Structural Design of Two Fluorine–Beryllium Borates BaMBe2(BO3

Communication pubs.acs.org/IC

Structural Design of Two Fluorine−Beryllium Borates BaMBe2(BO3)2F2 (M = Mg, Ca) Containing Flexible Two-Dimensional [Be3B3O6F3]∞ Single Layers without Structural Instability Problems

Shu Guo,†,‡ Xingxing Jiang,†,‡ Mingjun Xia,† Lijuan Liu,*,† Zhi Fang,†,‡ Qian Huang,†,‡ Ruofei Wu,†,‡ Xiaoyang Wang,† Zheshuai Lin,† and Chuangtian Chen† †

Beijing Center for Crystal Research and Development, Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China S Supporting Information *

grown crystal was poor. According to Meng’s report, the relatively rigid [Be6B6O15]∞ double layers were separated by large Sr atoms, which is “uncomfortable” for Sr atoms and results in structural instability problems of SBBO.25 Hence, how to resolve the structural instability problem and maintain the structure advantages is a very interesting subject. In a previous study, two strateges were adopted to overcome the structural instability problem and maintain the structural advantages of SBBO. In 2011, Huang et al. utilized the relatively small alkali-metal atoms to improve the small space between the double layers of SBBO, generating a series of new structures MM′Be2B2O6F (M = Na and M′ = Ca; M = K and M′ = Ca, Sr) with [Be6B6O12F3]∞ double layers.21 Then, in 2016, Zhao et al. developed a SBBO-like compound, K3Ba3Li2Al4B6O20F, which exhibits a “comfortable” (or large) space [Li2Al4B6O20F]∞ double layer and maintains the structural advantages of SBBO.27 In this work, two fluorine−beryllium borates, BaMgBe2(BO3)2F2 (BMBBF; CCDC 1541463) and BaCaBe2(BO3)2F2 (BCBBF; CCDC 1541466), were rationally designed by introducing the relatively large Ba atoms into the [Be6B6O15]∞ double layers to break the “uncomfortable” environment of SBBO. As expected, these two compounds inherit the moderate birefringence and short UV absorption edge of SBBO. Furthermore, BMBBF and BCBBF are kinetically stable according to the structural stability analysis. Single crystals of BMBBF and BCBBF were obtained by a flux method from a high-temperature solution (see the Supporting Information and Figure S1). Then differential scanning calorimetry (DSC) analysis was used to reveal the thermal stability of BMBBF and BCBBF. As shown in Figure 1a, the DSC curve of BMBBF shows one sharp endothermic peak at about 1040.9 °C in the heating process but two small exothermic peaks at about 850.7 and 873.2 °C in the cooling process. As shown in Figure 1b, the DSC data of BCBBF show an endothermic peak around 991.7 °C in the heating curve and a sharp exothermic peak at about 805.3 °C. Moreover, the powder X-ray diffraction (XRD) patterns of residues of these two compounds after DSC were totally different from those before measurement, indicating that these two compounds melt incongruently (Figure S2).

ABSTRACT: Molecular structural design is a compelling strategy to develop new compounds and optimize the crystal structure by atomic-scale manipulation. Herein, two fluorine−beryllium borates, BaMgBe2(BO3)2F2 and BaCaBe2(BO3)2F2, have been rationally designed to overcome the structural instability problems of Sr2Be2B2O7 (SBBO). When relatively large Ba atoms were introduced, the [Be6B6O15]∞ double layers of SBBO were successfully broken, generating flexible [Be3B3O6F3]∞ single layers. Also, the strategy adopted in this work has many implications in understanding the structural chemistry and designing novel optical functional materials in a beryllium borate system.

B

eryllium borates, benefiting from their good transmission in ultraviolet (UV; λ < 400 nm) and deep-UV (λ < 200 nm) regions, have been considered to be good candidates as UV or deep-UV optical materials.1−6 From the viewpoint of structure− property relationships, beryllium borates with layered structure are an important family of materials because of their deduced interesting properties, such as large birefringence and secondharmonic-generation (SHG) coefficient.7−11 In recent decades, a large number of beryllium borates with layered structure were found and proven to be potential UV/deep-UV nonlinear-optical (NLO) or birefrigent materials, such as Be 2 BO 3 F,12,13 KBe2BO3F2,9,10,14 RbBe2BO3F2,15,16 and BaBe2BO3F3 with [Be2BO3F2]∞ single layers,17 NaBeB3O6,18 Na2CsBe6B5O15,19 Na2Be4B4O11, and LiNa5Be12B12O3320 with [Be2BO3O2]∞ planar layers, NaCaBe2B2O6F with [Be6B6O12F3]∞ double layers,21 and Sr2Be2B2O7 (SBBO)22,23 and Ba2Be2B2O724 with [Be6B6O15]∞ double layers. Among them, SBBO was first found by our group in 1995. Structurally, it exhibits two-dimensional (2D) [Be3B3O6]∞ planar layers, which are further bridged by O atoms to construct [Be6B6O15]∞ double layers. As reported, SBBO exhibits a moderate birefringence (Δn ∼ 0.062 at 1064 nm), and the UV absorption edge is down to ∼165 nm,22 which is considered to be a good candidate as a deep-UV NLO material. Unfortunately, the structure of SBBO is not fullly solved and the structure is unstable, which is a structural polymorphism problem.25,26 Therefore, there is still a lack of high-opticalquality SBBO crystals for further measurement because the © XXXX American Chemical Society

Received: June 26, 2017

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DOI: 10.1021/acs.inorgchem.7b01627 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

bond-valence-sum calculations for BMBBF and BCBBF, the structure model is valid without a large strain in the structure. To further evaluate the structural stability of these two compounds, phonon dispersion of the two compounds was performed by first-principle calculations and used to reflect the interatomic interaction.28 In Zhao’s report, the SBBO structure is not in a kinetically stable state according to the negative (imaginary) phonon eigenvalues.27,28 As shown in Figure 3,

Figure 1. DSC curves for (a) BMBBF and (b) BCBBF.

The structures of BMBBF and BCBBF were determined by single-crystal XRD, and the phase purity of the title compounds was confirmed by powder XRD (Figure S3). BMBBF and BCBBF are isostructural and crystallize in the same trigonal crystal system with a space group of P3̅c1. Herein, the structure of BMBBF is discussed in detail as an example (Figure 2c,e). In the asymmetric unit, Ba, Mg, B, Be, O, and F occupy only crystallographically unique positions. Specifically, each B atom is surrounded by three O atoms to form [BO3] triangles with d(B−O) = 1.377(2) Å and a O−B−O angle of 120.0(1)°. Also, Be atoms are found to coordinate to three O atoms and one F atom to form a distorted [BeO3F] tetrahedron with d(Be−O) = 1.625(3) Å and d(Be−F) = 1.622(12) Å. The [BeO3F] tetrahedron connected with three [BO3] triangles via three corner-sharing O atoms to form a 2D [Be3B3O6F3]∞ infinite layer in the ab plane (Figure 2e). Also, the neighboring [Be3B3O6F3]∞ single layers are alternately bridged by Ba−F and Mg−O bonds. The Mg atoms are all six-coordinated to form a [MgO6] polyhedron with d(Mg−O) = 2.111(2) Å. Also, the Ba atoms are found to coordinate to six F and six O atoms to form a [BaO6F6] polyhedron. The bond lengths of Ba−O and Ba−F are 3.015(2) and 2.696(1) Å, respectively. Detailed crystallographic information is listed in Tables S1−S3. The total bond valences for the Ba, Be, Mg, Ca, B, O, and F atoms were calculated and are listed in Table S3, which indicate that the Ba, Be, Mg, B, O, and F atoms are in oxidation states of 2+, 2+, 2+, 3+, 2−, and 1−, respectively. According to the results of the bond length and

Figure 3. Phonon dispersion of (a) BMBBF and (b) BCBBF.

eigenvalues for all phonon modes are positive, which confirmed the structural stability of BMBBF and BCBBF theoretically. Furthermore, the residue factors R(F) for BMBBF and BCBBF are 0.031 and 0.023 (Table S1), respectively, which are much smaller than that of SBBO (0.043). Hence, the crystal structures of the title compounds successfully overcome the problem of structural instability (layer twinning) of SBBO. Structurally, the following two steps were used to elucidate our structural design strategy. First, the [BeO3F] groups were used to substitute the site of the [BeO4] groups in the [Be6B6O15]∞ double layers of SBBO (Figure 2d,e), generating [Be3B3O6F3]∞ single layers. Second, the Ba2+−F−-bridged bonds are introduced to the spaces among the adjacent single layers mentioned above, which break the original [Be6B6O15]∞ double layers of SBBO. Also, the distances of the neighboring single layers (4.895 Å for BCBBF and 5.031 Å for BMBBF) are larger than that of of double layers of SBBO (3.917 Å), which is “comfortable” for Ba atoms and conducive to improving the problem of stacking fault in the crystal. Moreover, the bond lengths of Ba−F (2.696 Å for

Figure 2. Structural evolution from SBBO to BCBBF and BMBBF. (a) Ball-and-stick model of SBBO. (b) Ball-and-stick model of BCBBF. (c) Ball-andstick model of BMBBF. (d) [Be3B3O9]∞ layer. (e) [Be3B3O6F3]∞ layer. B

DOI: 10.1021/acs.inorgchem.7b01627 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

results demonstrate that BCBBF and BMBBF have the potential to be deep-UV birefringent materials, and more work is still ongoing. Also, the strategy of optimizing the crystal structure by atomic-scale manipulation is effective in the design of new deepUV birefrigent and NLO materials with good performances.

BMBBF and 2.742 Å for BCBBF) and Be−F (1.622 Å for BMBBF and 1.621 Å for BCBBF) are not enlarged, which provides strong interlayer bonding between the single layers. From the viewpoint of structure−property relationships, the relatively small alkaline-earth metals Mg and Ca were introduced in the residual spaces of the single layers and reduced the interlayer spacing (3.131 Å for BCBBF and 2.643 Å for BMBBF), which is narrower than that of SBBO (3.739 Å) and maintained the high density of the [BO3] groups. Because BMBBF and BCBBF inherit similar structural units of SBBO, they deserve to have comparable optical properties. To verify our prediction, first-principles calculations were performed based on density functional theory methods and the refractive indices n (and the birefringence Δn) were obtained, as plotted and summarized in Figure 4. As expected, the refractive index curves of all three

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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.7b01627. Details of data analysis methods, photographs of as-grown BMBBF and BCBBF crystals, DSC curves, powder, experiemental, and calculated XRD patterns, and structure refinement (PDF) Accession Codes

CCDC 1541463 and 1541466 contain 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 Author

*E-mail: [email protected].

Figure 4. Calculated refractive indices of (a) BMBBF and (b) BCBBF.

ORCID

Shu Guo: 0000-0002-2098-8904 Mingjun Xia: 0000-0001-8092-6150 Zhi Fang: 0000-0001-5002-4842 Zheshuai Lin: 0000-0002-9829-9893

compounds display strong anisotropy and tend to have n0 > ne, indicating that they are negative uniaxial crystals. According to theoretical calculations, the magnitudes of birefringence (Δn = n0 − ne) of BMBBF and BCBBF are about 0.062 and 0.061, respectively, at 1064 nm, which are comparable to that of SBBO (Δn = 0.062 at 1064 nm; Table S4). Furthermore, the transmittance spectra of BMBBF and BCBBF are measured using a Lambda 900 UV−vis−near-IR spectrophotometer from 185 to 2500 nm. As shown in Figure 5,

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by grants from the National Natural Science Foundation of China (Grants 51402316, 51502307, and 51702330 ), National Key R&D Program of China (Grant 2016YFB0402103), and National Instrumentation Program (Grant 2012YQ120048).

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DOI: 10.1021/acs.inorgchem.7b01627 Inorg. Chem. XXXX, XXX, XXX−XXX