Pb4B6O13 has been recorded (Figure S7). No obvious weight loss is found in the thermogravimetric (TG) curve, and differential scanning calorimetry (DSC) analysis reveals two endothermic peaks (around 478 and 564 °C) in the heating curve. In addition, it is found that the solidified melt after the DSC experiment changes to βPb6B10O21 instead of the initial Pb4B6O13 phase. The results suggest that Pb4B6O13 melts incongruently and herein needs a suitable flux to grow single crystals. As a polar lead borate, Pb4B6O13 is expected to show SHG properties. Using the Kurtz−Perry method,39 we evaluated the SHG performance of the powder sample of Pb4B6O13 under a Q-switched Nd:YVO4 laser. As shown in Figure 2a, the plot of

distances and O−B−O angles ranging from 1.38(5) to 1.56(5) Å and from 104(3)° to 116(4)°, respectively. The three BO3 triangles and three BO4 tetrahedra link together by sharing O atoms to generate the [B6O12] fundamental building blocks (FBBs) (Figure 1a). According to the classification rule

Figure 1. (a) [B6O12] FBB and its topological structure (triangles, BO3 groups; squares, BO4 groups). (b) ∞(B6O12)6− 2D layer with an 18MR structure. (c) Simplified topological model of the ∞(B6O12)6− 2D layer (blue nodes, [B6O12] FBBs). (d) 3D framework of Pb4B6O13. The unit cells are indicated by black lines. (e) One-dimensional 1 ∞ [OPb2] chain.

proposed by Xue et al.,31 the topological structure of the [B6O12] FBB belongs to the “8”-shaped-ring FBB form, and the algebraic description could be given as 6:∞2[⟨2Δ3T⟩8 + Δ]. By adopting the topological description, one can easily find some borates with similar [B6O12] FBBs, e.g., β-CaB2O432 and TlB2O3(OH)·H2O33 (Figure S2). Interestingly, the modes of connection of BO3 triangles and BO4 tetrahedra in the [B6O12] FBB of Pb4B6O13 are unique. Moreover, the [B6O12] FBBs connect with each other by sharing four terminal atoms (two O2 and two O4 atoms), forming an ∞(B6O12)6− twodimensional (2D) branched layer with an 18-membered ring (18MR) (Figure 1b). Topologically, with [B6O12] FBBs as nodes, the connection mode of the ∞(B6O12)6− branched layer can be simplified as a (3, 6) plane net with a Schläfli symbol of {36.46.53} (Figure 1c). To the best of our knowledge, the 6− 2D layer in Pb4B6O13 is new to borate chemistry. ∞(B6O12) As shown in Figure 1d, the ∞(B6O12)6− layers further link with other adjacent layers through Pb cations to build the threedimensional (3D) crystal structure of Pb4B6O13. There are four types of Pb-centered polyhedra, that is, Pb1O6, Pb2O5, Pb3O7, and Pb4O8 polyhedra (Figure S3). The Pb−O bond lengths vary from 2.18(3) to 2.99(2) Å for Pb4B6O13. The bond length variation indicates significant distortion for all PbOn (n = 5−8) polyhedra owing to the stereoactivity of lone electron pairs on the Pb2+ cations.29 It should be noted that the “additional” O1 atom does not participate in strong covalent B−O bonds but bonds with only nearby Pb1 and Pb2 atoms to form an OPb4 tetrahedron (Figure 1e). The trans-edge-sharing OPb4 tetrahedra further

Figure 2. (a) Powder SHG curve of Pb4B6O13. (b) SHG intensities of Pb4B6O13 with LBO as a reference.

the SHG response versus particle size of the Pb4B6O13 powders is presented. It is clear that Pb4B6O13 is phase matchable as the SHG intensity increase with the increase in particle size. Pb4B6O13 exhibits an SHG efficiency close to that of LBO (approximately ∼3 times that of KDP) in the same particle size range of 150−200 μm (Figure 2b). As suggested by the anionic group theory,6 the anionic groups are the major contributor to the SHG effect. However, the cation contribution cannot be completely excluded from consideration when strong cation− B

DOI: 10.1021/acs.inorgchem.8b03356 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry anionic interaction is involved.40 To shed light on the structure relationship, the dipole moment calculation,41 which is derived from the Debye equation and the bond valence theory, was adopted. We calculated the direction and magnitude of the dipole moments of BO3, BO4, and PbOn groups in Pb4B6O13 and summarized the contribution of each group to the polarization in the unit cell (Table S6). The dipole moments of “additional” OPb4 tetrahedra are also taken into consideration, though they are ignored in the typical cation-centered dipole moment calculation. As shown in Table S6, the dipole moments of all groups are parallel to the a−c plane, while the y components completely cancel out due to the symmetry operation of the c glide plane. The calculated dipole moments of PbOn polyhedra and OPb4 tetrahedra vary from 0.05 to 0.23 × 10−18 esu cm Å−3, which are comparable to and larger than those of BO3 triangles. The results indicate that the distorted PbOn polyhedra or OPb4 tetrahedra may contribute significantly to SHG. We carried out density functional theory calculations to further explore the electronic structures and the origin of their optical properties. The band structure of Pb4B6O13 is given in Figure S8. The calculated direct band gap is 2.06 eV, which is smaller than the experimental value, as expected from the error of the exchange-correlation energy. The partial density of states of Pb4B6O13 was calculated and is plotted in Figure S9. It indicates that the valence band maximum (energy region from −10.0 to 0 eV) hybridizes the orbitals of O-2p and Pb-6s and -6p, while the conduction band minimum (0−8.0 eV) mainly consists of Pb-6p orbitals. Therefore, the hybridization of the O-2p and Pb-6s and -6p orbitals determines the band gap of Pb4B6O13. According to the Kleinman symmetry of point group m, there are six non-zero independent NLO coefficients, and the values of calculated NLO coefficients are −0.98, 1.85, 0.07, 0.50, − 3.90, and −1.04 pm/V for d11, d12, d13, d15, d24, and d33, respectively. The calculated results further confirm the large SHG of Pb4B6O13. To probe the possible electronic origin of SHG, we further adopted the SHG-weighted electron density analysis.42 The virtue electronic (VE) progress that contributes dominantly to the SHG coefficient is summed up by the SHG-weighted factor and then highlighted in the real space. Because d24 is the largest coefficient, it was then resolved as two electronic states: the occupied and unoccupied orbitals (Figure 3). From the visualization, it is clear that the regions with maximal density, i.e., main contributors to the SHG effect, are the atoms involving the formation of planar BO3 groups, distorted PbOn polyhedra, and OPb4 tetrahedra. Therefore, the electronic origin of the SHG effect can be described as the synergy of these structural units. Interestingly, the SHG densities around O1 atoms (center of the OPb4 tetrahedron) are more significant than other oxygen atoms, suggesting that the OPb4 tetrahedron may have a considerable contribution. This is further evidence that the introduction of the OPb4 tetrahedron would benefit SHG enhancement. In summary, we reported a new lead oxyborate, Pb4B6O13, exhibitng an unprecedented ∞(B6O12)6− 2D branched layer structure. The title compound also shows a band gap of 3.21 eV and a strong SHG response close to that of LBO (∼3 × KDP standard). The calculations of the dipole moments, electronic structures, and SHG density indicate that the enhanced SHG response may be derived from the cooperative effect of the π-delocalized BO3 groups, the SCALPs of Pb2+ cations, and distorted OPb4 tetrahedra. In addition, the

Figure 3. SHG density maps of the (a) occupied and (b) unoccupied orbitals in the VE process of the SHG tensor d24.

theoretical analysis also suggests that the contribution of OPb4 tetrahedra is substantial, and therefore, these groups can be considered a new type of NLO-active unit.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03356. Experimental and computational details, crystallographic information tables, infrared and ultraviolet−visible− near-infrared diffuse reflectance spectrum, TG/DSC curves, and electronic structures of Pb4B6O13 (PDF) Accession Codes

CCDC 1879643 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ying Wang: 0000-0001-6642-543X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (51602341). REFERENCES

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DOI: 10.1021/acs.inorgchem.8b03356 Inorg. Chem. XXXX, XXX, XXX−XXX