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Rational Design of Deep-Ultraviolet Nonlinear Optical Materials in Fluorooxoborates: Toward Optimal Planar Configuration Fei Liang,†,‡ Lei Kang,† Pifu Gong,† Zheshuai Lin,*,†,‡ and Yicheng Wu† †

Center for Crystal Research and Development, Key Lab Functional Crystals and Laser Technology of Chinese Academy of Sciences, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China ‡ University of Chinese Academy of Sciences, Beijing 100190, PR China S Supporting Information *

Materials design based on first-principles calculations provides an efficient way to discover new materials with good performance. In fact, several good NLO materials have been designed and predicted in our previous studies.17−20 In this work, we focus on the rational design of DUV NLO materials in the fluorooxoborates system. It is because the fluorooxoborate groups are stable and nonpoisonous and show good compatibility with known large π-conjugated groups. Very recently, Pan’s group introduced the Be-free (BO3F)4−, (BO2F2)3−, and (BOF3)2− fundamental building blocks (FBBs) in borates and obtained new materials that balance the multiple criteria for DUV NLO generation.21 Therefore, it would be promising to combine fluorooxoborate units and planar π-conjugated groups into one compound for exploring a new DUV NLO material. To date, however, the members in this materials family are very rare, and the relevant materials design is very desirable. On the basis of the above consideration, a series of rare-earth borate compounds, LnB4O6(OH)2Cl (Ln = Pr, Nd, Ce), attracts our attention. They were first reported as possible NLO materials by Belokoneva in 2002 and refined by AlbrechtSchmitt’s and Mi’s groups in 2012.22−24 Figure 1a displays the crystal structure of LnB4O6(OH)2Cl. All of these compounds crystallize in the monoclinic Cc space group and are featured with the layered structure of polyborate [B4O6(OH)2]2− sheets parallel to the c-axis. The polar anionic groups are composed of [4:(2Δ + 2T)] FBBs, including two planar (BO3)3− triangles and two tetrahedral [BO3(OH)]4− units. These FBBs are linked to form zigzag borate chains and are further interconnected with each other by common O atoms, resulting in a 2D layer with nearly equilateral triangular 9-membered rings within the layer (Figure 1b). The rare-earth atoms just reside at the center of 9-membered rings, and Cl atoms locate in interlayer space individually. It is noted that all boron and oxygen atoms (except for OH terminals) are laid in the nearly planar sheets of polyborates, which would supply a good parent model for further molecule engineering design. The optical nonlinearity of the crystals is superior to that of the K[B5O6(OH)4]·2H2O (KB5) crystal (dij ∼ 0.1 × ADP) by an order of magnitude.22 However, the bandgap of LnB4O6(OH)2Cl is too small to be applied in the DUV region, owing to the presence of valence electrons of rare-earth and Cl (Figure S1). So there is a

eep-ultraviolet (DUV, λ < 200 nm) coherent light becomes increasingly urgent owing to its great applications in many scientific and technical fields, such as semiconductor manufacturing, laser photolithography, and superhigh-resolution photoemission spectroscopy.1−3 Up to now, the most effective method to produce high quality DUV lasers is cascaded frequency conversion by DUV nonlinear optical (NLO) crystals.4 However, due to the harsh conditions (i.e., wide bandgap, large NLO effect, and birefringence) for DUV NLO generation, KBe2BO3F2 (KBBF) is the only practically usable material to date that can generate coherent light of wavelengths below 200 nm by the direct second harmonic generation (SHG).1 In DUV NLO materials, large NLO effects and birefringence can be expected when the planar π-conjugated building units, such as (BO3)3−, (CO3)2−, (NO3)− , and (B3O6)3− groups, are arranged in the optimal manner.5 For example, KBBF features a quasi-two-dimensional (Q2D) crystal structure with alternated anionic layer and potassium along the c axis. Every (BO3)3− unit is linked to six (BeO3F)5− units by the bridged O atoms, thus constructing a compact [Be2BO3F2]− anionic layer.1 So KBBF shows strong optical anisotropy and possesses large birefringence (Δn = 0.088@400 nm), which leads to the shortest SHG phase-matching (PM) output wavelength (λPM ∼ 161 nm) in all known NLO materials to date.1 In addition, high spatial density and coparallel alignment of π-conjugated (BO3)3− groups in KBBF deduce a relatively large SHG coefficient (d11 = 0.47 pm/V).1 Moreover, the dangling bonds of the oxygen anion in planar (BO3)3− units are almost eliminated by Be−O bonds, thus resulting in the ultrawide bandgap (Eg ∼ 8.40 eV), corresponding the cutoff wavelength (λcutoff) of 147 nm, of KBBF.1,6 Unfortunately, KBBF suffers a strong layering tendency that originates from the weak K−F ionic interactions between the adjacent [Be2BO3F2]− layers, which causes a great difficulty in the growth of thick crystals along the c-axis. Meanwhile, the highly poisonous BeO adopted in the synthesis of KBBF is not environmentally friendly. Owing to these obstacles, it is urgently demanded that new DUV NLO materials be discovered that preserve the merits of KBBF while overcoming the demerits.7−11 In order to accelerate advanced materials discovery, the structural data mining combined with high-performance computational simulations has become a research hotspot in materials science,12−15 as also essentially proposed by the “Materials Genomics Initiative”.16

D

© 2017 American Chemical Society

Received: July 26, 2017 Revised: August 18, 2017 Published: August 18, 2017 7098

DOI: 10.1021/acs.chemmater.7b03162 Chem. Mater. 2017, 29, 7098−7102

Communication

Chemistry of Materials

Figure 1. Crystal structural evolution diagram from LnB4O6(OH)2Cl (a, b), to BaB4O6F2 (c, d), to KB4O6F-I (e, f). Parts a, c, and e are viewed along the a axis, and parts b, d , and f are viewed along the c axis. The A and B sites represent the molecular tailoring position from KBF-I to KBF-II. The blue, pink, cyan, purple, brown, red, white, and light green balls represent Ln, Cl, Ba, K, B, O, H, and F atoms, respectively.

replaced by the charge-compensating K+ ions. This leads to the discovery of another hypothetical compound KB4O6F (named as KBF-I) (Figure 1e,f). The main structural difference between BBF and KBF-I is that the spatial density of planar (BO3)4− units in KBF-I is 25% higher than that in BBF. This would result in larger birefringence of KBF-I than that of BBF since the 2D [B4O6F]− layer maintains a near-perfect planar configuration in KBF-I (Figure 1f). The first-principles computations are employed to investigate the physiochemical properties in BBF and KBF-I (for details see Supporting Information, Tables S1−S3). The calculated phonon vibrational spectra show that none of the imaginary phonon mode existed, thus indicating that these two compounds are both kinetically stable (Figure S2). In addition, the calculated elastic coefficients demonstrate that they are also mechanically stable under ambient conditions (Table S4).27 Both results clearly demonstrate that BBF and KBF-I can be synthesized under suitable experimental conditions. The calculated optical properties of BBF and KBF-I are listed in Table 1. BBF and KBF-I both possess large bandgap (>7.5 eV), moderate SHG effect (dij ∼ 1.0 × KDP), and large birefringence (Δn > 0.06). However, the dispersion curves of refractive indices show that the shortest SHG PM output wavelength λPM in BBF is only 180 nm (Figure S3), and it

tremendous need to tailor LnB4O6(OH)2Cl for satisfying multiple requirements of DUV NLO materials. Herein, we predict three new DUV NLO materials, BaB4O6F2, KB4O6F-I, and KB4O6F-II, by rational molecular tailoring design. The three compounds are proven to be kinetically stable and mechanically stable. In particular, through adjusting the number and position of (BO3F)4− units, a perfect planar configuration with high density (B3O6)3− groups is obtained in KB4O6F-II. Based on the first-principles calculations, the optical properties of KB4O6F-II are obtained (λcutoff ∼ 160 nm, dij > 1 × KDP, Δn = 0.098@400 nm), which exhibits the excellent DUV NLO capability almost exceeding that of the KBBF crystal. At the first step in our molecular engineering design based on LnB4O6(OH)2Cl, the following two guidelines are considered: (i) In order to enlarge the bandgap into the DUV, the d−f/f−f electron transitions need to be avoided. The rare-earth elements should be replaced, preferably by the alkali metal and alkali-earth cations. Meanwhile, the isolated halide Cl− anions should also be removed to enlarge the bandgap, which has been verified to be feasible in our previous studies (Ba5P6O20 vs Ba3P3O10Cl).25 (ii) The OH− groups in [BO3(OH)]4− units should be replaced by F− ions. First, B− F bonds usually contribute to the enlargement of the bandgaps in borates because the F atom has a larger electronegativity than the oxygen atom. The strongly ionic B−F bonds benefit to the transparency of DUV light.1,6 Second, the resulted (BO3F)4− units usually exhibit larger microscopic polarizability anisotropy than that of (BO4)5− or B(OH)4−,21 which is preferable to produce a larger SHG effect and birefringence.26 Directly following the above guidelines, a hypothetical compound BaB4O6F2 (BBF) is designed. The structure of BBF (Figure 1c,d) belongs to the monoclinic Cc space group and exhibits a similar layer motif with LnB4O6(OH)2Cl. In view of BBF, the Ln3+ ions in LnB4O6(OH)2Cl are replaced by Ba2+, while Cl− ions are removed entirely. All of the hydroxyl units are substituted by F atoms, hence forming strongly anisotropic (BO3F)4− units. The two (BO3F)4− units and planar (BO3) units are then linked by common O vertices, thus connecting into the 2D near-planar [B4O6F2]2− layer, which is favorable for obtaining large birefringence. Based on the structure of BBF, the further rational molecular tailoring design can be performed; half of the terminal F atoms in the (BO3F)4− units are sheared out, and all Ba2+ cations are

Table 1. Comparison of Bandgaps (and λcutoff (nm)), SHG Coefficients, Birefringence Values (Δn@400 nm), and the Shortest SHG Output Wavelengths (λPM) of BBF, KBF-I, and KBF-IIa space group

Eg (eV) (λcutoff)

BBF

Cc

7.80b (158)

KBF-I

Cc

7.67b(161)

KBF-II

P31c

7.70b(161)

KBBF

R32

8.45c (147)

SHG coefficients (pm/V) d11 = −0.42; d12 = 0.57; d13 = −0.06; d15 = 0.35; d24 = −0.21; d33 = −0.05 d11 = −0.50; d12 = 0.51; d13 = −0.07; d15 = 0.22; d24 = −0.14; d33 = 0.23 d15 = 0.02; d22 = −0.51; d33 = 0.17 d11 = ±0.47c

Δn (λPM) 0.065 (180) 0.087 (161) 0.098 (161) 0.088c (161)

a

The experimental values of KBBF are listed as reference. bThe Eg values are calculated by PBE0 hybrid exchange-correlation functionals. c Experimental values.1 7099

DOI: 10.1021/acs.chemmater.7b03162 Chem. Mater. 2017, 29, 7098−7102

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Chemistry of Materials

which can lead to a strong optical anisotropy, i.e., large birefringence. The geometry optimization reveals that KBF-II belongs to the trigonal P31c space group. In this structure the optimal 2D (B4O6F)− layers differ from those in KBF-I and consist of planar (B3O6)3− and tetrahedral (BO3F)4− units, while K atoms are located in the interlayer voids (Figure 2a,b). The (B3O6)3− units have aligned arrangement in every (B4O6F)− layer and have a slight torsion between adjacent layers around the 3-fold axis by approximately 34° (Figure 2c). A similar torsion is also found for borate rings in the crystal structure of β-BBO (∼9° and 20°).29 First-principles calculations show that KBF-II is kinetically and mechanically stable (Table S4 and Figure S2) so can be obtained in experiment as well. We further predict that KBF-II also has a large bandgap (7.70 eV) and high SHG coefficient (d22 = 0.51 pm/V). More importantly, the birefringence of KBF-II increased to 0.098 at 400 nm, and the shortest PM wavelength is down to the UV absorption edge of 161 nm (Figure S4). Compared with monoclinic KBF-I, trigonal KBF-II is a uniaxial crystal, which is more convenient for practical DUV applications. As a promising DUV NLO material, KBF-II is believed to have several important advantages: (i) The first is good DUV transparency. It is known that the dangling bond of O atoms in (B3O6)3− units limits the transparent capacity in the DUV region,6 such as in the case of BBO (Eg ∼ 6.51 eV or λcutoff ∼ 190 nm),29 which cannot be applied for the sixth-harmonic generation of Nd:YAG laser. In KBF-II, the dangling bonds of (B3O6)3− are largely eliminated by the sp3 hybridization of B− O σ-bonds in (BO3F)4− units (see Figure S5). As a result, the DUV absorption wavelength of KBF-I is significantly blueshifted to 161 nm (i.e., Eg ∼ 7.70 eV). (ii) The second is strong SHG effect. High spatial density of (B3O6)3− units, even slightly higher than that of KBBF, is very favorable for obtaining strong SHG effect in KBF-II. Theoretical calculations exhibit that (B3O6)3− groups make the dominate contribution to d22 and in particular the terminal O atoms in (B3O6)3− units make a major contribution to total SHG coefficients (Table S5 and Figure S6). (iii) The third is large birefringence. The phase-matchable condition of known DUV borate materials is summarized in Table S6. The birefringence of KBF-II is larger than that of KBBF, thus leading to smaller PM angle θ and larger effective SHG coefficient deff. For example, in the type-I SHG process (converting 354.6 to 177.3 nm, setting ψ = 0°), the deff (deff = d11 × cos θ) of KBBF is 0.19 pm/V (PM angle is 65.3°) while that (deff = d15 × sin θ − d22 × cos θ) of KBF-II is 0.41 pm/V (PM angle is 36.6°). So KBF-II will have a high energy conversion efficiency in the NLO process. (iv) The fourth is reinforced interlayer interaction. Equally important, good crystal growth habit is essential to DUV functional optical material. It is well-known that the layer habit of KBBF seriously impedes its commercialization. The interlayer distance of KBFII (5.27 Å) is shorter than that of KBBF (6.25 Å) (Figure S7). The adjacent (B4O6F)∞ layers are connected by the (KO6F) polyhedra (Figure 2d) with K−O bond length of 2.652/2.905 Å and K−F bond length of 2.571 Å. According to Coulomb’s law, the interaction force strength between K and O is nearly twice stronger than that of K and F atoms in KBBF. The enhanced interlayer binding forces suggest an improved growth habit for KBF-II compared with KBBF. (v) In the synthesis of KBF-II, highly poisonous BeO raw material is avoided. So it is environmentally friendly, convenient, and safe in the synthesis process.

cannot be applied for the sixth-harmonic generation of Nd:YAG lasers (@177.3 nm) by direct SHG. In comparison, the birefringence of KBF-I is enlarged to be 0.087 in virtue of increased number of planar (BO3)3− groups in the near-perfect 2D (B4O6F)− layer, which is close to KBBF (Δn ∼ 0.088). As a result, the λPM in KBF-I is blue-shifted to 161 nm (Figure S3), almost the same as that in KBBF (λPM ∼ 161 nm).28 This suggests that KBF-I is a possible deep-UV NLO material for the sixth-harmonic generation of Nd:YAG lasers. Nevertheless, KBF-I is a biaxial monoclinic crystal in the m (Cs ) point group, which is inconvenient for practical applications. We note that for the planar (BO3)3− unit itself it has the higher symmetry of the trigonal D3h point group, and if three (BO3)3− units are linked together into a planar (B3O6)3− unit, the same symmetry can be maintained. In comparison, if two (BO3)3− units and one (BO4)5− unit are connected into a nonplanar (B3O7)5− unit, the symmetry would be reduced to the C2v point group. In KBF-I, two planar (BO3)3− units and one tetrahedral (BO3F)4− unit are linked into the nonplanar (B3O6F)4− motif, and the imperfect coplanar arrangement of (BO3)3− units reduces the crystallographic symmetry. If the position of (BO3F)4− could be regulated by adopting a different linking mode between (BO3)3− and (BO3F)4− groups, the optimal coplanar arrangement of the π-conjugated (B3O6) 3− groups would be constructed with improved structural symmetry. Therefore, we propose to move the F atom in the (BO3F)4− groups in KBF-I from the trimer position (site A in Figure 1f) to the adjacent monomer position (site B). Through this simple but important atomic manipulation, the FBBs change from nonplanar (B3O6F)4− and planar (BO3)3− in KBF-I (Figure 1e,f), to planar (B3O6)3− and tetrahedral (BO3F)4− units (Figure 2a,b) in a new phase of KB4O6F (named as KBF-II). Since the microscopic symmetrical characteristic of (B3O6)3− is higher than that of the (B3O6F)4− group, a higher crystal symmetry can be expected in the KBF-II phase. Moreover, all (BO3)3− units in this phase are exactly located in one plane,

Figure 2. (a) Crystal structure of KBF-II viewed along the b axis. (b) Planar (B4O6F)− single layer in KBF-II. (c) Crystal structure of KBF-II viewed along the c axis (K atoms omitted for clarity). The torsion angle between adjacent layer is 34°. (d) Coordination environment of K atoms. The purple, brown, red, and light green balls represent K, B, O, and F atoms, respectively. 7100

DOI: 10.1021/acs.chemmater.7b03162 Chem. Mater. 2017, 29, 7098−7102

Communication

Chemistry of Materials ORCID

In addition, we also consider other possibilities with the selections of the alkali metal (A) site in KBF-II (A = Li, Na, Rb, Cs, NH4). It is found that, except for Li+, other compounds are all kinetically stable. Our calculations reveal that their optical properties strongly depend on the c parameter in the unit cell, i.e., the spatial density of (B3O6)3− units (the detailed results are listed in Table S7). In particular, (NH4)B4O6F is a very promising DUV NLO material. Contrary to the case that the spherical K+ ion has little electronic overlap with neighboring F− anions in KBF, the conical NH4+ has quite a large electronic overlap with F− anions in (NH4)B4O6F (Figure S8), indicating the stronger interlayer binding interaction in the latter crystal, which would benefit single crystal growth.19 Due to the relatively weak thermal stability of fluorooxoborates, the hydrothermal method might be a preferable approach to synthesize and grow the materials designed in this work. We hope that these investigations may encourage exploratory synthesis of fluorooxoborates and allow for the rational design of new NLO materials, especially for those used in UV and DUV applications. Notably, while this manuscript was in preparation, an independent experimental study on a similar topic was published in which Pan’s group synthesized a new fluorooxoborate (NH4)B4O6F (ABF) with orthogonal Pna21 symmetry.30 ABF features a 2D wave-like (B4O6F)− layer that consists of (BO3)3− and (BO3F)4− units, analogous to KBF-I and KBFII, and the layers are further linked by the NH4+ cations through hydrogen bonds. ABF exhibits a wide DUV transparent range and large birefringence that satisfies a frequency doubling short down to 158 nm, which clearly demonstrates its excellence as a DUV NLO crystal. The experimental results on ABF are quite comparable to the predicted DUV output capability in our designed compound (e.g., λPM = 161 nm in KBF-II), thus indicating our molecular design strategy is feasible and credible. In summary, we design three possible NLO materials in fluorooxoborates combining rational tailoring design and first principle calculations. A perfect planar configuration with high density (B3O6)3− groups is achieved in the trigonal KB4O6F phase (KBF-II). The calculated results demonstrate that it possesses a DUV SHG capability (λcutoff ∼ 160 nm, dij > 1 × KDP, Δn = 0.098), which almost exceeds the performance of KBBF. More importantly, the KB4O6F structure is predicted to be kinetically and mechanically stable. It is convincing that synthesis of KBF-II in experiments would significantly prompt the discovery of new DUV NLO fluorooxoborates and have great implications on the search for new functional NLO materials.

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Fei Liang: 0000-0002-4932-1329 Pifu Gong: 0000-0002-4656-1554 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by China “863” project (No. 2015AA034203) and the National Natural Science Foundation of China under Grant Nos. 91622118, 91622124, 11174297, and 51602318.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b03162. Computational methods, crystallographic data, phonon spectra, and refractive indices of BBF, KBF-I, and KBF-II, and PDOS and SHG density analysis of KBF-II (PDF)

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*(Z.L.) E-mail: [email protected]. 7101

DOI: 10.1021/acs.chemmater.7b03162 Chem. Mater. 2017, 29, 7098−7102

Communication

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DOI: 10.1021/acs.chemmater.7b03162 Chem. Mater. 2017, 29, 7098−7102