CaB5O7F3: A Beryllium-Free Alkaline-Earth Fluorooxoborate

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

CaB5O7F3: A Beryllium-Free Alkaline-Earth Fluorooxoborate Exhibiting Excellent Nonlinear Optical Performances Zhizhong Zhang,†,‡ Ying Wang,*,† Bingbing Zhang,† Zhihua Yang,† and Shilie Pan*,† †

CAS Key Laboratory of Functional Materials and Devices for Special Environments, Xinjiang Technical Institute of Physics & Chemistry, CAS; Xinjiang Key Laboratory of Electronic Information Materials and Devices, 40-1 South Beijing Road, Urumqi 830011, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

Recent research indicates that chemically benign fluorooxoborates could be optimal system for discovering new DUV NLO materials. Fluorooxoborates possess two unique merits: First, the dangling bonds of BO3 groups are eliminated by B− O/F groups (such as BO3F, BO2F2, and BOF3), resulting in the broaden DUV transmittance range. 19 In addition, the introduction of F atoms with large electronegativity also make blue-shift the absorption edge.20 For example, a series of fluorooxoborates have been reported with absorption edges below 190 nm, such as Li 2 B 6 O 9 F 2 , 21,22 Na 2 B 6 O 9 F 2 , 23 Na3B3O3F6,24 K3B3O3F6,25 SrB5O7F3,26 and BaB4O6F2.27,28 Second, the B−F covalent bonds not only increases the structural diversity of the B−O anionic frameworks, but also help to enlarge the SHG responses and birefringence.21,22,29,30 Very recently, our group reported a series of NLO-active fluorooxoborates in the AB4O6F (A = alkali metal or ammonium) family: NH4B4O6F,31 CsB4O6F,32 RbB4O6F,33 CsKB8O12F2,33 and CsRbB8O12F2.33 All the AB4O6F family materials possess large band gap, large SHG responses, and moderate birefringence to meet the phase-matching requirement in the DUV region, which are considered to be promising DUV NLO materials. In this contribution, we further investigated the alkaline earth fluorooxoborates system, and discovered a new DUV NLO material: CaB5O7F3. It keeps the advantageous structural traits of KBBF. The broad transparency range, large NLO responses, and appropriate birefringence make CaB5O7F 3 another prospective DUV NLO material. Polycrystalline CaB5O7F3 was synthesized through conventional molten-salt method (see details in the Supporting Information). The purity of the CaB5O7F3 phase was checked by powder X-ray diffraction (Figure S1). Single crystals of CaB5O7F3 were obtained through high-temperature method in a closed system. The crystal structure data were collected and analyzed by single-crystal X-ray diffraction (Table S1). The elemental analysis by EDX spectrum gives Ca:B:O:F molar ratio of approximately 1:5:7:3 (Figure S2). CaB5O7F3 belongs to the polar space group Cmc21 (No. 36). Bond valence calculations34,35 (Ca, 2.009; B, 3.049−3.077; O, 1.994−2.112; F, 0.952−1.026) demonstrate that the structural model is reasonable (Table S4). As shown in Figure 1, CaB5O7F3 features two-dimensional (2D) layered structure, in

ABSTRACT: Noncentrosymmetric (NCS) fluorooxoborates are one of the most attractive systems for discovering the new deep-ultraviolet (DUV) nonlinear optical (NLO) materials. Here, we report the formation, crystal structure, and optical properties of a new NCS alkaline earth fluorooxoborate: CaB5O7F3. It exhibits excellent NLO performances including large band gap (8.75 eV), large second harmonic generation intensity (2 × KH2PO4 and 0.4 × β-BaB2O4, under 1064 and 532 nm radiation, respectively), and moderate birefringence (0.07 @ 1064 nm) that enables frequency doubling down to 183 nm. It demonstrates that CaB5O7F3 is a promising DUV NLO material.

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eep-ultraviolet (DUV) nonlinear optical (NLO) materials are intriguing interest because of their promising applications in sophisticated laser technologies such as laser medical treatment, material micromachining, laser photolithography.1−5 For a practical DUV NLO material, the following characteristics are indispensable: (i) large transparency window with short DUV cutoff edge (band gap >6.2 eV); (ii) large second-harmonic generation (SHG) response(>1 × KH2PO4); (iii) suitable birefringence to satisfy the DUV phase-matching (PM) condition.6,7 Unfortunately, these requirements can hardly be met simultaneously, and it is still very difficult to design DUV NLO material with all of the stringent requirements.8−12 Borates are the most attractive candidates for DUV NLO materials. However, available borate-based NLO materials can barely satisfy the need of DUV laser application. For example, LiB3O5 (LBO)13 and CsB3O5 (CBO)14 possess short cutoff edges. However, their low birefringence (Δn < 0.07) prohibits them to be applied in the DUV region. Although β-BaB2O4 (βBBO)15 exhibits both absorption edge (185 nm) and larger birefringence (Δn ∼ 0.11), its shortest PM second-harmonic wavelength is limited to 205 nm and the large walk-off angle reduces the SHG efficiency. Up to now, KBe2BO3F2 (KBBF) is the only available DUV NLO crystal material practically deployed in DUV laser source. However, its intrinsic layer growth habit has greatly limited the productions and applications.16−18 Therefore, the search of new beryllium-free DUV NLO materials has attracted increasing interest in recent years. © XXXX American Chemical Society

Received: February 28, 2018

A

DOI: 10.1021/acs.inorgchem.8b00531 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

output, respectively (Figure 2). The observed intensity of powder SHG is comparable with those of AB4O6F family (0.8−

Figure 2. Result of powder SHG measurement of CaB5O7F3 at 1064 (a) and (b) 532 nm laser radiation, respectively. Figure 1. (a) Coordination environment of Ca2+. (b) (B5O9F3)6− fundamental building block of CaB5O7F3. (c) The 2D wave-like [B5O7F3]∞ layer viewing along the b axis. Structural comparison between (d) CaB5O7F3 and (e) KBBF.

3 × KDP),31−33 and other DUV NLO materials, such as KBBF (1.2 × KDP),16,17 K3Ba3Li2Al4B6O20F (1.5 × KDP),38 and RbBa2(PO3)5 (1.4 × KDP).39 In order to analyze the relationship between structure and properties, we carried out the first-principles calculations by using the CASTEP package.40 The calculated band gap by generalized gradient approximation (GGA) is 7.05 eV (Figure S6). Since the band gap is always underestimated by the GGA method, we also use the HSE06 method to more precisely predict the band gaps of CaB5O7F3. The calculated band gap by HSE06 is 8.75 eV, which is larger than well-known DUV NLO crystals, such as KBBF (8.43 eV)16−18 and LBO (8.00 eV).13 The partial density of states (PDOS) indicate that the B 2s, 2p states show a wide hybridization with O 2p as well as F 2p states from −10 to 0 eV, indicating that the BO3 and BO3F groups are the main contributor of the top of the valence band (VB) (Figure 3a). While for the bottom region of the conduction band (CB), the 2p states of the central B and 3d states of the central Ca atoms take the major contributions. It illuminates that the [B5O7F3]∞ layers determine the band gap of CaB5O7F3. In addition, since birefringence is an essential parameter for satisfying the DUV phase-matching (PM) conditions, we calculated the linear refractive indexes. CaB5O7F3 possesses a birefringence of Δn = 0.07 at 1064 nm. By using the method proposed by Zhang and Yu et al.,41 we plot the refractive indices dispersion curves for CaB5O7F3 (Figure 3b). The shortest type I PM SHG limit of CaB5O7F3 is as short as 183 nm, suggesting that it may be capable of generating the DUV laser by a direct SHG process. The SHG coefficients (dij) of CaB5O7F3 were calculated by length-gauge formalism.42,43 Under the restriction of Kleinman’s symmetry, CaB5O7F3 possesses three independent and nonzero SHG coefficients. The calculated SHG coefficients of CaB5O7F3 are d15 = d31 = −0.94 pm/V, d24 = d32 = 0.38 pm/V, and d33 = 0.73 pm/V. In addition, the SHG density analysis21,44 was adopted to evaluate the origin of NLO-active electron states and units. Two components, that is, virtual-electron (VE) and virtual-hole (VH), are considered in this approach. The SHG density of the largest tensor d31 is drawn in Figure 3. In the dominant VE process, the primary contribution of occupied states to d31 comes from the nonbonding O-2p and F-2p in VB, revealing that the contributions of SHG are mostly derived from BO3 and BO3F groups in occupied states. From structural point of view, it also indicates that the [B5O7F3]∞ layers are the NLO-active substructure in CaB5O7F3. Meanwhile, the major contributions of unoccupied states to d31 are mainly determined

which wave-like [B5O7F3]∞ layers are stacking along the (010) direction, and the Ca2+ cations locate in the 18 member ring (18-MR) to keep charge balance. Three unique B atoms exhibit two forms of coordination: three-coordinated BO3 and fourcoordinated BO3F. The planar BO3 triangles are regular with the B−O distances ranging from 1.355(3) to 1.380(3) Å, while the BO3F tetrahedra are distorted with the B−F bonds varying from 1.417(4) to 1.432(5) Å and B−O bonds in the range of 1.439(3)−1.531(3) Å. These bond lengths are in normal range and are comparable with other fluorooxoborates.31−33 The functional building block (FBB) of CaB5O7F3 is the (B5O9F3)6− group, which is composed of three BO3F tetrahedra and two BO3 triangles (Figure 1b). To our best knowledge, the (B5O9F3)6− FBB is new in borate chemistry. The (B5O9F3)6− groups further connect with each other, generating a 2D [B5O7F3]∞ layer extending in the ac plane (Figure 1b). It is interesting to point out that CaB5O7F3 inherits the favorable structural features of KBBF, i.e., the dangling bonds of all BO3 groups are eliminated by the connection of BO3F groups. In addition, because the Ca2+ cations tend to reside in the layers, the interlayer spacing of two adjacent [B5O7F3]∞ layers in CaB5O7F3 is ∼4.20 Å (vs 6.25 Å in KBBF), expecting a significantly enhanced interlayer force (Figure 1d). Thus, CaB5O7F3 shows a more compact structure in comparison with KBBF, which is essential to overcome the layering crystal habit of the latter. The IR spectrum and peak assignments of CaB5O7F3 are shown in Figure S3. The peak assignments are comparable to recently reported fluorooxoborates,22,31−33,36 which confirm the rationality of the structural model. The thermal behaviors are presented in Figure S4, demonstrating that CaB5O7F3 melts incongruently and is stable up to 640 °C. The UV−vis−NIR diffuse reflectance spectrum shows no obvious absorption in the range of 180−2600 nm, and the cutoff edge of the title compound is below 180 nm (Figure S5), which indicates that CaB5O7F3 can be used in the DUV region. CaB5O7F3 is expected to show SHG effect because of the asymmetric structure. We perform powder SHG measurements by using the Kurtz-Perry method37 under incident laser wavelength at 1064 and 532 nm, respectively. It indicates that CaB5O7F3 has an SHG efficiency of approximately 2 × KDP and 0.4 × β-BBO, for 532 and 266 nm frequent-doubling B

DOI: 10.1021/acs.inorgchem.8b00531 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

[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

*(Y.W.) E-mail: [email protected]. *(S.P.) E-mail: [email protected]. ORCID

Ying Wang: 0000-0001-6642-543X Bingbing Zhang: 0000-0002-1334-5812 Shilie Pan: 0000-0003-4521-4507 Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the West Light Foundation of the CAS (Grant No. 2015-XBQN-B-11), the National Natural Science Foundation of China (Grant Nos. 51602341, 51425206, 91622107), the Natural Science Foundation of Xinjiang (Grant No. 2016D01B061), the National Key Research Project (Grant Nos. 2016YFB1102302, 2016YFB0402104), the National Basic Research Program of China (Grant No. 2014CB648400), and Xinjiang Key Research and Development Program (Grant No. 2016B02021).

Figure 3. (a) Projected density of states for CaB5O7F3. (b) Calculated refractive index dispersion curves of CaB5O7F3. Type I PM, i.e., nx(ω) = ny(2ω) occurs when the solid line crosses the dotted line. (c−f) SHG density maps of the (c) VE occupied, (d) VE unoccupied, (e) VH occupied, and (f) VH unoccupied orbitals of the SHG tensor d31.

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by 3d orbital of Ca, indicating that the Ca atom also has considerable contribution to SHG effects. The B−O and B−F anti-σ bonds also give some slight contribution to d31 in CB. This is, however, different from the AB4O6F family, in which the A-site cation has almost no contributions.31−33 In summary, a new NCS alkaline earth metal fluorooxoborate crystal, CaB5O7F3, has been obtained through high-temperature method in a closed system. CaB5O7F3 features a large band gap and strong NLO response (2 × KDP). Importantly, it also exhibits suitable birefringence to ensure the shortest PM second-harmonic wavelength down to the DUV region (∼183 nm). Our theoretical approach indicates that the SHG origin of CaB5O7F3 comes from the synergistic effect of [B5O7F3]∞ layers and Ca cations. Very recently, we found that M2B10O14F6 (M = Ca, Sr) moieties have been reported by another group.45 Thus, we believe that the distinct point of view for these excellent compounds in both works will benefit the design of prospective DUV NLO materials.

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REFERENCES

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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.8b00531. Experimental sections, crystallographic data tables, optical property measurements, and DFT calculation results(PDF) Accession Codes

CCDC 1826239 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 data_ C

DOI: 10.1021/acs.inorgchem.8b00531 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b00531 Inorg. Chem. XXXX, XXX, XXX−XXX