Tin Fluorooxoborates MB2O3F2 (M

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Rational Design of the First Lead/Tin Fluorooxoborates MB2O3F2 (M=Pb, Sn): Containing Flexible Two-Dimensional [B6O12F6]# Single Layers with Widely Divergent Second Harmonic Generation Effects Min Luo, Fei Liang, Yunxia Song, Dan Zhao, Ning Ye, and Zheshuai Lin J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 21, 2018

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Rational Design of the First Lead/Tin Fluorooxoborates MB2O3F2 (M=Pb, Sn): Containing Flexible Two-Dimensional [B6O12F6]∞ Single Layers with Widely Divergent Second Harmonic Generation Effects Min Luo†, #, Fei Liang§, ‡, #,Yunxia Song†, ‡, Dan Zhaoƾ, Ning Ye*, † and Zheshuai Lin§, * †

Key Laboratory of Optoelectronic Materials Chemistry and Physics, Collaborative Innovation Center for Optoelectronic Semiconductors and Efficient Devices, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China § Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡

University of Chinese Academy of Sciences, Beijing 100190, China Department of Physics and Chemistry, Henan Polytechnic University, Jiaozuo, Henan 454000, China Supporting Information Placeholder ƾ

ABSTRACT: Molecular engineering design is a productive strategy on atomic-scale to optimize crystal structure and develop new-functional materials. Herein, the first lead/tin fluorooxoborates, MB2O3F2 (M=Pb, Sn), were rationally designed by employing the nonlinear optical crystal Sr2Be2B2O7 (SBBO) as a parent model. Compared with the rigid [Be6B6O15] ∞ double layers in SBBO, MB2O3F2 have flexible two-dimensional [B6O12F6]∞ single layer, which not only keeps the NLO-favorable layered structure but also overcomes the structural instability issues of SBBO. Both compounds exhibited desired short UV cutoff edge. Interestingly, MB2O3F2 exhibit widely divergent second harmonic responses, although they are isostructural and both contain stereochemically active lone-pair cations. Our first-principles calculations revealed that the SHG difference is mainly attributed to the different anisotropies of Pb and Sn SHG-active orbitals, which make constructively and destructively contribution to the SHG effects in PbB2O3F2 and SnB2O3F2, respectively.

The exploration of ultraviolet (UV, λ < 400 nm) nonlinear optical (NLO) materials with strong second harmonic generation (SHG) response (>10×KDP) is of great academic interest because a larger NLO coefficient usually means a higher frequency conversion efficiency.1 To date, an effective strategy for acquiring enhanced SHG response is to introduce or combine NLO-active structural units with the asymmetric crystal structure, including introduction or combination distorted polyhedra with a d0 cation center, d10 cations centered polyhedra with large polar displacement, or stereochemically active lone-pair (SCALP) cations2 with the asymmetric crystal structure. However, an obvious issue of units mentioned above is the red-shift of absorption edge, thus causing the material unsuitable for application in the UV region. Therefore, it is always a great challenge to design UV NLO materials that can achieve a subtle balance between significant SHG response and wide UV transparency. It is well-known that the introduction of tetrahedral groups such as BO4 and PO4 in the compound can effectively increase the band gaps of the crystals to shift the absorption edge to the deep-UV region, such as LiB3O5,3 CsB3O5,4 SrB4O7,5 KH2PO4 (KDP),6 LiCs2PO4,7 Ba3P3O10Cl,8 Ba5P6O20,9 RbBa2(PO3)5,10 and BPO4.11

Recently, new oxyfluoride tetrahedral fundamental building blocks, [BOxF4-x](x+1)- (x = 1, 2, 3) with high anisotropy, large band gap and remarkable strong hyperpolarizability, have been considered among the best structural units for designing the UV NLO materials.12 As a result of intensive studies, many excellent fluorooxoborate NLO crystals with large band gaps have been reproted, including Li2B6O9F2,12c AB4O6F (A= NH4, Rb, Cs),12d-f ACsB8O12F2 (A=K, Rb),12e and M2B10O14F6 M=(Ca, Sr).12g These prompted us to investigate whether we could introduce NLOactive structural units, such as a SCALP cation Pb2+ or Sn2+, into fluorooxoborates to create a new type of NLO materials with the desired balance between strong SHG effect and large band gap. Besides, to compensate for the adverse effects of NLO-active structural units on band gap maximally, the material structure also should meet one basic requirement, namely, no lone electron pairs on O atoms in the crystal structure. To date, the [Be2BO3F2]∞ single layers in KBe2BO3F2 (KBBF)13 and [Be2B2O7]∞ doublelayers in SBBO14 are two classical structure units for achieving large band gap. However, both SBBO and KBBF crystals suffer various serious issues associated with their structures, such as the weak interlayer bonding force in KBBF and the structural polymorphism problem in SBBO. Therefore, in fluorooxoborate system, the molecular design to modify the structures of KBBF or SBBO is critical for the development of new high-performance UV NLO materials. In this work, by adopting the SBBO as a parent model, we replaced Sr2+ cation with a SCALP cation (Pb2+ or Sn2+) to enhance SHG response. Meanwhile, BO3F units were utilized to modify [Be2B2O7]∞ double-layers, which not only overcomes the structural instability issues but also increases band gap. Thus, systematic explorations on the Pb(Sn)-B-O/F systems resulted in two novel fluorooxoborates, MB2O3F2 (M= Pb, Sn) (PBOF and SBOF), among which SBOF is one of few crystalline tin borates. As expected, both compounds possessed an inherited and optimized NLO-favorable layered structure of SBBO, but they showed wildly different optical properties. Compared with SBOF, PBOF exhibited the desired large band gap and excellent NLO properties.

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Figure 2 Powder SHG measurements at 1064 nm.

Figure 1 Structural evolution from SBBO to PBOF. (a) [Be6B6O15]∞ layer. (b) [B6O9F6]∞ layer. (c) The structure of SBBO. (d) The structure of PBOF. PBOF and SBOF are isostructural and belong to a noncentrosymmetric space group P31m. Therefore, only PBOF structure, as an example, will be discussed in detail. The asymmetric unit consists of one crystallographically independent Pb, B, O, and F atoms each. Specifically, each B atom is coordinated to three O atoms and one F atom to form a distorted [BO3F] tetrahedron with B-F bond lengths of 1.46(3) Å and B–O bond lengths of 1.458(8) Å. The [BO3F] tetrahedra connect with each other via cornershared O atom to form a two-dimension infinite [B6O12F6]∞ layer in the a-b plane (Figure 1b). Also, the adjacent [B6O12F6]∞ single layers are bridged by Pb-O/F bonds. Each Pb atom is surrounded by three O atoms and six F atoms to form a [PbO3F6] polyhedron, and all [PbO3F6] polyhedra are aligned parallelly along the c-axis (Figure S4). The structural evolution from SBBO to MBOF is illustrated in Figure 1. Our structural design strategy can be elucidated as the following three steps: (1) To obtain a large SHG response, the sites of Sr2+ cations are substituted with the SCALP cations, Pb2+/Sn2+, which have similar ionic radii; (2) To address the structural instability issue of SBBO-type structure, the F atoms are employed to act as “scissors” to cut off the bridged O atoms in the Be2O7 dimers which connect the adjacent layers ([Be3B3O9]∞) of the rigid [Be6B6O15]∞ double layers. Meanwhile, the planar [BO3] trigonal groups are all replaced by [BO3F] groups. These two molecular design considerations, thus, generate the flexible [B6O9F6] ∞ single layers in MBOF. Notably, the substitution between [BO3] and [BO3F] groups is similar to that between [BO3] and [PO4] groups in some borates or phosphates, such as the structural evolution from CsZn4(BO3)(PO4)2 to CsZn4(PO4)3.15 Also, the distance between the adjacent flexible single layers in PBOF/SBOF (4.708/4.768 Å) is greater than that of rigid double layers of SBBO (3.917 Å), providing a “comfortable” space for Pb/Sn atoms. The increased distance is conducive to solve the problem of stacking faults in crystal, which can be verified by not only the calculated results of phonon dispersions (Figure S5) but also the decreased structural converge factor from SBBO (> 0.065) to PBOF/SBOF (0.0240/0.0187); (3) To achieve the additional SHG enhancement, that is, make full use of the hyperpolarizability of all BO3F groups, the tailored single layers alternately rotate 180°, thus resulting in all BO3F groups being aligned in the same direction.

The UV-Vis-NIR diffuse reflectance spectra showed that PBOF and SBOF exhibited relatively short UV absorption edges of ∼ 220 and 250 nm, respectively (Figure S6), which are far shorter than those of recently reported materials containing NLO-active structural units, such as Pb3Mg3TeP2O14 (250 nm),16 Pb2(NO3)2(H2O)F2 (300 nm),17 Pb2BO3I (330 nm),18 Bi3TeBO9 (385 nm)19 and K3[V(O2)2O]CO3 (420 nm).20 As MBOF (M=Pb, Sn) are non-centrosymmetric, they are expected to be NLO active. Therefore, powder SHG measurements on them were performed under 1064 nm laser radiation, which revealed that PBOF and SBOF were type-I phase-matchable at 1064 nm and exhibited very different SHG efficiency of 13×KDP and 4×KDP, respectively for PBOF and SBOF (Figure 2). The SHG response of PBOF is comparable with (and even greater than) those of excellent Pb-containing UV NLO materials, such as Pb3Mg3TeP2O14 (13.5×KDP),16 CsPbCO3F (13.5×KDP),21 Pb2(NO3)2(H2O)F2 (12×KDP)17 and Pb2BO3X (X=Cl, Br, I) (9~10 × KDP).2e, 18, 22

Figure 3 Total and partial density of states of (a) PBOF and (b) SBOF. To investigate the microscopic mechanism of enhanced band gaps in PBOF and SBOF, the first-principles investigations were performed. The calculated density of states (DOS) projected on the constituent elements of MBOF are displayed in Figure 3 (and the band structures shown in Figure S7), from which several important characteristics can be deduced: (i) The energy states -10 to 0 eV are mainly composed of B 2p, O 2p, F 2p and Pb 6s/Sn 5s orbitals, and quite a large hybridization among B 2p, O 2p, and F 2p orbitals indicates the strong covalent interaction between the atoms in the tetrahedral BO3F groups. The valence band (VB) maximum is occupied by O 2p, F 2p and Pb 6s/Sn 5s orbitals. Notably, compared with the Pb 6s electrons, Sn 5s electrons exhibit stronger isolation and make more contribution to the top of VB, which narrows the bandgap of SBOF by 0.8 eV. (ii) The conduction band (CB) minimum is predominantly contributed

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Journal of the American Chemical Society from Pb 6p/Sn 5p orbitals with slight hybridization with O and F 2p orbitals. Clearly, Pb 6p and Sn 5p electrons make almost same contribution to this region. (iii) The higher levels (> 9 eV) mainly consist of B 2p, O 2p and F 2p orbitals, indicating that the intrinsic band gap of BO3F group (> 9 eV) is larger than that of conjugated π-orbital BO3 unit (~8.0 eV). Therefore, these results demonstrate that our design strategy based on the introduction of BO3F tetrahedra to broaden band gap is exactly feasible.

Figure 4 Electronic charge density maps from the orbitals near the band gaps in PBOF and SBOF. (a) and (b) from the top of VB (-0.5 eV – 0 eV), (c) and (d) from the bottom of CB (5eV – 7 eV), and (e) and (f) from their superposition in PBOF and SBOF, respectively. Our first-principles calculations shown that the largest SHG coefficients d33 of PBOF and SBOF are 6.65 pm/V and 1.36 pm/V (see Table S6), in good agreement with the experimental measurements. Remarkably, both experimental and calculated results demonstrate that the SHG response of PBOF is several-fold larger than that of SBOF. In order to elucidate the cause of SHG difference between PBOF and SBOF, we calculated the respective contribution of the constituent ions to the SHG responses by adopting an atom-cutting technique.23 It is revealed that the SHG contributions from Pb2+ and Sn2+ cations are totally opposite: Pb2+ cations give constructive contribution, while Sn2+ cations give destructive contribution, to the SHG effects (also see Table S6). The microscopic mechanism of the SHG difference for PBOF and SBOF are illustrated in the electronic charge density maps near band gap, since the virtual electronic transitions between these states dominantly determine the NLO properties in crystal. Figures 4a and 4b display the charge densities from the top of VB which are mainly composed of the lone-pair 6s and 5s electrons of Pb2+ and Sn2+ cations, respectively. Clearly, the mono-capped 6s2/5s2 lone-pair electrons on Pb2+/Sn2+ cations accumulate at the bottom of atoms (opposite the c-axis). The spatial density of Sn 5s electrons is much larger than that of Pb 6s electrons, suggesting much stronger stereo-chemical activity of Sn 5s electrons. Figures 4c and 4d display the charge densities from the bottom of CB, which manifest almost the same amount of contribution from Pb 6p and Sn 5p orbitals, respectively. Notably, the distortion directions of the charge densities from Pb 6p orbitals (and Sn 5p orbtials) in 4c (and 4d) are antiparallel to those from Pb 6s electrons (and Sn 5s electrons) in 4a (and 4b). The superposition of charge densities from the top of VB and the bottom of CB (Figure 4e and 4f), therefore, exhibits very different distortion directions around Pb2+ and Sn2+ cations. Namely, the distortion directions of the NLO-active charge densities for Pb2+ cations are along the caxis. In comparison, those for Sn2+ cations are opposite the c-axis. As the NLO-active charge densities of the BO3F tetrahedra are

along the c-axis for both PBOF and SBOF, the orbitals around Pb2+ cations enhance, while those around Sn2+ cations counteract, the overall SHG effect. Therefore, PBOF exhibits much larger NLO response compared with SBOF. It should be emphasized that the mechanism in this work is some different with the cases in Pb2B5O9I and Pb2(BO3)(NO3), in which the enhancement of SHG effects are mainly attributed to the active 6s2 lone-pair electrons.24 In addition, we also calculated the “flexibility indices” in the [PbO3F6] and [SnO3F6] polyhedra using a chemical bonds flexibility analysis25. It is shown that the flexibility indices of PbO/F chemical bonds (0.136/0.038) are larger than that of Sn-O/F bonds (0.121/0.024). This means that the SHG response can be more enhanced by the [PbO3F6] polyhedra in PBOF than by the [SnO3F6] polyhedra in SBOF, since the more “flexible” the chemical bonds in the microscopic groups are, the larger the SHG effect in a crystal will be, provided that the respective microscopic second-order susceptibilities are additively superposed. In summary, a molecular engineering method was successfully applied to the design of the first lead/tin fluorooxoborate NLO materials, MB2O3F2 (M=Pb, Sn), among which PbB2O3F2 exhibits a remarkably strong SHG response, a short UV absorption edge, and type-I phase-matchability. Moreover, it has been demonstrated that the different s electrons activity on Pb and Sn are significantly responsible for the optical nonlinearity. It is anticipated that molecular structural design based on excellent crystal structures can be harnessed to develop other structuredriven functional materials.

ASSOCIATED CONTENT Supporting Information The CIF data and additional tables and figures as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected] [email protected]

Author Contributions #

M. L. and F. L. contributed equally.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant Nos. 51425205, U1605245, 91622118 and 51602309), the National Key Research and Development Plan of Ministry of Science and Technology (Grant No. 2016YFB0402104) and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB20000000). L. M. is grateful for the support from Chunmiao Project of Haixi Institute of Chinese Academy of Science.

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