Computer-Assisted Design of a Superior Be2BO3F Deep-Ultraviolet

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

Computer-Assisted Design of a Superior Be2BO3F Deep-Ultraviolet Nonlinear-Optical Material Qiang Bian,†,‡ Zhihua Yang,*,† Yanchao Wang,§ Miriding Mutailipu,†,‡ Yanming Ma,*,§ and Shilie Pan*,† †

CAS Key Laboratory of Functional Materials and Devices for Special Environments, and Xinjiang Key Laboratory of Electronic Information Materials and Devices, Xinjiang Technical Institute of Physics & Chemistry of Chinese Academy of Sciences, 40-1 South Beijing Road, Urumqi 830011, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § State Key Laboratory of Superhard Materials, Department of Physics, Jilin University, Changchun 130012, China S Supporting Information *

in (Be2BO3F2)∞ layers, which yield large SHG and suitable birefringence for generating coherent DUV light.15 However, the large intercalated K atoms induce a wide interlayer spacing (6.25 Å) and thus weak interlayer bonding between the adjacent (Be2BO3F2)∞ layers.16 This causes a severe layering tendency in the structure. Furthermore, substitution of the K atoms by heavier Rb and Cs atoms increases the interlayer spacing to 6.61 Å in RbBe2BO3F2 and 7.04 Å in CsBe2BO3F2.17,18 To reduce structural layering, one strategy is to remove the interlayers of the K atoms from the KBBF crystal, which would strengthen the interlayer bonding. Therefore, the design of superior DUV-NLO materials from alkali-metal-free Be2BO3F (BBF) comprising only light elements is desirable. Our focus on BBF satisfies the fundamental criteria that are essential for good DUV-NLO materials. First, borates comprising light elements are transparent to light down to short wavelengths because the light elements’ small atomic radii and large electronegativities are beneficial to shift the cutoff edge to the DUV region.19−21 Second, the coparallel distribution of the planar BO3 groups in borates induces a relatively large birefringence,22−24 which favors a short phase-matching edge for generating short DUV coherent light. Third, the coparallel BO3 groups are aligned in the same orientation, which is crucial for producing large SHG coefficients.11,17,18,25 Fourth, the chemical bonds formed between light elements are generally short and strong, suggesting that borates containing only light elements might prevent the layering tendency and thus enable the growth of large crystals. Crystalline BBF was first synthesized in 1978.26 Two crystalline phases have been reported: C2 (α-BBF) and R3̅c (βBBF). The SHG response of α-BBF is about one-quarter of that of KH2PO4, which is too small to be useful as a DUV-NLO material.27 β-BBF crystallizes in centrosymmetric space groups, violating the Kleinman symmetry requirements for SHG.28 Thus, BBF structures with good DUV-NLO properties have remained elusive. In this work, we use the swarm intelligence-based CALYPSO (Crystal structure AnaLYsis by Particle Swarm Optimization) method29−31 for structure prediction, as implemented in the

ABSTRACT: Deep-ultraviolet (DUV) nonlinear-optical (NLO) materials generating coherent DUV light by a direct second-harmonic-generation (SHG) process have long been pursued as industrially useful lasers. For several decades, KBe2BO3F2 (KBBF) has been regarded as the best DUV-NLO material; it is characterized by a short DUV phase-matching edge of 161 nm and a large SHG coefficient of 0.47 pm/V. However, it suffers a strong layering tendency, hindering the growth of large crystals for commercial use. Here, we use a computer-aided swarm structure searching technique to design an alternative DUV-NLO material with a new atmospheric-pressure phase Be2BO3F2 with a P6̅2c space group (γ-BBF) that outperforms the DUV-NLO properties of KBBF. The predicted DUV phase-matching edge and SHG coefficient of γ-BBF are 152 nm and 0.70 pm/V, respectively. The structure of γ-BBF reduces the layering tendency compared with KBBF because of the absence of K atoms in the γ-BBF crystal. Our work paves the way for superior DUV-NLO materials that can be grown as large crystals for commercial applications.

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eep-ultraviolet (DUV) nonlinear-optical (NLO) crystals are key materials for producing coherent DUV ( 0.1) will result in serious walk-off and self-focused effects, reducing the SHG efficiency. Therefore, a moderate birefringence value (i.e., ∼ 0.07−0.1) is desirable for satisfying the phasematching condition in the DUV region.38 Optical property calculations (Figure 2b) show γ-BBF to be a negative uniaxial crystal with a moderate birefringence value of about 0.086 at 400 nm, which is somewhat larger than that of KBBF (Δn ≈ 0.078 at 400 nm). This appropriate birefringence makes γ-BBF suitable for DUV phase matching. The physical cause of the birefringence is mainly the parallel arrangement of the BO3 units. B

DOI: 10.1021/acs.inorgchem.8b00557 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

The predicted high DUV-NLO performance of γ-BBF is very encouraging. γ-BBF is the simplest form of the KBBF structure and thus retains the structural features of the KBBF crystal,11,25 in which the BO3 groups are coplanar and aligned to produce moderate birefringence and a large SHG coefficient. This structure type is believed to be the optimal configuration for DUV harmonic generation.14,15 Following the design concepts of γ-BBF, fluoroborates composed only of light elements (e.g., lithium fluorooxoborate42 and beryllium fluoroborates) should be screened to identify more DUV-NLO materials. In summary, a new atmospheric-pressure phase of γ-BBF with space group P6̅2c was uncovered by computer-assisted swarm structure searching. The structure has two-dimensional (BeBO3)∞ layers parallel to each other with bridging F atoms between them, forming a three-dimensional framework. The parallel distribution of the BO3 units in the (BeBO3)∞ layers causes an appropriate birefringence value (Δn ≈ 0.086 at 400 nm), which leads to short DUV phase matching. As a result, γBBF features the shortest SHG wavelength (152 nm) among the known DUV-NLO materials, thus providing the widest optical wavelength region (152−200 nm) for generating coherent DUV light. The BO3 units of the (BeBO3)∞ layers in γ-BBF have a coparallel alignment and are densely packed, increasing the SHG efficiency to 1.5 times that of KBBF. Furthermore, the polar Be− F covalent interactions linking the adjacent (BeBO3)∞ layers are short and strong, greatly reducing the layering tendency. The predicted γ-BBF crystal is likely to be able to grow to sizes large enough for practical applications. Also, new phases including metastable phases might also be synthesized under the atmospheric-pressure or high-pressure conditions. Our work provides an alternative route to seeking superior DUV-NLO materials that can provide large crystals.

γ-BBF exhibits a high SHG conversion efficiency. The calculated SHG coefficient is 0.70 pm/V, which is ∼1.8 times larger than that of KH2PO4 (0.39 pm/V) and ∼1.5 times larger than that of KBBF39 (0.47 pm/V). Importantly, the NLO-active BO3 units in γ-BBF have an aligned coparallel arrangement. This is beneficial for the superposition of second-order nonlinear polarizability. In α-BBF, the NLO-active BO3 has an antiparallel distribution, greatly reducing the SHG efficiency. SHG density calculations (Figure S3) for γ-BBF show that the spheroidal charge cloud is mainly localized on the opposite sides of the B and O atoms in the planar BO3 units, indicating that the 2p orbitals of the B and O atoms dominate the SHG process. Moreover, the number density of BO3 units for γ-BBF (13.7 × 10−3) is much denser than that of KBBF (9.42 × 10−3). This explains the origin of the enhanced SHG efficiency in γ-BBF. To generate coherent light effectively by SHG, the phasematching condition of ne(λ/2) ≤ no(λ) is required for a negative uniaxial crystal, where ne(λ/2) and no(λ) are the minimum and maximum of the refractive indices at wavelengths of λ/2 and λ, respectively.33 γ-BBF has moderate birefringence, which is favorable for a short phase-matching edge of 152 nm (Figure 2b). To the best of our knowledge, this is the shortest SHG wavelength among all known DUV-NLO materials.17−19,39−41 As a result, γ-BBF provides the widest optical wavelength region (152−200 nm) for generating coherent DUV light by a direct SHG response. Growing large crystals of NLO materials is necessary for their practical use. γ-BBF has reduced the layering tendency relative to KBBF. In γ-BBF, the overlap population found by Mulliken population analysis (Table S2) for Be−F (0.35 e) is much larger than that for K−F (0.08 e) in KBBF and close to that of Be−O (0.36 e). Such a high overlap population indicates a strong covalency in the Be−F bonds, similar to Be−O bonds. Given the covalent nature of Be−O bonds, the Be−F bonds in γ-BBF appear to be strong covalent interactions. They are also much shorter (1.588 Å) than the K−F bonds (2.756 Å) in KBBF. As a result, the adjacent layers in γ-BBF ((BeBO3)∞ layers) are linked by the polar Be−F covalent interactions, and those in KBBF [(Be2BO3F2)∞ layers] are linked by the K−F ionic interactions. This will inevitably result in much stronger interlayer interactions in γ-BBF than in KBBF (Figure 3). Note that γ-BBF has an interlayer bonding behavior similar to that of β-BBF. Its interlayer spacing (4.21 Å) and Be−F distance are nearly identical with those of β-BBF (Figure S4). Large β-BBF crystals have been grown experimentally (7.0 × 5.0 × 0.7 mm3).28 Similarly, once synthesized, it may be possible to grow large γBBF crystals.

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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.8b00557. Computational details and additional data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Zhihua Yang: 0000-0001-9214-3612 Yanchao Wang: 0000-0003-4518-925X Miriding Mutailipu: 0000-0002-1331-0185 Shilie Pan: 0000-0003-4521-4507 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 11474353, 11774414, 51425206, and 91622107), National Basic Research Program of China (Grant 2014CB648400).

Figure 3. Structural evolution between the adjacent layers from KBBF to γ-BBF. After removal of the K atoms from KBBF, the shorter, stronger Be−F covalent interactions connect the adjacent (BeBO3)∞ layers in the predicted γ-BBF structure, which greatly reduces the layering tendency. C

DOI: 10.1021/acs.inorgchem.8b00557 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.8b00557 Inorg. Chem. XXXX, XXX, XXX−XXX