AZn2BO3X2(A = K, Rb, NH4; X = Cl, Br): New Members of KBBF

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AZn2BO3X2(A = K, Rb, NH4; X = Cl, Br): New Members of KBBF Family Exhibiting Large SHG Response and the Enhancement of Layer Interaction by Modified Structures Guangsai Yang,†,‡ Pifu Gong,§ Zheshuai Lin,§ and Ning Ye*,† †

Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § Beijing Center for Crystal R&D, Key Lab of Functional Crystals and Laser Technology of Chinese Academy of Sciences, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China S Supporting Information *

ABSTRACT: A new category of five KBBF-analogy nonlinear optical (NLO) materials, AZn2BO3X2 (A = K, Rb, NH4; X = Cl, Br), are developed by the tetrahedron substitution of BeO3F for ZnO3X from KBe2BO3F2 (KBBF). They preserve the structural merits of KBBF, consisting of the infinite planar [Zn2BO3X2]∞ layers. Optical measurements on this series of NLO crystals reveal that they are phase-matchable in the visible and UV region with powder second-harmonic generation (SHG) responses being more than twice that of isostructural KBBF. First-principles calculations and atomcutting analysis were carried out to demonstrate that enhanced SHG response originates from the cooperative effect of coparallel [BO3] triangles and distorted ZnO3Cl/Br tetrahedra. The theoretical calculations and experimental results show that AZn2BO3X2 exhibits a less-developed layer habit compared with KBBF. Especially, because of the existence of relatively strong hydrogen bond between NH4+ groups and [Zn2BO3Cl2]∞ layers, NH4Zn2BO3Cl2 crystal exhibits the best growth behavior along the c axis. These results show that they may have prospects as a kind of UV nonlinear optical material.

1. INTRODUCTION

highly toxic BeO in their synthesis and crystal growth complicates the detailed investigation of their properties. Based on the relationship between structure and property, the excellent properties of KBBF stems from [Be2BO3F2]∞ layers in which the NLO-active [BO3]3− groups adopt a coplanar configuration promoting birefringence and SHG effects.20 Given that the layer structure feature of the KBBF family is still the most conducive to produce UV harmonic generation, attempts to overcome layer growth tendency by an innovative structural modulation or molecular engineering design based on the structure of KBBF may be a good strategy for exploration of new materials. With respect to the crystal growth mechanism, crystals prefer to grow along the direction with the larger binding energy. This is mainly due to the fact that the binding energy of [Be2BO3F2]∞ intralayer is far greater than that of K+−F− interlayer. KBBF suffers from serious layer growth habit along c direction. The ratio (Eintra/Einter) between the binding energies of intra- and interlayers can be viewed as an important index employed to evaluate crystal growth capacity along the c direction, namely, the larger the ratio,

1−4

Nonlinear optical (NLO) materials have attracted continuous intensive attention owing to their ability to control and produce coherent light at a variety of difficult-to-access wavelengths through common solid-state lasers. For decades, many NLO materials such as β-BaB2O 4(β-BBO),5 LiB3O5(LBO),6 CsLiB6O10(CLBO),7 CsB3O5(CBO),8 KTiOPO4,9 AgGaS2,10 and ZnGeP211 have been discovered and used commercially. Although these materials can satisfy the frequency-conversion requirements from the near-ultraviolet (UV) to the near-infrared wavelength, the direct generation of shortwave UV (λ < 300 nm), particularly for deep-UV (λ < 200 nm) coherent light by second harmonic generation (SHG) has remained especially challenging, yet crucial to lots of advanced laser and photonic technologies.12−14 For years, many borate, phosphate to carbonate, and even miscellaneous NLO materials have been reported.15,16 Currently, however, only two materials, KBBF (KBe2BO3F2) and RBBF (RbBe2BO3F2), are capable of generating deep-UV light through sixth harmonic generation of 1064 nm radiation (i.e., 177.3 nm radiation).17,18 Unfortunately, single-crystal growth of KBBF suffers from a serious layering tendency, hindering their practical applications.19 Also, the need for © XXXX American Chemical Society

Received: October 6, 2016 Revised: November 28, 2016

A

DOI: 10.1021/acs.chemmater.6b04272 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials the more obvious is the layering growth tendency. Thus, we may achieve the goal of improving crystal growth weaknesses by enhancing the interlayer interaction or weakening the intralayer binding energies. The conventional methods only considered reinforcing layer interaction for the purpose of improving growth habit by introducing bridging atoms (such as Sr2Be2B2O7(SBBO)21 and NaCaBe2B2O6F22) or linking adjoining layers with B−O groups (such as NaBeB3O6,23ABe2B3O7(A = K, Rb),23 and Na2CsBe6B5O15.24). However, despite the improvement of layer growth habit to some extent, these crystals do not possess the optical advantage of KBBF.25,26 We believe that weakening intralayer binding energies in [Be2BO3F2]∞ by substituting other elements with ionic character for Be with large covalence may be an innovative idea for relieving layering habit and meanwhile maintaining the structural feature of KBBF. In our work, we attempted to to substitute Zn for Be in KBBF according to the following reasons: first, Zn2+ prefers to form ZnO4 tetrahedra in the zincoborates, such as CsZn2B3O7,27,28 and the substitution of Zn for Be in beryllium borates eliminates toxicity issues from beryllium oxide powders during synthesis; second, the replacement of the relatively strong Be−O bonds with Zn−O bonds may reduce the intralayer interaction, which will be beneficial to improve layering habit; and third, introducing distorted Zn tetrahedra may promote an enhanced contribution to the SHG response. Because of the larger radius of Zn than Be, using Zn as replacement of Be in KBBF structure would lead to the lattice expansion, lager halogen may be the substitution for F− to maintain structural stability. Guided by these ideas, we successfully obtained a novel series of borates, namely, KZn 2BO 3Cl 2 (KZBC), RbZn 2 BO 3 Cl2 (RZBC), KZn2BO3Br2 (KZBB), and RbZn2BO3Br2 (RZBB). The microscopic structures of these compounds are isostructural with KBBF possessing the NLO-favorable structural features. Afterward, for the purpose of strengthening chemical binding between layers, on the basis of the predictions and calculations of first-principles,29 we successfully designed and synthesized ammonia zinc halogen borates analogous to the compounds above, NH4Zn2BO3Cl2 (NZBC), by substituting NH4+ groups for A-site cationic. Interestingly, the modified structures by Asite cationic and X-site halide anions enhance the interaction between layers and result in better single-crystal growth behavior. Remarkably, this series of materials exhibit the largest SHG response in the KBBF family, approximately more than 2 times that of KBBF (∼1.24× KDP). The first-principles calculations and atom-cutting analysis indicate that the SHG enhancement mainly originates from the ZnO3Cl/Br tetrahedra. Herein, we will report the synthesis, crystal growth, structures, and thermal behaviors as well as the linear and nonlinear optical properties of AZn2BO3X2 (A = K, Rb, NH4+; X = Cl, Br) in this paper.

ACl + 2ZnX 2 + B2O3 = AZn2BO3X 2 + BCl3↑ (A = K, Rb; X = Cl, Br)

(i)

2ACl + ZnX 2 + B2O3 + 3ZnO = 2AZn2BO3X 2 (A = K, Rb; X = Cl, Br)

(ii)

Single crystals of KZBC, RZBC, KZBB, and RZBB were obtained using one of two methods shown below: (i). High-temperature solution method. The KZBC and RZBC crystals were grown from a mixture of ACl (A = K, Rb), ZnCl2, and B2O3 in a molar ration of 7:2:2. The mixtures were heated to 720 °C and kept for 10 h to ensure that the solution was homogenized. The homogenized melt solution was then slowly cooled at 5 °C/h until colorless, block crystals appeared on the matrix, and the solution was allowed to cool to room temperature at a rate of 50 °C/h. For KZBB and RZBB, the polycrystalline samples were mixed thoroughly with ABr (A = K, Rb) and B2O3 at a molar ratio of the polycrystalline samples: ABr:B2O3 = 1:5. The mixtures were heated in a platinum crucible to 670 °C and held at this temperature for 10 h. The temperature was reduced to 500 °C at a rate of 4 °C/h and cooled to room temperature at a rate of 10 °C/h. Colorless block crystals grew as regular hexagonal-shaped prisms. (ii). Solvothermal techniques. Crystals of KZBC, RZBC, KZBB, and RZBB could also be grown by solvothermal techniques under subcritical conditions. The reaction mixture of AX (A = K, Rb; X = Cl, Br), ZnX2(X = Cl, Br), ZnO, and H3BO3 in a molar ratio of 4:2:1:1, were sealed in an autoclave equipped with a Teflon liner (23 mL). The autoclave was closed, heating at 220 °C for 5 days, and followed by slowly cooling to ambient temperature at a rate of 3 °C/h. The reaction products were separated from the mother liquid by filtration and washed with deionized water and ethanol and then dried in the air. Colorless, transparent, hexagonal prism-shaped crystals were subsequently determined to be KZBC, RZBC, KZBB, and RZBB, and these compounds were obtained in approximately 90, 97, 85, and 70% yields based on Zn, respectively. Then the obtained solid products were put back into reaction kettles for further large crystal growth using AX (A = K, Rb; X = Cl, Br) and H3BO3 mixed solution as mineralizer. When the holding time at 220 °C was extended to more than 500 h, larger crystals (Figure 6) were obtained with a cooling rate of 2 °C·h−1. Preparation of NZBC. Due to the decomposition of ammonium compounds at high temperature, crystals of NZBC were grown only by solvothermal techniques. Under the above hydrothermal conditions, single crystal of NZBC was synthesized by using NH4Cl, ZnCl2, ZnO, and H3BO3 in a molar ratio of 16:8:3:2; Colorless, transparent, hexagonal prism-shaped crystals were obtained in approximately 85% yields for NZBC based on Zn, respectively. The obtained products were utilized to grow large crystals using a NH4Cl and H3BO3 mixed solution as mineralizer, holding time at 220 °C for more than 500 h. We obtained a crystal with dimensions 6 × 5 × 0.8 mm3 (Figure 6) with a cooling rate of 2 °C·h−1. Single-Crystal X-ray Diffraction. A transparent block of crystal was mounted on a glass fiber with epoxy for single-crystal diffraction analysis. Diffraction data were collected using monochromatic Mo Kα radiation at 296 K on a Rigaku Mercury CCD diffractometer. A hemisphere of data was collected using a narrow-frame method with ω-scan mode. The data were integrated using the CrystalClear program, and the intensities were corrected for Lorentz polarization, air absorption, and absorption attributable to the variation in the path length through the detector face plate. Absorption corrections based on the Multiscan technique were also applied. The structures were determined by direct methods and refined by difference Fourier maps and full-matrix least-squares fitting on F2 using SHELXL-2014. The TWIN command is used in the following refinement. The program PLATON was used to check the structure, and no other higher symmetry was found. Crystal data and structure refinement

2. EXPERIMENTAL SECTION Reagents. ZnO, ZnCl2, ZnO, ZnBr2, KCl, RbCl, NH4Cl, KBr, RbBr, and H3BO3 were purchased from Shanghai Titan Scientific Co., Ltd. All starting materials were analytical grade from commercial sources. Preparation of KZBC, RZBC, KZBB, RZBB. Polycrystalline samples of KZBC, RZBC, KZBB, and RZBB were synthesized by a conventional solid-state reaction method based on one of the following reactions: B


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Chemistry of Materials Table 1. Crystal Data and Structure Refinement for AZn2BO3X2 (A = K, Rb, NH4; X = Cl, Br) formula formula mass (amu) crystal system space group a (Å) c (Å) α (deg) γ (deg) V(Å3) Z ρ (calcd) (g/cm3) temp (K) λ (Å) F(000) μ (mm−1) θ (deg) index range

Rint R/wR (I > 2σ(I)) R/wR (all data) GOF on F2 absolute structure parameter largest diff peak and hole (e/Å−3) a

KZn2BO3Cl2

NH4Zn2BO3Cl2

RbZn2BO3Cl2

KZn2BO3Br2

RbZn2BO3Br2

299.55 trigonal R32 4.9624(6) 26.386(5) 90 120 562.71(18) 3 2.652 293(2) 0.71073 A 426 7.587 2.32 to 27.50 −6 ≤ h ≤ 5 −6 ≤ k ≤ 6 −33 ≤ l ≤ 33 0.0351 0.0197/0.0389 0.0215/0.0394 1.034 0.04(2) 0.536 and −0.447

274.5 trigonal R32 4.952(6) 27.05(5) 90 120 574.5(17) 3 2.415 293(2) 0.71073 402 6.895 4.52 to 27.52 −6 ≤ h ≤ 5 −6 ≤ k ≤ 6 −34 ≤ l ≤ 33 0.0651 0.0449/0.1012 0.0460/0.1016 1.05 0.14(8) 0.824 and −1.070




R(F) = Σ∥Fo| − |Fc∥/Σ|Fo|. wR(Fo2) = [Σw(Fo2 − Fc2)2/Σw(Fo2)2]1/2. ground and sieved into the same particle size ranges, respectively. The samples were pressed between glass microscope cover slides and secured with tape in 1 mm thick aluminum holders containing an 8 mm diameter hole. They were then placed in a light-tight box and irradiated with a pulsed laser. An interference filter (530 ± 10 nm) was used to select the second harmonic for detection with a photomultiplier tube attached to a RIGOL DS1052E 50-MHz oscilloscope. This procedure was then repeated using the standard nonlinear optical materials KDP and BBO, and the ratio of the second-harmonic intensity outputs was calculated. No index-matching fluid was used in any of the experiments. Computational Method. To further investigate the optical properties, the first-principles calculations for the title compounds are performed using the plane-wave pseudopotential method33 implemented in CASTEP package.34 The optimized norm-conserving pseudopotentials35 in the Kleinman−Bylander form36 for all the elements are used, in which H 1s1, N 2s22p3, K 3s23p64p1, Rb 4s24p65p1, Zn 4s23d10, B 2s22p1, O 2s22p4, Cl 3s23p5, and Br 4s24p5 electrons are treated as the valence electrons, respectively. The Perdew, Burke, and Ernzerhof (PBE) functionals37 of generalized gradient approximation (GGA) are adopted to describe the exchange− correlation (XC) functionals. The kinetic energy cutoffs of 900 eV are chosen for the title compounds. Monkhorst−Pack k-point meshes38 with densities of 5 × 5 × 2 and 5 × 5 × 1 points in the Brillouin zone are chosen for the four alkali metal contained compounds and the one containing NH4+, respectively. The cell parameters and atomic positions are optimized using the quasi-Newton method.39 Meanwhile, dispersion correction for DFT (DFT-D)40 is adopted to deal with the noncovalent forces such as hydrogen bonding and van der Waals interactions especially between NH4+ and surrounding ions in NH4Zn2BO3Cl2. The PBE0 functionals41 are chosen to accurately predict the experimental bandgaps. Our previous works42 reveal that the above computational methods are sufficiently accurate for the present purposes. The scissor-factors-corrected LDA method are used to calculated the second-order susceptibility χ(2), that is, the SHG coefficiency dij, based on the formula developed by Lin et al.43 In order to analyze the contribution of an ion (or ionic group) to the n th order susceptibility

information for the title compounds are listed in Table 1, and their final refined atomic positions and isotropic thermal parameters, selected bond lengths are listed in Tables S1 and S2 in the Supporting Information. Powder X-ray Diffraction. Powder X-ray diffraction (PXRD) measurements for polycrystalline materials were performed at room temperature on a Miniflex600 powder X-ray diffractometer using Cu Kα radiation (λ = 1.540598 Å) in the angular range of 2θ = 5−65° with a scan step width of 0.05° and a fixed time of 0.2s. No impurities were observed, and the powder XRD patterns for the pure powder samples of all compounds showed good agreement with the calculated XRD patterns from the single-crystal models. The differences between the measured and calculated intensities can be attributed to the effects of preferred orientation. (Figure S1). Thermal Analysis. Thermogravimetric analyses of all compounds were performed on a NETZSCH STA449F3 simultaneous analyzer. Reference (Al2O3) and crystal samples (5−10 mg) were enclosed in Al2O3 crucibles and heated from 30 to 1300 °C at a rate of 10 °C/min under a constant flow of nitrogen gas. UV−vis Diffuse Reflectance Spectroscopy and UV Transmittance Spectroscopy. Optical diffuse reflectance spectra were measured using a powder sample with BaSO4 as the standard of 100% reflectance on a PerkinElmer Lamda-950 UV/vis/NIR spectrophotometer scanning in the range of 190−1500 nm at room temperature. Reflectance values were converted to absorbance using the Kubelka− Munk function.30,31 The UV transmittance spectrum of the all crystals was collected from 190 to 300 nm using a PerkinElmer Lamda-950 UV/vis/NIR spectrophotometer. Crystals with a certain size were used for the measurement without polishing. Second-Harmonic Generation. The SHG measurements were measured on a Q-switched Nd: YAG solid-state laser of wavelength 1064 nm, which were evaluated using the Kurtz−Perry powder technique32 Because SHG efficiencies have been shown to depend on the following particle size ranges: 25−45, 45−62, 62−75, 75−109, 109−150, and 150−212 μm.To make relevant comparisons with known SHG materials, crystalline KDP (KH2PO4) and β-BaB2O4 (BBO) used as the references for visible and UV SHG were also C


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Chemistry of Materials χ(n), the real-space atom-cutting technique43 is adopted, in which χ(n)(A) = χ (n) (all ions except A are cut). In addition, the binding energy of the intra- and interlayers are calculated to value the growth tendencies of the title compounds.

illustrated in Figure 2. AZBX features similar layered structural units composed of [Zn2BO3X2]∞ layers, which can be regarded as a variant of [Be2BO3F2]∞ layers of KBBF by substituting BeO3F tetrahedra for ZnO3Cl tetrahedra. Within [Zn2BO3X2]∞ and [Be2BO3F2]∞ layers, [BO3]3− groups adopt a nearly coplanar and aligned arrangement, which is established to be a favorable mode for generating SHG responses and birefringence, indicating that they are likely to share the optical advantages of KBBF. Compared with KBBF, AZBX exhibits a slight increase in the a and b unit cell parameters (4.427 versus 4.962−4.982 Å for KBBF and AZBX, respectively), while the interlayer distances along the c-axes of AZBX are significantly longer than that of KBBF, which are the function of the bond distances associated with the larger size of ZnO3X tetrahedra (AZBX: 8.795−9.733 Å; KBBF:6.248 Å). Thus, the unit cell volumes of AZBX are much larger than that of KBBF (AZBX: 562.71−627.7 Å3; KBBF: 318.14 Å3), and the number density of [BO3] groups of the former smaller than the later. Besides, in all the structures of AZBX, planar six-member Zn2BO3 rings almost keep the same size, while the ZnO3X groups are slightly elongated along the c-axis and increase in c-axis with the A-site ion size increase from K+ to NH4+ and Rb+ and the E-site ion size do from Cl− to Br−. The bond valence sums (BVS)44 for all the crystals confirm the oxidation state of every atom. These results show good agreement with the expected oxidation states. Thermal Analysis. The thermal behaviors of measurements were carried out with ground crystals of title compounds. As shown in Figure S2, the TGA curves demonstrate that KZBC, KZBB, RZBC, and RZBB do not show weight loss until 800 °C. The decomposition of them occurred in the interval 800−1300 °C, and experimental values of weight loss agree basically with theoretical numeral ones. The XRD patterns of melted residues, which mainly displayed Zn3(BO3)2 peak, were clearly different from that of original crystals. The decomposition can be written as follows:

3. RESULTS AND DISCUSSION Crystal Structure. AZn2BO3X2 (A = K, Rb, NH4; X = Cl, Br) (AZBX) crystals were all isostructural to KBBF and crystallize into noncentrosymmetric structures with chiral space group R32 (155). Hence, only the structure of RZBC will be discussed in detail as a representation. The crystal structure of RZBC is illustrated in Figure 1. In the asymmetric unit, B, Zn,

AZn2BO3X 2 (A = K, Rb; X = Cl, Br) → A 2ZnCl4↑ + Zn3(BO3)2

Figure 1. Crystal structure of RbZn2BO3Cl2. (a) Unit cell. (b) Fundamental building blocks [ZnO3Cl]5− and [BO3]3−. (c) Infinite anionic [Zn2BO3Cl2]∞ layer consisting of triangular [BO3] groups and ZnO3Cl tetrahedra. (viewed along the c axis). (d) Cationic layer consisting of edge-connected RbCl6 octahedra.

As for NZBC, it exhibits decomposition in three steps with the heating process. The first step happened at the temperature range of about 250−450 °C, corresponding to the loss of NH4Cl. The second step can be assigned to the removal of ZnCl2, and the last step is the further decomposition of Zn3(BO3)2 and the residue is ZnO. The analysis of the PXRD pattern of residues at different temperature demonstrates the process of melting behavior of it (Figure S5). The decomposition reaction is

O, Rb, and Cl occupy only one crystallographically unique position. The B atoms are bonded to three O atoms to form a normal triangular [BO3]3− with B−O bond length of 1.354 Å and O−B−O bond angles of 120.0(1)°. The Zn atoms coordinated to three O atoms and one Cl atom are found to form an irregular ZnO3Cl tetrahedron with the Zn−O distance of 1.959 Å and Zn−Cl distance of 2.258(1) Å, respectively. Each flat [BO3] triangle is connected to six ZnO3Cl tetrahedra by sharing all basal oxygen atoms to form a 2D anionic [Zn2BO3Cl2]∞ layers in the a−b plane. The apical Cl atoms of ZnO3Cl tetrahedra alternately point upward and downward from the [Zn2BO3Cl2]∞ layer. Rb+ cations are located in the space between the anionic [Zn2BO3Cl2]∞ layers to maintain charge balance: they are all surrounded by six Cl-atoms to form RbCl6 octahedra. In connectivity terms, the materials may be described as the anionic [Zn2BO3Cl2]∞ layers formed by [BO3] and ZnO3Cl and the cationic layers consisting of edgeconnected RbCl6 octahedra. The crystal structures of AZBX are analogous to that of KBBF, and the structural evolution, from KBBF to AZBX is

NH4Zn2BO3Cl 2 → Zn2BO3Cl + NH4Cl↑

(1)

Zn2BO3Cl → 1/2Zn3(BO3)2 + 1/2ZnCl 2↑

(2)

1/2Zn3(BO3)2 → 3/2ZnO + 1/2B2O3↑

(3)

The results indicate that the title compounds all melt incongruently. Therefore, they must be grown under the melting temperature by flux methods or hydrothermal technique. UV Transmittance Spectroscopy and Diffuse Reflectance Spectroscopy. The optical diffuse-reflectance spectra of powder sample of all titled compounds in the region of 180− 1200 nm are shown in Figure S3. On the basis of diffusereflectance spectra, they were converted to absorbance based on the Kubelka−Munk function. The results reveal that the D


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Chemistry of Materials

Figure 2. Structural evolution from KBBF to AZBX.

(Cl− and Br−) give greater contribution to the band gaps of AZBC compounds than A-site cations (NH4+, K+, Rb+), and it seems that the lighter elements are conducive to increase band gaps. The measurements demonstrate that the optical transparency of AZBX has potential advantage for frequency generation in the UV region. NLO Properties. The curves of SHG signal measured by means of the powder Kurtz−Perry method with an incident laser of 1064 and 532 nm are shown in Figure 4. The plots of the SHG intensity versus particle size of the AZBX powders suggest that all the materials can achieve type I phase matching in both the visible and UV region according to the rules proposed by Kurtz and Perry. The powder SHG measurements for AZBX, using the KDP sample (visible) and BBO (UV) as a reference, revealed the SHG responses from 2.6 to 3.01 times that of KDP standard and from 0.44 to 0.55 times of BBO standard (Table 2) Considering the NLO coefficient of BBO is about 6.6 times as large as deff (KDP) which is 0.26 pm/V,46 the relative SHG intensities in the visible region and the UV region coincide with each other. According to the anionic group theory,47 the SHG effects of KBBF-type compounds mainly originate from a geometrical addition of the microscopic second-order susceptibility of the planar [BO3] groups. To investigate the relationships between the macroscopic NLO properties and microscopic geometry alignment of NLO-active [BO3] groups, the calculations reported in ref 48 were performed. From the derivation process of the formulas and calculation method described in detail in the Supporting Information, we can infer that the NLO coefficient is proportional to density of the [BO3] groups (n/V) and the structural criterion (C). The results of the calculations were displayed in Table 2. It was noted that the structural criterion C for all compounds is equal to 1, which means that all [BO3] groups are planar and aligned in the same orientation to maximize their contribution to the SHG effects. Thus, as the method mentioned above, the major determinant of SHG response differences among the series of AZBX crystals was the number density of the coparallel [BO3] triangles. However, it is difficult to be explained that AZBX compounds exhibit larger SHG efficiency than KBBF by only

band gap of NZBC, KZBC, RZBC, KZBB, and RZBB are approximately 6.67, 6.42, 6.25, 6.01, and 5.79 eV, with UV cutoff edges of 186, 193, 198, 206, and 214 nm, respectively. In order to accurately investigate the short absorption edge, transmittance spectroscopy measurements were performed on single crystals of AZBC, KZBC, RZBC, and KZBB (RZBB was too small to be measured). Figure 3 indicates that these crystals

Figure 3. UV transmittance spectroscopy of NZBC, KZBC, RZBC, and KZBB. The absorption edges increase in the order of NZBC (190 nm) < KZBC(194 nm) < RZBC (198 nm) < KZBB (209 nm).

are transparent down to the UV spectral region and exhibit absorption edges below 205 nm and the absorption edges increase in the order of NZBC (190 nm) < KZBC(194 nm) < RZBC (198 nm) < KZBB (209 nm). In particular, comparing the absorption edges of Cl−-containing compounds, NZBC has the shortest absorption edge, which is as short as 190 nm, whereas KZBC and RZBC have slightly red-shifted absorption edges (i.e., 194 and 198 nm, respectively), resulting from the heavier A-site ion (K+, Rb+) replacing the lighter counterpart NH4+ ion. Also, the absorption edges of KZBB and RZBB are further red-shifted to 209 and 214 nm, thereby associating with the E-site Br-anion. It is also suggested that the E-site anions E


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Figure 4. SHG measurements of (a) KZBC, RZBC, NZBC and (b) KZBB and RZBB ground crystals with KDP as the reference and the laser at 1064 nm wavelength; SHG measurements of (c) KZBC, RZBC, NZBC and (d) KZBB and RZBB with BBO as the reference and the laser at 532 nm wavelength.

Table 2. NLO Effects of KZBC, NZBC, RZBC, KZBB, and RZBB crystals 45

KBBF KZBC NZBC RZBC KZBB RZBB

SHG response (visible) (KDP)

SHG response (UV) (BBO)

structural criterion C

densities of the [BO3] (n/V) (Å−3)

(n/V)C (Å−3)

1.24 3.01 2.82 2.85 2.68 2.53

0.55 0.48 0.50 0.47 0.44

1 1 1 1 1 1

9.43 5.34 5.25 5.18 4.99 4.78

9.43 5.34 5.25 5.18 4.99 4.78

correlation (XC) energy.49 Meanwhile, the partial density of states (PDOS) projected on the constitutional atoms of the title compounds are shown in Figure S4, from which the following electronic characteristics are shown: the states lower than 10 eV are mainly composed of the isolated inner-shell states of K 3s23p6 (H 1s1, N 2s22p3, Rb 4s24p6), B 2s22p1, O 2s2, and Cl 3s2 (Br 4s2), which have little interaction with neighbor atoms. The upper part of the valence bands (VB) and the bottom of the conduction bands (CB) are attributed to the orbitals of zinc, oxygen, boron and chlorine or bromine, and thus, the states on the both sides of the band gap mostly consist of those from the [BO3] and [ZnO3Cl/Br] groups. Because the optical response of a crystal mainly originates from the electronic transitions between the VB and CB states close to the band gap,50 the [BO3] and [ZnO3Cl/Br] groups determine the optical properties in both crystals, in accordance with the

considering the number density of [BO3] groups, because AZBX have smaller density of the [BO3] triangles than KBBF. The enhanced SHG intensity of AZBX may be attributed to other NLO-active groups like distorted ZnO3Cl or ZnO3Br tetrahedra with d10 Zn2+ cations, which will be discussed in the next section. Origin of the SHG Enhancement. To better understand the origin of the optical properties in the series of NLO materials and the reason why they exhibit an enhanced SHG efficiency in comparison with KBBF, the first-principles density functional theory (DFT) calculations were performed. First, we computed the electronic band structures for each member of these series. The band gap calculated by PBE0 functionals are approximately 5.7 eV for the compounds which are basically consistent with experimental results within the error range allowed, while those calculated by LDA are about 3.5 eV. This is because the discontinuity of exchange− F


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Chemistry of Materials Table 3. First-Principles Nonlinear Optical Properties and the Atom-Cutting Results for AZBX (d11 = −d12) K+/Rb+/NH4+ [BO3] [ZnO3Cl/Br] total original

d11 d11 d11

KZBC

RZBC

NZBC

KZBB

RZBB

0.0221 0.2525 0.2453 0.5199 d11 = 0.4273

00146 0.2438 0.3061 0.5645 d11 = 0.438

0.0091 0.1935 0.2120 0.4146 d11 = 0.347

0.0148 0.2413 0.2806 0.5376 d11 = 0.429

0.0128 0.2354 0.2948 0.5430 d11 = 0.4298

anionic group theory proposed by Chen51 for the ultraviolet NLO crystals. On the basis of the electronic band structures, the firstprinciples calculation of nonlinear optical properties for the title compounds are listed in Table 3. Because AZBX compounds belong to the trigonal crystal system with a space group of R32, under the restriction of Kleinman symmetry,52 there are three equivalent nonzero components of the SHG tensor d11 = −d12 = −d26.53 In general, the calculated SHG effects are in basically good agreement with the experimental results, which verifies the validity of the pseudopotential methods employed. As the results shown, all the calculated values are about 0.44 pm/V, which are comparable with that of KBBF, although the densities of [BO3] groups in the title compounds are smaller than that in KBBF. The strong SHG efficiency of AZBX can be interpreted by analyzing the contribution of individual groups. Thus, the realspace atom-cutting analysis was performed. As shown in Table 3, remarkably, the ZnO3Cl/Br tetrahedra integrated with the anionic [BO3] groups make the dominant contribution to the SHG tensors, while the contribution of A+ (K, Rb, NH4) cations is negligibly small. It is worth noting that ZnO3Cl/Br tetrahedra were first confirmed as an NLO-active group to provide notably to the SHG intensity attributed to the polarization from distorted tetrahedra. Specifically, the contribution of the [ZnO3Cl/Br] groups to the SHG effect is significantly larger than that from the aligned [BO3] in these five materials. As a comparison, the dominant contribution to the SHG effect of KBBF mainly originates from [BO3] groups, whereas BeO3F groups in KBBF make too little contribution to ignore. Thus, the SHG mechanism of AZBX compounds is different from that of KBBF, although they have similar structural features. In other words, [ZnO3Cl/Br] groups are the origin of the SHG enhancement, which lead to the double SHG responses as compared with KBBF. It is the first reported that [ZnO3Cl/Br] groups can be as the NLO-active structural units in NLO materials. Enhancement of Layer Interaction by Modified Structures and Single-Crystal Growth Behavior. Better single-crystal growth behavior of NLO materials plays a crucial role in the practical application. KBBF has a great difficulty in the growth of thick crystals because of layering tendency. From the point of view of microscopic crystal structure, the layer tendency of KBBF crystal is originated in the weak ionic binding of K+−F− between the [Be2BO3F2]∞ layers and relatively much stronger intralayer binding energy. Thus, enhancing the interlayer interaction or weakening intralayer binding energy by utilizing other ionic or group as substitution of K+−F− between interlayers or Be−F and Be−O in intralayer might be beneficial to the growth of the crystal. During the process of KZBC crystal growth, it was also found that KZBC has a layering tendency to some extent. To improve the layering growth habit, we attempt to introduce different A-site cations (K, Rb) and E-site (Cl, Br) anions into interlayers to

regulate the layer interaction, yielding varying single-crystal growth behaviors, namely, different average crystal sizes and thickness in the c direction (see Figure 6) observed in the

Figure 5. Electronic densities of NZBC and RZBC.

experiment. Furthermore, considering the similarity of NH4+ and K+ (Rb+) in ionic radius and valence electronic property, the A-site cation replacement of NH4+ for K+ directed to the appearance of NZBC crystal. Interestingly, compared with the four compounds mentioned above, the layering growth tendency and the crystal growth behavior of NZBC have been greatly improved. During our preliminary trials of crystal growth, NZBC crystals with the dimensions of 6.0 × 5.0 × 0.8 mm3 (Figure 6) were obtained, much thicker than the former compounds. To evaluate the interlayer interactions and explain different growth tendencies for the title compounds observed in experiment, the binding energies of the intra- and interlayers for the compounds are calculated, and the results are listed in Table 4. The ratio (Eintra/Einter) between the intra- and interlayers binding energies is an important index employed to evaluate crystal growth capacity along the c direction in the series of AZBX compounds, namely, the larger the ratio, the more obvious is the layering growth tendency. It is concluded that the layering growth tendencies increase in the order of NZBC < RZBC < KZBC < KZBB < RZBB, which is agreement with the experimental observation. The experimental and calculated results reveal that A-site cations and E-site anions play an important role in adjusting layer interaction. Due to the larger electronegativity of Cl− ion than that of Br− ion, the layer interaction of Cl-containing compounds significantly stronger G


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Figure 6. Photos of AZBX crystals. The thickness along the c direction decrease in the order of NZBC < RZBC < KZBC < KZBB < RZBB.

observed in the trials of crystal growth. The experimental and calculated results showed that layer interaction significantly enhanced and the layer growth habits has been improved from AZn 2 BO 3 Br 2 (K, Rb), AZn 2 BO 3 Br 2 (A = K, Rb) to NH4Zn2BO3Cl2. Overall, AZn2BO3X2 have better growth habit compared with KBBF. Because of the relatively strong chemical binding present between NH 4 + groups and [Zn2BO3Cl2]∞ layers, NH4Zn2BO3Cl2 crystal exhibit better growth behavior along the c axis. These series of crystals may have potential application in the short-wave UV NLO field. Future efforts will be devoted to the growth of bulk single crystals and the characterizations of optical properties for these crystals.

Table 4. Calculated Intra- and Interlayer Binding Energies of AZBX binding energies

NZBC

RZBC

KZBC

KZBB

RZBB

KBBF

interlayer intralayer Eintra/Einter

4.195 38.78 9.24

3.735 39.11 10.47

3.735 39.11 10.47

3.465 38.52 11.11

3.46 38.53 11.14

4.265 54.2 12.71

than that of Br-containing compounds. Besides, AZBX possess a higher binding energy (Eintra/Einter: 9.24−11.14) than that of KBBF (Eintra/Einter: 12.71), indicating the improvement of layer growth habit of AZBX in comparison with KBBF. Furthermore, in the process of the synthesis and crystal growth, raw material ZnO used in AZBX has larger solubility in solvent than highly toxic BeO used in KBBF, which means the easiness of crystal growth of AZBX, compared to KBBF in terms of crystal growth conditions. Specially, it is obvious that NH4Zn2BO3Cl2 exhibits great progress in the nonlayering tendencies. To further investigate this, the electronic density distributions are plotted for NH4Zn2BO3Cl2 and RbZn2BO3Cl2 (Figure 5). It is shown that the spherical Rb+ has little electronic overlap with neighboring Cl− anions in RbZn2BO3Cl2, whereas the conical NH 4+ has quite large electronic overlap with Cl− in NH4Zn2BO3Cl2, indicating stronger chemical bond interaction in the latter crystal. So the existence of hydrogen-bonding between NH4+ and Cl− in the structure enables NZBC to possess a higher binding energy than that of other title compounds, which significantly enhances the layer interaction and greatly improves the crystal behavior.

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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.chemmater.6b04272. TG traces, PXRD patterns, PDOS figures, optical diffuse reflectance spectra, and additional tables (PDF) X-ray date of NH4Zn2BO3Cl2 (CIF) X-ray date of KZn2BO3Cl2 (CIF) X-ray date of KZn2BO3Br2 (CIF) X-ray date of RbZn2BO3Br2 (CIF) X-ray date of RbZn2BO3Cl2 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

4. CONCLUSIONS We designed and obtained a new family of NLO-active borate crystal materials, namely, AZn2BO3X2 (A = K, Rb, NH4+; X = Cl, Br). They possess KBBF-type structure, consisting of the infinite planar [Zn2BO3Cl(Br)2]∞ layers. Powder secondharmonic generation (SHG) measurement on the series of crystals reveals they are all phase-matchable in both visible and UV region with measured SHG efficiencies of about 2.53−3.01 times as large as that of KDP, and the short-wavelength absorption edge range is between 190 and 214 nm from AZn2BO3Cl2 (A = NH4, K, Rb) to AZn2BO3Br2(A = K, Rb). These features promise these kinds of borate crystals great applications as UV NLO materials. Moreover, the SHG effects of these compounds are more than double that of structurally analogous KBBF. Using first-principles calculations and atomcutting analysis, we investigated that enhanced SHG response originates from the cooperative effect of coparallel [BO3] triangles and distorted ZnO3Cl/Br tetrahedra to improve layer growth tendency, and we introduced different A-site cations (K, Rb, NH4) and E-site (Cl, Br) anions into interlayers to regulate the layer interaction, leading to different single-crystal behaviors

ORCID

Ning Ye: 0000-0001-8904-4576 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the National Natural Science Foundation of China (Grant Nos. 91222204 and 51425205) and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB20000000).

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AZn2BO3X2(A = K, Rb, NH4; X = Cl, Br): New Members of KBBF

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