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Structural Modulation of Anionic Group Architectures by Cations to Optimize SHG Effects: A Facile Route to New NLO Materials in the ATCO3F (A = K, Rb; T=Zn, Cd) Series Guangsai Yang, Guang Peng, Ning Ye, Jiyang Wang, Min Luo, Tao Yan, and Yuqiao Zhou Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03890 • Publication Date (Web): 15 Oct 2015 Downloaded from http://pubs.acs.org on October 20, 2015
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Chemistry of Materials
Structural Modulation of Anionic Group Architectures by Cations to Optimize SHG Effects: A Facile Route to New NLO Materials in the ATCO3F (A = K, Rb; T=Zn, Cd) Series Guangsai Yang,†,‡ Guang Peng,† Ning Ye,*,†,‡ Jiyang wang,§ Min Luo,† Tao Yan,† Yuqiao Zhou† †
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. ‡ College of Materials Science and Engineering, Fujian Normal University, Fuzhou, Fujian 350108, P. R. China § State Key Laboratory of Crystal Materials and Institute of Crystal Materials, Shandong University, Jinan, Shandong 250100, P. R. China. ABSTRACT: A new series of alkali-transition metal fluoride carbonates (KCdCO3F, RbCdCO3F, KZnCO3F, and RbZnCO3F) have been synthesized under subcritical hydrothermal conditions. All crystals are isostructural with the acentric space group P-6c2 (188). They were structurally characterized by X-ray single crystal diffraction and exhibited the stacking of alternating [AF]∞(A=K, Rb) and [TCO3]∞(T=Zn,Cd) layers connecting adjacent layers by infinite T−F−T(T=Zn, Cd) chains parallel to c-axis. We found that all [TCO3](T=Zn, Cd) building units aligned perfectly parallel in any given layer, but the rotation from one layer to the next resulted in the nonparallel arrangement of [CO3] groups between two adjacent [TCO3]∞(T=Zn, Cd) layers. In this work, the relative rotation of [CO3] groups between two successive layers was successfully controlled by introducing cations of different sizes into the structures, which led to different relative rotation angles of [CO3] groups, yielding varying second harmonic generation (SHG) effects for each fluoride carbonates. The SHG measurement indicates these compounds are all phase-matchable materials in both the visible and the UV region, and the experimental SHG responses are approximately 4.58, 2.84, 1.76, and 0.83 times that of KH2PO4 (KDP) for KCdCO3F, RbCdCO3F, KZnCO3F, and RbZnCO3F, respectively., All new compounds exhibit wide transparent regions ranging from the UV to the near IR, which suggest that they are promising UV NLO materials. In addition, the differences of the structures and NLO properties of A1+M2+CO3F-type crystals were summarized, and their structural design ideas and methods with respect to the structural modulation of anionic group architectures by cations to optimize SHG effects were detailed.
1. INTRODUCTION Using nonlinear optical (NLO) crystals1-3 for UV frequency conversion has become an attractive area of research due to the increasing demand of UV laser science and technology. Over the past few decades, numerous attempts and efforts have been made to create new UV NLO materials with high NLO coefficients and wide UV transparency. Many excellent UV NLO crystals are primarily borate crystals,4-15 which owe their NLO behavior to various boron-oxygen anionic groups such as [B3O6]3- in β-BaB2O4(BBO),16 [B3O7]5− in LiB3O5(LBO),17 [BO3]3− in KBe2BO3F2(KBBF),18 and Sr2Be2B2O7(SBBO),19 and [BO4]5− in SrB4O7.20 The planar [BO3]3− anion is believed to be the best NLO basic structural unit for UV and deep-UV light generation because of their relatively large microscopic second-order susceptibility and moderate birefringence.21-22 Analogous to the [BO3]3− anion, the [CO3]2− anion is found to have a similar trigonal planar geometry with π-conjugated molecular orbitals, which is desirable for good NLO micro-structural units. Thus, materials with [CO3]2− groups have drawn particular interests, and several new NLO carbonate crystals containing alkaline, alkaline earth, or rare earth cations, such as CsNa5Ca5(CO3)8, Na4La2(CO3)5,23 and Na3Re(CO3)3(Re=Y, Gd),24 have been discovered under hydrothermal conditions. In particular,
fluoride carbonates, which have been known to exist mainly in minerals,25 have garnered considerable attention because they have a layered topology, and a low melting point, so that they are easily synthesized by hydrothermal techniques or by flux methods. In recent years, our laboratory has been focused on the study and design of new NLO crystals in the fluoride carbonate family. A new series of fluoride carbonate crystals have been synthesized, including ABCO3F (A = K, Rb, Cs; B= Ca, Sr, Ba),26 Na8Lu2(CO3)6F2, and Na3Lu(CO3)2F2,27 K2.70Pb5.15(CO3)5F3,28 RbPbCO3F,29 CsPbCO3F,29-30 and RbMgCO3F,31 which are verified to be very promising for UV NLO materials with a wide transparency for practical applications by experimental measurements and theoretical calculation.32-33 In addition, transition metal fluoride carbonates, KCuCO3F,34 BaMCO3F2 (M=Mn, Cu and Zn), 3536 and Ba2Co(CO3)2F237 have also been discovered, among which only KCuCO3F is both NCS and polar, and the others are centrosymmetric. The design of SHG materials by selecting anions that are able to produce high nonlinearities (such as [CO3]2-) was considered to be a very good strategy, however it does not guarantee that the essential requirements for SHG materials, such as noncentrosymmetry, will be fulfilled 38 Therefore, the first challenge for the design of new NLO material based on the [(CO3)xFy]n− anion groups is prevent these anions from
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Chemistry of Materials
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crystallizing in a centrosymmetric arrangement with respect to each other. Furthermore, on the basis of the relationship between the structure and overall NLO properties, structural criteria39 for high-performance nonlinear optical material includes the parallel alignment of the anionic groups with large second-order microscopic susceptibilities to have them work in concert. This arrangement provides the maximum contribution for macroscopic NLO coefficients rather than having them canceling out each other, and has been experimentally confirmed to be reasonable for designing new large NLO effects inorganic materials. Therefore, the second challenge we faced was to obtain the optimal arrangement of the anionic groups for large SHG response. Further structural analysis of known fluoride carbonates revealed the essential role of cations in modulating the arrangement of the [CO3]2- groups, which was strongly related to the cation size and variable coordination environment of the cations. The effects of cations on the alignment of [CO3]2- in the alkaline-alkaline earth fluoride carbonate crystals ABCO3F(A=K, Rb, Cs; B=Ca, Sr, Ba) have been discussed.12 In this family, it was found that the different alignments among the [CO3]2- groups originated from the coordination behaviors of the countercations, which can be distinguished by the ratios of ionic radii between the A(K, Rb, Cs) and B(Ca, Sr, Ba) cations. With the ratio range from 1.16(Cs+/Ba2+) to 1.66 (Cs+/Ca2+) corresponding to the variable coordination number of countercations, the alignment of [CO3]2- groups changed from noncoplanar to coplanar to fully coparallel, and finally to coplanar but not fully coparallel. These different arrangements resulted in different SHG effects due to the structural modulation of the anion groups. Remarkably, the ABCO3F-type compounds possessing smaller size countercations, such as alkaline-earth metals Be, and transition metals Zn and Cd, characterized by higher radius ratios of cations(A+/B2+), have not been reported, likely due to the low decomposition temperature of MCO3(M=Be, Zn, Cd) compared with the other alkaline-earth carbonates. In particular, we have been very interested in investigating the association of d10 cations (Zn2+ and Cd2+) and fluoride carbonate anions to design new NCS compounds. Apart from the main reason stated above, there are other important factors which are as follows:(i) From the structural analysis of the known transition metal fluoride carbonate BaZnCO3F2, the coordination geometry in ZnO3F2 with equatorial Zn-O bonds could produce a unique bonding pattern in the [Zn(CO3)]∞ layers where [CO3] groups align parallel. Unfortunately, [CO3]2- groups rotated to the opposite direction in successive layers leading to the formation of a centrosymmetric structure. Thus, Zn2+ as countercation was a suitable choice for the parallel arrangement of [CO3]2- groups, however the requirement is to prevent them from rotating in the overlying layers. From the aspect of molecular design, the replacement of suitable cations controlling the rotation of [CO3]2- groups might be feasible. (ii) It is well-known that the Jahn-Teller distortion could occur in Zn2+ or Cd2+ with fully occupied d orbitals.40 Such examples include Cd4ReO(BO3)3 (Re=Y, Gd, Lu),41 Cd5(BO3)3F,42 Cs3Zn6B9O21,43 and BaZnBO3F, 44all of which possess NCS or polar structures and exhibit large NLO coefficients. So d10 cations (Zn2+, Cd2+) introduced into fluoride carbonate might be beneficial to increase the odds of creating new NCS compounds with large SHG responses. Based on the above points, our investigation of the d10 (Zn2+, Cd2+)-A+(K+,Rb+,Cs+)-CO3-F system resulted in the discovery
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of a series of new NLO fluoride carbonate crystals, namely KZnCO3F, RbZnCO3F, KCdCO3F, and RbCdCO3F. Interestingly, all the crystals are NCS nonpolar and isostructural, and they exhibit similar structure framework structures, but different arrangements of [CO3]2- groups. Herein, crystal structures, thermal behaviors, spectra, NLO properties and structure-property relationships of these fluoride carbonates are reported. In addition, we demonstrate that cations have a dramatic influence on the alignment of [CO3]2- anionic groups and can significantly modulate the crystal structure.
2. EXPERIMENTAl SECTION Reagents. KF·3H2O, KF, CdCl2, K2CO3, RbF, Rb2CO3, and Zn(NO3)2·6H2O were purchased from Shanghai Titan Scientific Co., Ltd and used as received. The purity of all the reagents is greater than or equal to 99.9%. Synthesis. Crystals of KCdCO3F, RbCdCO3F, KZnCO3F, and RbZnCO3F were grown by solvothermal techniques under subcritical conditions. For KCdCO3F, the reaction mixture of 0.917 g of CdCl2 (5.00x10-3 mol), 2.884 g of KF·2H2O (3x102 mol), 1.383 g K2CO3 (1x10-2mol)，and 5 ml H2O 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 product were separated from the mother liquid by filtration and washed with deionized water and ethanol and then dried in the air. Under the same conditions, single crystal of RbCdCO3F was synthesized by using 1.100 g of CdCl2 (6.00x10-3mol), 4.179 g of RbF (4.00x10-2mol), 2.309 g of Rb2CO3 (1.00x10-2mol), and 5 ml H2O, and that of KZnCO3F by using 1.190 g of Zn(NO3)2·6H2O (4.00x10-3mol), 1.923 g of KF·2H2O (2x10-2mol), 1.383 g of K2CO3 (1x102 mol) and 5 ml H2O, and that of RbZnCO3F by using 1.190 g of Zn(NO3)2·6H2O (4x10-3mol), 3.13 g of RbF(3x10-2mol). Colorless, transparent, hexagonal prism-shaped crystals subsequently determined to be KCdCO3F, RbCdCO3F, KZnCO3F, and RbZnCO3F, and these compounds were obtained in approximately 90%, 80%, 85% and 70% yields based on Cd and Zn, respectively. Polycrystalline KCdCO3F and RbCdCO3F were synthesized by conventional solid-state reactions. A mixture of KF(RbF) and CdCO3 in a molar ratio of 1:1 were thoroughly ground and placed in the platinum crucible that was heated to 250℃ in flowing CO2 gas, held for 5d, and then cooled to room temperature. The materials were confirmed to be pure by power X-ray diffraction. With respect to Zn compounds, attempts at synthesizing pure polycrystalline had failed due to the instability of ZnCO3. Single-Crystal X-ray Diffraction. A transparent block of crystal was mounted on a glass fiber with epoxy for singlecrystal diffraction analysis. Data was collected at room temperature on a Rigaku Mercury CCD diffractometer with graphite-monochromatic Mo Kα radiation (λ= 0.71073 Å). A hemisphere of data was collected using a narrow-frame method with the ω-scan mode. The date was integrated using the CrystalClear program, with the intensities corrected for Lorentz polarization, air absorption, and absorption attributable to variation in the path length through the detector faceplate. Absorption correction was also applied based on the Multiscan technique. The structures were determined by direct methods and refined by difference Fourier maps and full-
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Chemistry of Materials
Table 1. Crystal Data and Structure Refinement for KCdCO3F, RbCdCO3F, KZnCO3F, RbZnCO3F formula
KCdCO3F
RbCdCO3F
KZnCO3F
RbZnCO3F
formula mass(amu)
230.52
276.89
183.48
229.85
crystal system
hexagonal
hexagonal
hexagonal
hexagonal
space group
P-6c2
P-6c2
P-6c2
P-6c2
a(Å)
5.1349(13)
5.2109(7)
5.0182(6)
5.1035(9)
c(Å)
8.846(4)
9.0645(19)
8.355(2)
8.619(4)
α(deg)
90
90
90
90
γ(deg)
120
120
120
120
202.00(12)
213.16(6)
182.21(5)
194.41(10)
3
V(Å ) Z
2
2
2
2
ρ(calcd)(g/cm3)
3.790
4.314
3.344
3.926
temp(K)
293(2)
293(2)
293(2)
293(2)
λ(Å)
0.71073 A
0.71073
0.71073
0.71073
F(000)
212
248
176
212
µ(mm-1)
6.327
16.364
7.761
18.641
θ(deg)
4.58 to 27.47
4.50 to 27.42
4.69 to 27.24
4.61 to 27.43
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