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Shu Guo, Xingxing Jiang, Lijuan Liu, Mingjun Xia, Zhi Fang, Xiaoyang Wang, Zheshuai Lin, and Chuangtian Chen Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b04403 • Publication Date (Web): 05 Dec 2016 Downloaded from http://pubs.acs.org on December 6, 2016

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BaBe2BO3F3: A New KBBF-type Deep-ultraviolet Nonlinear Optical Material with Reinforced [Be2BO3F2]∞ Layers and Short Phase-matching Wavelength Shu Guo,†,‡ Xingxing Jiang,†,‡ Lijuan Liu,∗,† Mingjun Xia,† Zhi Fang,†,‡ 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, China. ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China. . ABSTRACT: Deep-ultraviolet (DUV) nonlinear optical (NLO) materials are the crucial components of solid-state lasers to produce DUV coherent light (λ < 200 nm). To date, KBe2BO3F2 (KBBF) and RbBe2BO3F2 (RBBF) are the only two applicable DUV NLO materials. However, the weak K+–F– and Rb+–F– ionic bonds between the adjacent [Be2BO3F2]∞ layers limit their applications. In this work, a new KBBF-type DUV NLO material, BaBe2BO3F3 (BBBF), was obtained through molecular engineering design using KBBF as a model. Interestingly, the two-dimensional [Be2BO3F2]∞ layers of BBBF are bridged via relatively stronger Ba2+– F− bonds, which is beneficial to improve the layering growth tendency. The optical transmittance spectrum revealed that its UV cutoff wavelength is far below 200 nm. According to the results of theoretical calculations, the large birefringence (∆n = 0.081 at 200 nm) of BBBF is sufficient for phase-matching in the DUV region. These results indicate that BBBF is the third NLO material that could produce second-harmonic generation below 200 nm besides KBBF and RBBF.

Deep-ultraviolet (DUV) coherent light sources (λ < 200 nm), which are always rare resources, have attracted much attention because of the significant role played in semiconductor photolithography, high density storage, laser micromachining, material processing, photochemical synthesis and scientific equipment.1-4 As is well accepted, the most efficient way to produce practical DUV laser is using solid-state lasers with nonlinear optical (NLO) crystal through cascaded secondharmonic generation (SHG) conversion. Hence, an appropriate DUV NLO crystal that can produce SHG below 200 nm is the key to generating high quality DUV lasers. According to the anionic group theory proposed by Chen,5-8 a good DUV NLO crystal should possess the following three optical properties: 1) wide transparent range in the UV region with the UV cut-off wavelength far below 200 nm; 2) a moderate birefringence (∆n ∼ 0.08) to ensure the phase-matching ability; 3) a sufficient second-order nonlinear coefficient. Specifically, the first two conditions are indispensable for a DUV NLO material. Taking the famous LB3O5 (LBO) crystal as an example,9 the transmittance spectrum is wide and the UV absorption edge is down to about 155 nm. However, the relatively small birefringence (∆n ∼ 0.04) limits its SHG phasematching wavelength (276 nm). Currently, KBe2BO3F2 (KBBF)10 and RbBe2BO3F2 (RBBF)11 are the only two applicable DUV NLO crystals owing to their outstanding optical properties. They both have short absorption edges down to 150 nm, a moderate birefringence (∆n ~ 0.08 at 200 nm) and a large SHG effect comparable to that of KH2PO4 (KDP). It's worth mentioning that KBBF first breaks down the “200-nm wall” for generating DUV coherent light sources through a simple SHG method and opened a new window into laser be-

low 200 nm.12-14 Hence, the KBBF was called the China’s crystal cache15 and the DUV lasers have been successfully used in advanced scientific instruments such as angle resolved photoemission (ARPES) and photoemission electron microscopy (PEEM).3, 16 As mentioned above, the NLO properties for KBBF are excellent. But the as-grown crystals are plate-like and the individual layers are weakly bounded due to the weak K+–F– ionic force in the structure. It is very difficult to grow thick crystal along c-axis and therefore unfavourable to obtain high-power coherent DUV output.17 Hence, the DUV applications still await improved nonlinear optical crystals. Recently, a series of beryllium borates such as NaBeB3O6,18 NaSr3Be3B3O9F4,13 Na2CsBe6B5O15,19 Na2Be4B4O1120, 20 LiNa5Be12B12O33 and Sr3[(BexB1−x)3B3O10][Be(O1−xFx)3] (x = 0.30),21 and beryllium-free materials including K3B6O10Cl,22 Rb3Al3B3O10F,23 and Ba3(ZnB5O10)PO4,24 were reported as potential DUV NLO crystals. Among them, the UV cut-off wavelength (λcutoff < 200 nm) was focused on, however, the birefringence, as an important parameter to achieve the practical DUV output laser by a direct SHG technique, was rarely mentioned. Actually, the further experimental and theoretical calculation results revealed that the above NLO materials are unable to meet the DUV SHG phase-matching conditions due to a relatively small birefringence or large refractive indices dispersion.25-27 Thus, the structure of the KBBF family is still the optimal configuration for DUV harmonic generation. From the viewpoint of structure–property relationship, the outstanding NLO properties of KBBF come from the perfect arrangement of [BO3] and [BeO3F] groups in the twodimensional (2D) infinite [Be2BO3F2]∞ co-planar layers, which are in favour of generating a relatively large SHG coefficient

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and birefringence.28-29 Just like “DNA” in biology, the [Be2BO3F2]∞ layers, as the brilliant building blocks in crystal structure, perfectly inherited the excellent optical properties of KBBF family. Hence, it is ideal to design a material with reinforced [Be2BO3F2]∞ planar layers to keep the excellent NLO properties and have less layer growth habit simultaneously. Guided by this idea, the divalent alkaline-earth cation was first introduced into the space between 2D [Be2BO3F2]∞ units to reinforce the bonding interaction and we have successfully synthesized a new non-centrosymmetric beryllium fluoride borate BaBe2BO3F3 (BBBF). It shows better growth habit, moreover it has a short UV cut-off edges (λcutoff < 185 nm), large birefringence (∆n = 0.081 at 200 nm) and moderate refractive dispersion, which indicates that BBBF is a potential DUV NLO material. Considering the highly-toxic of BeO, the well-ventilated environment was configured in an independent laboratory. The single-crystal of BBBF crystals were grown by spontaneous crystallization in a flux system of BaCO3/BaF2/BeO/NaF/NaBF4/H3BO3 with molar ratios of 1-4:14:2-3:3-4:1-2:4-5. The single-crystal X-ray diffraction (XRD) was used to determine the crystal structure. As shown in Figure 1, the differential scanning calorimetric (DSC) curves of BBBF shows one sharp endothermic peak at about 997.9 °C in the heating process but two small exothermic peaks at about 894.5 and 797.3 °C in the cooling process, respectively. Moreover, the powder XRD of residues after DSC was mainly BaF2 and totally different from before measuring, indicating that the BBBF melts incongruently. Therefore, large crystal should be grown by the flux method below the decomposition temperature.

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are one unique Ba atom, three unique B atoms, three unique O atoms, five unique F atoms and two unique Be atoms. The crystal structure of BBBF is depicted in Figure 2a. Each of the B atom coordinate to three O atoms to form a planar [BO3] triangle (Figure 2b), with d(B–O) = 1.360(3) – 1.374(3) Å and O–B–O bond angles covering the range of 119.7(1) – 120.0(1). And the Be atoms are bound to three O atoms and one F atom to form a distorted [BeO3F] tetrahedral, with O– Be–O angles between 107.9(11) and 110.1(8). The structure units of [BO3] triangle and [BeO3F] tetrahedral are connected via corner-sharing O atoms to form the 2D planar layers [Be2BO3F2]∞ in ab plane (Figure 2c), which eliminates the three dangling bonds of [BO3]. Each Ba atom of BBBF is eight-fold coordinated with d(Ba–F) = 2.591(1) – 3.110(5) Å to build a [BaF8] polyhedron. The detailed crystallographic information was listed in Table S1-S3. The total bond valence for Ba, Be, B, O and F atoms were calculated and listed in Table S3, which indicate that the Ba, Be, B, O and F atoms are in oxidation states of +2, +2, +3, –2 and –1, respectively. The results of inductively coupled plasma optical emission spectrometer (ICP-OES) for BBBF is Ba/Be/B =1.1:2.2:1.0, which is consistent with the compositions deduced by single-crystal XRD analysis (see SI).

Figure 2. (a) Unit cell of BBBF. (b) Basic building blocks [BeO3F], [BaF8] and [BO3] of BBBF. (c) The project of NLOactive [Be2BO3F2]∞ units of BBBF along c axis.

Figure 1. (a) DSC curves and (b) calculated and experimental powder XRD patterns for BBBF.

BBBF crystallizes in a hexagonal crystal system with a chiral space group of P63 (No.173), which is in agreement with the powder SHG measurements. In the asymmetric unit, there

To gain a better understanding of the relationship between BBBF and KBBF, the structure of two compounds were compared in Figure 3a. The neighboring [Be2BO3F2]∞ layers of BBBF are bridged via Ba1–F4 and Ba1–F5 ionic bond with the interlayer spacing of 6.995 Å (Figure 3a). Interestingly, the linkage of eight-coordinate [BaF8] was found in beryllium borates for the first time. The [BaF8] polyhedron are interconnected with each other mainly via F1, F2 and F3 atoms to build [BaF] layer in ab plane (Figure 3c). The structure of BBBF can be described as the [Be2BO3F2]∞ layers and [BaF] layers are alternately stacked and interconnected by F––Ba2+– F– bonds to build a three dimensional (3D) network.

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Chemistry of Materials Table 1. Comparison of the Optical Properties and Interlayers bonding for BBBF and KBBF family. lengths ||,∗ ∆n# λcutoff (Å) (nm) BBBF Ba12+–F4– 2.664 2.14 0.081a < 185a 3.031 1.65 3.110 1.57 Ba12+–F5– 2.753 2.00 2.735 2.03 2.757 1 0.088b 147c KBBF K+–F– RBBF Rb+–F– 2.834 0.95 0.082b 150d CBBF Cs+–F– 2.964 0.91 0.076b 151e ∗ # In multiple of – . λ=200 nm. aThis Work. bDate from Re 25. cDate from Re 10. dDate from Re 11.eDate from Re 28. species

Figure 3. (a) Structural evolution from KBBF to BBBF. (b) [KF] layer of KBBF. (c) [BaF] layer of BBBF.

Among KBBF family, alkali metal atoms were reside in the space between adjacent [Be2BO3F2]∞ layers and coordinate with six F atoms to act as bridged bond. As mentioned, the layering tendency of KBBF is attributed to the weak K+–F– bond. Considering the A+–F– (A+ = K+, Rb+, Cs+) and Ba2+–F– bond are basically ionic, the electrostatics force of interaction was used to evaluate the interlayer connections of KBBF family and BBBF.21, 28 The magnitude of electrostatics force of interaction were calculated based on Coulomb’s law29 using the follow equation, || 

 | |

(1)

where ke is the electrostatic constant, q1 and q2 are the magnitude of the charged quantity, respectively, and r is the distance between two point charges. As the Ba1–F4 and Ba1–F5 are serve as bridged bond of [Be2BO3F2]∞ planar layers, the electrostatics force of interaction of Ba1–F4 and Ba1–F5 were carefully calculated. Fortunately, the electrostatics force of interaction calculation reveals that the Ba2+–F– ionic bond is stronger than A+–F– of KBBF family. The calculated | – | and | – | are largerthan that of |– | (Table 1). Therefore, Ba2+–F– bonds serves as better linkage of [Be2BO3F2]∞ layers compared with weak K+–F– ionic bonds in KBBF, which is favorable to improve the layering tendency. In our preliminary trial, crystals with the size of 3.0 × 3.0 × 0.4 mm3 have been obtained (Figure S1), which are thicker than the grown KBBF crystals at early growth stage. Guided by the idea of strengthening the interlayers bonding, the linkage of [Be2BO3F2]∞ layers in BBBF has potential to be substituted by other bridged bond such as M2+–F– (M=Sr, Ca Mg) and the further synthesis experiments are under way. The SHG intensity for BBBF was ~ 1/10 times that of KDP in the same range of 180 – 250 µm (Figure S2). And the SHG intensity is proportional to the square of SHG coefficient. Therefore, the SHG coefficient is about 0.32 × KDP. Based on the anionic group theory,5-7 the different orientation of [BO3] groups among adjacent [Be2BO3F2]∞ layers weakens the SHG effects of BBBF (Figure S3). A size of 3.0 × 3.0 × 0.4 mm3 BBBF crystal was used to measure the transmission spectrum by a Lambda 900 UV–vis–NIR spectrophotometer in the wavelength range of 185 – 800 nm. As respected, the UV absorption edge of BBBF is far below 200 nm (Figure 4a).

bonds

As a DUV NLO material, DUV SHG capability is particularly important. However, it is still a very complicated process to evaluate the DUV SHG ability of crystal when lacking of high-quality crystals for NLO properties measurements. Recently, Jiang et.al carried out the evaluation of 13 new promising alkali and/or alkaline earth NLO beryllium borates and revealed that the refractive-index dispersion along with birefringence play a key role in DUV SHG capability for the first time.25 In contrast, the optical properties and structure features of BBBF and other typical NLO alkali and/or alkaline earth beryllium borates are presented in Table 1 and Table S4. Among beryllium borates, some materials exhibit the KBBFlike [Be2BO3O2]∞ units, which were connected with multiple anionic groups such as [B3O6] in γ–KBe2B3O7,18 [BO3] in Na2CsBe6B5O15,19 [B2O5] in Na2Be4B4O11 and LiNa5Be12B12O3320. As reported, the layering growth habit of these materials were improved by strong connection of covalent bonds. However, the birefringence and the refractive indices dispersions limit the DUV phase-matching capability of these crystals above 200 nm.25

Figure. 4 (a) Measured transmittance spectra for BBBF. (b) Phase-matching capabilities for BBBF at 392 nm. The minimum refractive index at 196 nm (n1) located between the minimum (n2) and maximum refractive index (n3) at 392 nm indicates the fulfillment of the requirement of the phase-matching condition.

To further evaluate the DUV phase-matching capability of BBBF, first-principles studies are performed to elucidate the relationship between electronic structure and optical properties. The total and partial densities of states (DOS and PDOS) of BBBF are shown in Figure S3. According to our results, BBBF is an indirect band gap compound and the value of the energy band gaps is 8.41 eV (corresponding to λ = 147 nm). The electron transition of BBBF is mainly contributed by the inside excitation of the [BO3] groups among the 2D [Be2BO3F2]∞ layers, which is similar to KBBF family and resulted in a short wavelength edge (λcutoff < 185 nm). Hence, the

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UV transmission property of BBBF is comparable to KBBF. Then the refractive index were calculated based on the electronic structures. According to the theory calculations, BBBF exhibits a large birefringence (∆n = 0.081 at 200 nm), which is as good as KBBF and much larger than most of beryllium borates (Table 1 and Table S4). Moreover, further analysis of refractive-index dispersion was carried out to evaluate the DUV phase-matching ability. As shown in Figure 4b, n3(392 nm)> n1(196 nm) > n2(392 nm), indicating that BBBF is phase-matching under 200 nm. According to the theoretical calculation, the shortest SHG phase-matching wavelength is down to ~196 nm, which is shorter than that of the reported beryllium borates and beryllium-free materials, such as NaBeB3O6 (226 nm),25 NaCaBe2B2O6F (245 nm),25 NaSr3Be3B3O9F4 (233 nm),32 Na2CsBe6B5O15(452 nm),25 Na2Be4B4O11 (256 nm),25 LiNa5Be12B12O33 (258 nm),25 K3B6O10Cl (255 nm)33 and Ba3(ZnB5O10)PO4 (365 nm)27. Hence, BBBF is a true DUV NLO crystal besides KBBF and RBBF that can frequency doubling below 200 nm owing to the outstanding DUV phase-matching capability. In summary, a new KBBF-type DUV NLO material, BBBF, has been obtained through molecular engineering design by substituting monovalent K+ in KBBF with divalent Ba2+. Our analysis demonstrates that the Ba2+–F– bond serve as a new type of linkage of [Be2BO3F2]∞, which is stronger than the K+– F– ionic bond in KBBF and beneficial to improve the layering growth tendency. As respected, BBBF inherits the excellent optical properties of KBBF. Based on the first-principle calculations, the large birefringence (∆n = 0.081 at 200 nm) and moderate refractive-index dispersion are the key factors to ensure the shortest SHG phase-matching wavelength of BBBF down to the DUV region (~196 nm). These features makes BBBF a DUV NLO material and the more works are ongoing. We hope that the unique Ba2+–F– bridged bond, which was first found in beryllium borates, may be particularly useful for designing new NLO DUV materials.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Details of data analysis methods, photograph of as-grown BBBF crystals, density of states and partial density of states plots, crystallographic data (CIF). Deposition number CCDC 1504981 for BBBF.

AUTHOR INFORMATION Corresponding Author * [email protected]

Notes The authors declare no competing financial interest

ACKNOWLEDGMENT This work is supported by grants from the National Natural Science Foundation of China (No. 51132008, 61138004, 51402316, 51502307), and the National Instrumentation Program (No.2012YQ120048).

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Table of Contents artwork

ACS Paragon Plus Environment

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