Beryllium-Free KBBF Family of Nonlinear-Optical Crystals

Dec 7, 2016 - ... halide borates AZn2BO3X2 (A = Na, K, Rb; X = Cl, Br) have been obtained and were all structurally similar to crystals of the KBBF fa...
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Beryllium-Free KBBF Family of Nonlinear-Optical Crystals: AZn2BO3X2 (A = Na, K, Rb; X = Cl, Br) Qian Huang,†,‡ Lijuan Liu,† Xiaoyang Wang,† Rukang Li,*,† 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 the Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

BO3 groups, which further widens its transparence in the UV region.15 Nevertheless, the strong layer habit and the toxicity of BeO limit its applications due to difficulties in the growth process.16 Recently, it is interestingly observed that the coordination environments of Zn atoms in several zinc borates resemble that of the Be atom, although the ionic radii of the Zn and Be atoms differ greatly. For example, [Zn2(BO3)2F2]∞ and [ZnBO3F]∞ layers in KMZn2(BO3)2F (M = Ca, Cd) and BaZnBO3F are similar to that of [Be3B3O6]∞ in the SBBO NLO crystal. In 2015, a new natural mineral borate chloride, KZn2(BO3)Cl2, was found in the sublimates of active fumaroles by Pekov and Zubkova.17 Recognizing that Zn atoms are generally coordinated by four O atoms (tetrahedral) and the Zn−O bond length is also close to that of Be−O, we thought that linking the coplanar BO3 triangles with ZnO3Cl tetrahedra, forming [Be2BO3F2]∞-like layers, will partly maintain the optical properties of KBBF. Moreover, substitution of Zn for Be eliminates the toxicity issues inherent in the synthesis of KBBF from beryllium oxide powders. In addition, the distorted ZnO3Cl tetrahedra should provide an enhanced contribution to the SHG response. Following these ideas, a new series of NLO crystals, AZn2BO3X2 (A = Na, K, Rb; X = Cl, Br), were obtained. Their crystal structures consist of the targeted [Zn2BO3Cl2]∞ layers, which resemble the [Be2BO3F2]∞ layer in KBBF. The SHG test shows that their SHG effects are about 1.17 and 1.3 times that of KH2PO4 (KDP) for RbZn2BO3Cl2 (RZBC) and KZn2BO3Cl2 (KZBC), respectively, which are at the same level of KBBF. Because the title compounds are isostructural, the structures are illustrated by RZBC. Single-crystal X-ray diffraction (XRD) analysis reveals that RZBC crystallizes into the noncentrosymmetric trigonal space group R32 with the unit cell parameters of a = 4.9698(7) Å and c = 27.178(5) Å and exhibits a layered structure. The crystal structure of RZBC is depicted in Figure 1a, which is identical with that of the mineral KZBC reported in ref 17. One BO3 triangle and two ZnO3Cl tetrahedra are arranged in a hexaatomic ring and bridged by O atoms to generate the twodimensional infinite [Zn2BO3Cl2]∞ layer shown in Figure 1b. In the unit cell of the RZBC structure, each BO3 triangle is connected to six Zn-centered tetrahedra. Each Cl atom is shared by one Zn-centered tetrahedron and three edge-connected RbCl6 octahedra belonging to the cationic layer formed by Rb+ cations. In RZBC, the BO3 triangles adopt a completely coplanar

ABSTRACT: A series of a novel beryllium-free KBBF family of nonlinear-optical materials AZn2BO3X2 (A = K, Rb and X = Cl; A = Na, K, Rb and X = Br) were successfully synthesized through molecular engineering design, and single crystals of AZn2BO3Cl2 (A = K, Rb) were grown by a spontaneous nucleation technique from self-flux systems. As a representative for the halogen KBBF family of crystals, KZn2BO3Cl2 features the infinite lattice layer [Zn2BO3Cl2]∞ made up of BO3 and ZnO3Cl anionic groups, and the in-layer BO3 groups are completely coplanar and well-aligned. Besides, KZn2BO3Cl2 exhibits high transmittance in the range of 300−2000 nm with a UV-transmission cutoff of around 200 nm according to transmission spectra. The compounds of AZn2BO3Cl2 (A = K, Rb) are both phase-matchable with powder secondharmonic-generation efficiencies of 1.3 and 1.17 times that of KH2PO4 for KZn2BO3Cl2 and RbZn2BO3Cl2, respectively, which are similar to that of KBBF.

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onlinear-optical (NLO) crystals, which are key materials for solid-state lasers to produce coherent light through cascaded frequency conversions, have attracted considerable attention since the second-harmonic-generation (SHG) phenomenon was first observed in 1961.1 Concentrated researches for noncentrosymetric crystals as NLO materials have been done for decades.2 The anionic group theory,3 which assumes that the NLO properties of crystals are dominantly determined by the anionic groups, has been very successful in developing ultraviolet (UV) and deep-UV NLO crystals in borates. Over the years, a number of important UV NLO crystals have been discovered, including β-BaB2O4, LiB3O5, CsB3O5, CsLiB6O10, YCa4O(BO3)3, and Sr2Be2B2O7 (SBBO),4−8 which have been widely used in laser science and technology. More recently, the continuous search for new NLO materials, particularly for deep-UV applications, has aroused broad interest.9−13 The most successful deep-UV material is KBe 2 BO 3 F 2 (KBBF),14 which possesses moderate SHG coefficients and large birefringence. It is the unique crystal that can generate coherent light at wavelengths below 200 nm by direct SHG response. The excellent properties of KBBF mainly arise from [Be2BO3F2]∞ layers in its structure, which contains BO3 groups that adopt a coplanar configuration promoting birefringence and nonlinearity. The three terminal O atoms of the BO3 group are linked with Be atoms, eliminating three dangling bonds of the © XXXX American Chemical Society

Received: August 23, 2016

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DOI: 10.1021/acs.inorgchem.6b02044 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Figure 1. (a) Crystal structure of RZBC. (b) Infinite layers of (Zn2BO3Cl2)∞ in the RZBC structure.

Figure 2. DSC and TGA curves of the RZBC crystal.

configuration, which is favorable for generating large SHG responses and birefringence in borates. From both the composition and [Zn2BO3Cl2]∞ layering structure points of view, RZBC can be considered as a new member in the family analogous to KBBF. It is interesting to note that, in comparison to the KBBF family compounds, the unit cell dimensions of AZBX in the a and b directions are only slightly changed, whereas it is significantly increased in the c direction from the KBBF family to AZBX (Table S3). For example, it is found that KZBC (a = 4.9463 Å) compounds have an a axis similar to that of KBBF (a = 4.427 Å), but the c axis of KZBC (c = 26.348 Å) is about 1.4 times that of KBBF (c = 18.744 Å). The rigidity in the a and b directions may due to the strong covalent bonding character among the Zn−O− B−O bonds. The large increase in the c direction can be attributed to the size differences among the halogen ions. For a comparison of KZBC and KBBF, the ZnO3Cl tetrahedron in the former structure has three basal Zn−O bonds of 1.946 Å and one apical Zn−Cl bond of 2.256 Å, while the Be−O bond is 1.636 Å and the Be−F bond is only 1.52 Å for the BeO3F tetrahedron in the latter structure. In one unit cell, the total difference caused by six Zn−Cl bond distances, which are 0.736 Å higher than the Be− F bonds, is 4.416 Å. Besides, the Cl− ionic radius is 0.45 Å larger than that of F−,18 resulting in an increase of the distance between adjacent Cl layers. The distance of the adjacent Cl layers in KZBC is 3.101 Å, while it is 2.056 Å for adjacent F layers in KBBF. There are three interlayer spacings in one unit cell; consequently, the total difference along the c axis is 3.14 Å. Differential scanning calorimetry (DSC) measurements were carried out. The DSC curve exhibits two endothermic peaks on the heating curve at 812 and 980 °C, while an obvious weight loss was observed on the thermogravimetric analysis (TGA) curve, along with the first endothermic peak; therefore, the first peak probably is the decomposition temperature (Figure 2). The XRD pattern indicates that the residues were mainly Zn3B2O6 and RbCl instead of the original compound. Hence, bulk single crystals of the title compounds should be grown by flux methods below the decomposition temperature. From the transmission spectrum (Figure 3), the UVtransmittance cutoff edge of the KZBC crystal is located at about 200 nm. It is clear that there are no absorption peaks from 300 to 600 nm, and the transmittance of the level is about 60%. This low transmittance cannot be accounted only for the reflection losses of the two surfaces (10%), and additional losses may come from inclusions and scattering centers in the grown crystals, which seriously reduce the transmittance. The improvement in the crystal growth conditions may overcome the above

Figure 3. Transmission spectrum of KZBC.

problems, and RZBC may serve as a new NLO crystal in the UV−visible range. The excellent linear-optical and NLO properties of KBBFstructured materials were widely considered to originate from the [Be2BO3F2]∞ layer. Given that a highly similar [A2BO3X2]∞ is found in AZBC, the favorable linear-optical and NLO properties in AZBC can be envisaged. The plots of SHG intensity versus particle size of the RZBC and KZBC powders are shown and compared with that of KDP in Figure 4. Along with an increase of the particle size, the SHG effects gradually increase and ultimately remain unchanged in the range of 150−250 nm. These plots show that AZBC is clearly consistent with the phase-matching behavior based on the rule proposed by Kurtz and Perry.19 Their SHG effects are about 1.17

Figure 4. SHG intensity versus particle size curves at 1064 nm. B

DOI: 10.1021/acs.inorgchem.6b02044 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry ORCID

and 1.3 times that of KDP for RZBC and KZBC, respectively, indicating that the SHG responses of the two title compounds are comparable to that of KBBF. As one of the zinc borates, compared to CsZn2B3O710c and Cs3Zn6B9O21,9a RZBC has a layering structure that is more like that of the KBBF family and that causes the SHG effects to also be comparable to that of KBBF, whereas in the structures of CsZn2B3O7 and Cs3Zn6B9O21, the adjacent layers are connected by the [B3O6]3− groups, and that may result in increasing SHG effects. According to the anionic group theory and considerable relevant investigations, the SHG responses in the borates mainly originate from the B−O groups.3c The macroscopic SHG coefficients d11 can be calculated by summing the second-order polarizabilities (χ(2)) of the anionic groups BO3 for compounds of the KBBF family and AZBC (Table S4). It is clear that BO3 groups almost entirely contribute to the SHG coefficients in both KBBF and RBBF; nevertheless, the proportion of BO3 is only about half that for AZBX because of the reduction of the density of the BO3 groups in the AZBX structures. The observation of discrepancies between the calculations based on pure BO3 groups and the experimental results on AZBC crystals means that the contribution from the other anionic group, ZnO3Cl, cannot be completely neglected. Furthermore, from the calculation, one can conclude that the contribution to the SHG coefficients d11 of every ZnO3Cl group is about half that of the BO3 group. These results agree well with the observation that AZBC shows a KBBF level signal that is slightly higher than that of KDP according to the powder SHG tests. In conclusion, a new family of alkaline-metal borate halides, AZn2BO3X2 (A = Na, K, Rb; X = Cl, Br), have been obtained through molecular engineering. The title compounds were all structurally similar to crystals of the KBBF family. In their structures, the ab plane is the infinite layer [Zn2BO3Cl2]∞ made up of BO3 and ZnO3Cl anionic groups that are similar to the [Be2BO3F2]∞ layer in KBBF. In AZBC, the isolated triangles adopt a completely coplanar configuration, which favors large SHG responses and birefringence in borates. SHG measurements on powdered AZBC crystals reveal that they are phasematchable NLO crystals and the short-wavelength absorption edge of KZBC is down to 200 nm. Our preliminary results indicate that AZBC may be promising UV NLO crystals. We believe the molecular engineering procedures present in this work would provide a new path to search for the deep-UV NLO crystals.

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Qian Huang: 0000-0002-7284-2042 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by grants from the National Natural Science Foundation of China (Grants 51132008, 61138004, 51402316, and 51502307) and the National Instrumentation Program (Grant 2012YQ120048).

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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.6b02044. X-ray crystallographic data in CIF format for AZn2BO3X2 (CIF) Experimental details, specified physical characterization, anisotropic displacement parameters, and first-principles calculations (PDF)

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DOI: 10.1021/acs.inorgchem.6b02044 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b02044 Inorg. Chem. XXXX, XXX, XXX−XXX