Noncentrosymmetric Cubic Cyanurate K6Cd3(C3N3O3)4 Containing

Dec 13, 2017 - Single crystals of K6Cd3(C3N3O3)4 (1) were successfully grown via a solid-state cyclotrimerization reaction method from CdCl2 and KOCN...
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Noncentrosymmetric Cubic Cyanurate K6Cd3(C3N3O3)4 Containing Isolated Planar π‑Conjugated (C3N3O3)3− Groups Mingjun Xia,† Molin Zhou,†,‡ Fei Liang,†,‡ Xianghe Meng,†,‡ Jiyong Yao,† Zheshuai Lin,†,‡ and Rukang Li*,†,‡ †

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 100190, China S Supporting Information *

Ca 3 (C 3 N 3 O 3 ) 2 , 19 α-Sr 3 (C 3 N 3 O 3 ) 2 , 20 β-Sr 3 (C 3 N 3 O 3 ) 2 , 21 Eu3(C3N3O3)2,21 and ARE2(C3N3O3)2Cl (A = K, Rb, Cs; RE = La, Ce, Pr), were reported.22 Among them, both Ca3(C3N3O3)2 and β-Sr3(C3N3O3)2 compounds are isostructural to β-BBO; they all belong to the space group of R3c, while BCY is isotypic to the high-temperature phase of BaB2O4 (α-BBO), a commercial ultraviolet birefringence crystal. Li3Sr2(C3N3O3)2F, which crystallizes into space group of P63/m, is also isostructural with the deep-ultraviolet birefringence material Ba2Na3(B3O6)2F.23 Notably, Ca3(C3N3O3)2, α-Sr3(C3N3O3)2, and β-Sr3(C3N3O3)2 exhibit stronger second-harmonic-generation (SHG) responses than that of β-BBO under 800 nm irradiation in the microscopic powder SHG test experiments. In addition, previous theoretical investigations have demonstrated that it is the (C3N3O3)3− anionic group, similar to (B3O6)3−, that makes dominate contributions to large SHG responses in Ca3(C3N3O3)2 and βSr3(C3N3O3)2.24 In comparison with the (B3O6)3− group, the (C3N3O3)3− unit exhibits a shorter interatomic distance and stronger pπ−pπ interaction. In addition, C atoms in (C3N3O3)3− units can provide one electron-populated pz orbital to form stronger π-conjugated interaction in the 6-MR. Therefore, the (C3N3O3)3− group, serving as the fundamental building block in metal cyanurates, should be suitable as another distinctive NLOactive microscopic unit. We believe that more experiments and theoretical analyses are urgently needed to evaluate the potential applications of metal cyanurates. In addition, various strategies have been put forward to explore new NCS compounds by introducing NLO-active units including second-order Jahn− Teller d0 cations (Ti4+, Nb5+, Mo6+, etc.) with distorted octahedra, stereochemically active lone-pair cations (Pb2+, Te4+, I5+, etc.), d10 cations (Zn2+, Cd2+, etc.), and π-conjugated groups (BO3, B3O6, C3N3O3, etc.).25−27 Guided by the above idea, we introduced the d10 cation Cd2+ and π-conjugated (C3N3O3)3− anionic group into a structure. Accordingly, we grew single crystals of a new cyanurate with a composition of K6Cd3(C3N3O3)4 (1). It contains isolated planar π-conjugated (C3N3O3)3− groups and crystallized into a cubic system. Herein, its single-crystal structure, electronic structure, and NLO properties will be reported. Single crystals of 1 were obtained from the reaction of CdCl2 and KOCN with a molar ratio of 1:4. The mixture was put into a

ABSTRACT: Single crystals of K6Cd3(C3N3O3)4 (1) were successfully grown via a solid-state cyclotrimerization reaction method from CdCl2 and KOCN. To our best knowledge, it is the first inorganic compound containing isolated six-membered-ring (6-MR) anionic groups that crystallizes in the cubic system. In the structure, the basic 6-MR anionic unit is a planar π-conjugated (C3N3O3)3− group that is isoelectronic with the (B3O6)3− group, as observed in the benchmark nonlinear-optical (NLO) crystal β-BaB2O4 with strong second-harmonic-generation response. In addition, the electronic structure and linearoptical and NLO properties for 1 were also investigated by the first-principles calculation. The NLO coefficient (d14 = 1.17 pm/V) of 1 is about 3 times that of KH2PO4.

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oncentrosymmetric (NCS) compounds, having technological importance, such as nonlinear optics, electrooptics, pyroelectricity, piezoelectricity, and ferroelectricity, are of broad interest and have been widely employed in the fields of optoelectronics because of their abundant chemical and physical properties.1−4 Especially, borate crystals play vital roles in nonlinear-optical (NLO) materials because of their wide transparent range, high laser damage threshold, and rich structural types. β-BaB2O4 (β-BBO),5 LiB3O5,6 CsLiB6O10,7 and KBe2BO3F2 crystals8 are mostly used as NLO materials in the visible, ultraviolet, or deep-ultraviolet spectral range. In particular, the β-BBO crystal, which exhibits a large NLO coefficient (d22 = 2.2 pm/V), a short ultraviolet absorption edge (λcutoff = 189 nm), and a high resistance against laser damage (15 GW/cm2 at 1064 nm and 1.3 ns), has usually been utilized as second-, fourth-, and fifth-harmonic-generation neodymiumbased lasers. The outstanding NLO properties of the β-BBO crystal are mainly attributed to the planar π-conjugated (B3O6)3− groups according to anionic group approximation.9 Moreover, the combinations of planar (B3O6)3− groups and other constructing blocks such as BO3F groups also provide a versatile platform for discovering new deep-ultraviolet NLO materials.10−15 Recently, Meyer’s group found several cyanurates containing planar (C3N3O3)3− six-membered-ring (6-MR) units that are isoelectric with (B3O6)3− groups. The new metal cyanurates, including Pb 3 (C 3 N 3 O 3 ) 2 ·2H 2 O, 1 6 LiSr(C 3 N 3 O 3 ), 1 7 Li3Sr2(C3N3O3)2F,17 Ba3(C3N3O3)2 (BCY),18 © XXXX American Chemical Society

Received: November 8, 2017

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DOI: 10.1021/acs.inorgchem.7b02860 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry fused-silica tube in an argon-filled glovebox. The tube was sealed under a high vacuum of 10−3 Pa and then placed in a temperature-controlled furnace. The furnace was slowly heated to 693 K and held for 2 days to melt completely. Finally, by slow cooling of the furnace at a cooling rate of 3 K/h, small single crystals with block habit were obtained from the flux system after the remaining solvents were washed away with dry ethanol (Figure S1). The polycrystalline samples of 1 were synthesized at 520 K according to eq 1. As seen in Figure 1, the pure phase

Figure 1. Experimental and calculated powder X-ray diffraction patterns of 1.

samples can be successfully obtained by washing the mixture with dry ethanol. The as-synthesized crystalline product was stable in air. Thus, it is clear that 1 was obtained by cyclotrimerization of three linear (OCN)− units, forming cyanurate (C3N3O3)3− ions, according to the following reaction: (1)

Figure 2. (a) π-conjugated (C3N3O3)3− anionic group, (b) Cd atom coordinating to four (C3N3O3)3− anionic groups, and (c) threedimensional structure of 1 along the b axis.

Elemental analysis of the as-grown crystal, performed by energy-dispersive X-ray spectroscopy, indicated that the ingredients of 1 are K, Cd, C, N, and O, which is consistent with the chemical composition obtained from single-crystal structure X-ray diffraction determination (Figure S2), and the composition ratio of K:Cd was obtained in a molar ratio of 2.06:1, which is consistent with the theoretical ratio of 2:1. Compound 1 crystallizes in the NCS cubic space group I4̅3d with the cell parameters of a = 13.4017(15) Å and Z = 3 (Tables S1−S3). As shown in Figure 2a, the π-conjugated (C3N3O3)3− anionic groups in the structure, which are isoelectronic with the (B3O6)3− groups, are condensed by three (CN2O)3− units through corner-sharing N atoms. As expected, the bond lengths of C−N and C−O are 1.365(4)−1.372(4) and 1.254(4) Å, respectively, which are much shorter than those of the respective average bond distances of 1.40 and 1.32 Å in (B3O6)3− groups.28 The bond angles of the (C3N3O3)3− anionic group are in the range 118.7(3)−120.9(3)°, showing good coplanar characteristics. The above bond distances of C−N and C−O in the 6-MR are consistent with those of the reported cyanurates.16−22 As presented in Figure 2b, Cd atoms are connected with four (C3N3O3)3− groups through N atoms with equal bond distances of 2.253(3) Å, which is slightly shorter than the Ca−N bond distances [2.459(4)−2.472(3) Å] reported in Ca3(C3N3O3)2.19 Also, the [Cd(C3N3O3)4]10− blocks are further connected to form a three-dimensional framework. K atoms, surrounded by four O atoms, form two short [2.529(3) Å] and two long [2.833(3) Å] bonds in a distorted tetrahedral environment and occupy the voids in the three-dimensional network (Figure 2c).

The fingerprint region of the IR spectra of 1 is displayed in Figure S3. The peaks located at 1138, 1417, and 1518 cm−1 were assigned to the stretching vibrations of the C3N3O3 group, and the peaks around 621 and 821 cm−1 were attributed to the bending vibrations of the C3N3O3 group.18 According to the UV−vis−IR reflectance spectra, 1 has high transmittance in the range from 500 to 2500 nm (Figure S46). Also, the band gap was obtained as 5.23 eV, corresponding to the ultraviolet absorption edge of 237 nm (Figure S5). In order to gain better insight into the linear and NLO properties of 1, first-principles calculations based on density functional theory29 were performed using the CASTEP program.30 First, to investigate the structural stability, we adopted the linear response method to obtain the phonon dispersion of 1. It is well-known that the lattice vibrational properties can be deduced from the phonon dispersion, which intrinsically characterizes the interatomic interaction.31 The negative (imaginary) phonon eigenvalues indicate that the interatomic forces are not attractive at given reciprocal lattice points, and thus the crystal structure is not kinetically stable. Clearly, for 1, the eigenvalues for all phonon modes are positive, thus confirming their structural stability theoretically (Figure 3a). The electronic band structure of 1 has been calculated by both Perdew−Burke−Ernzerhof (PBE) and hybrid HSE06 functionals (Figure 3b,c). It has a direct band gap of 4.58 eV with a valence band maximum (VBM) and a conduction band minimum located at the G point. Generally, the calculated band gap by PBE is less than the experimental value because of

3CdCl 2 + 12KOCN → K 6Cd3(C3N3O3)4 + 6KCl

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DOI: 10.1021/acs.inorgchem.7b02860 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. (a) Calculated phonon dispersion of 1, (b) electronic band structure of 1 by the PBE functional, (c) electronic band structure of 1 by the HSE06 functional, and (d) total and partial DOS of 1.

the discontinuity of the exchange-correlation energy. The corrected band gap by hybrid HSE06 functions is obtained as 5.65 eV, in agreement with the experimental band gap of 5.23 eV. Figure 3d displays the density of states (DOS) and partial DOS of the respective species in 1. Clearly, the deep part of the valence band (VB) lower than −10 eV is mainly composed of strongly localized K 3p, N 2s, and O 2s orbitals. The upper part of the VB (−10 to 0 eV) consists of Cd 4d, C 2p, N 2p, and O 2p orbitals. Notably, the Cd 4d orbitals have a few hybrid components with N 2p (from −7.5 to −5 eV), which corresponds to the Cd−N bond in the crystal structure. In addition, C, N, and O orbitals exhibit strong hybridization around −5 eV, thus resulting in localized σ-type C−N/C−O bond and delocalized π-conjugated bonds in the (C3N3O3)3− 6-MR. The VBM is exclusively occupied by N 2p and O 2p orbitals, which are similar to those of α-Sr3(C3N3O3)2 and β-Sr3(C3N3O3)2. The bottom of the conduction band is mainly contributed from C, N, and O 2p orbitals, while the contributions of Cd and K atoms are nearly ignorable. Because the optical effects of a crystal are mainly determined by the optical transition between the electronic states close to the band gap, accordingly, it is anticipated that they are dominantly contributed from the (C3N3O3)3− groups, while the contributions from the orbitals of the Cd2+ and K+ cations are negligibly small. To investigate the electron distribution of the respective groups, the visualized total electron density and electron localized function (ELF) maps are shown in parts a and b of Figure 4, respectively. It is clear that the electron densities at the Cd−N bonds are much weaker than those of the C−N and C−O bonds. The Mulliken population of the Cd−N bond (0.35) is also smaller than those of the C−N (0.98) and C−O (1.03) bonds. A nonspherical density distribution is clearly presented

Figure 4. (a) Total electron density map of 1 at the (111) plane cutting through the (C3N3O3)3− anion group and the contour line values ranging from 0 (blue) to 1 (red). (b) ELF map of 1 at the (111) plane cutting through the (C3N3O3)3− anion group and the ELF value ranging from 0 (blue) to 0.3 (red).

around each N and O atom, which are composed of umbrellashaped N sp2 lone pairs and nonbonding O 2p electrons. Six parallel pπ electrons are uniformly distributed at the C3N3 ring, thus corresponding to a small π-conjugated configuration of π66. Three relatively weak pπ orbitals are localized at the C−O bond, which form a larger π-conjugated configuration of π12 9 in the (C3N3O3)3− anion. The B atom only provides an empty pz orbital in the (B3O6)3− unit, whereas the C atom can provide a pz orbital electron, which is favorable for obtaining stronger π-conjugated interaction in the (C3N3O3)3− 6-MR than that in the (B3O6)3− 6MR. Therefore, it is expected that the microscopic susceptibility of the (C3N3O3)3− group would be larger than that of the (B3O6)3− unit. Under the restriction of Kleinman’s symmetry, 1 has only one nonzero independent SHG coefficient because of its point group symmetry of 4̅3m. The NLO coefficient d14 is C

DOI: 10.1021/acs.inorgchem.7b02860 Inorg. Chem. XXXX, XXX, XXX−XXX

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Chem., Int. Ed. 2017, 56, 11860−11864. (e) Tian, N.; Zhang, Y. H.; Li, X. W.; Xiao, K.; Du, X.; Dong, F.; Waterhouse, G I. N.; Zhang, T. R.; Huang, H. W. Precursor-reforming protocol to 3D mesoporous g-C3N4 established by ultrathin self-doped nanosheets for superior hydrogen evolution. Nano Energy 2017, 38, 72−81. (2) (a) Chi, E. O.; Ok, K. M.; Porter, Y.; Halasyamani, P. S. Na2Te3Mo3O16: A new molybdenum tellurite with second-harmonic generating and pyroelectric properties. Chem. Mater. 2006, 18, 2070− 2074. (b) Gao, Z. L.; Yin, X.; Zhang, W. G.; Wang, S. P.; Jiang, M. H.; Tao, X. T. Electro-optic properties of BaTeMo2O9 single crystal. Appl. Phys. Lett. 2009, 95, 151107. (c) Zhang, W. G.; Li, F.; Kim, S. H.; Halasyamani, P. S. Top-Seeded Solution Crystal Growth and Functional Properties of a Polar Material-Na2TeW2O9. Cryst. Growth Des. 2010, 10, 4091−4095. (d) Zhao, P.; Cong, H.; Tian, X.; Sun, Y.; Zhang, C.; Xia, S.; Gao, Z.; Tao, X. Top-Seeded Solution Growth, Structure, Morphology, and Functional Properties of a New Polar Crystal - Cs2TeW3O12. Cryst. Growth Des. 2015, 15, 4484−4489. (3) (a) Zhao, S.; Kang, L.; Shen, Y.; Wang, X.; Asghar, M. A.; Lin, Z.; Xu, Y.; Zeng, S.; Hong, M.; Luo, J. Designing a Beryllium-Free DeepUltraviolet Nonlinear Optical Material without a Structural Instability Problem. J. Am. Chem. Soc. 2016, 138, 2961−2964. (b) Zhao, S.; Gong, P.; Luo, S.; Liu, S.; Li, L.; Asghar, M. A.; Khan, T.; Hong, M.; Lin, Z.; Luo, J. Beryllium-Free Rb3Al3B3O10F with Reinforced Interlayer Bonding as a Deep-Ultraviolet Nonlinear Optical Crystal. J. Am. Chem. Soc. 2015, 137, 2207−2210. (c) Huang, H. W.; Liu, L. J.; Jin, S. F.; Yao, W. J.; Zhang, Y. H.; Chen, C. T. Deep-Ultraviolet Nonlinear Optical Materials: Na2Be4B4O11 and LiNa5Be12B12O33. J. Am. Chem. Soc. 2013, 135, 18319−18322. (d) Huang, H. W.; Yao, J. Y.; Lin, Z. S.; Wang, X. Y.; He, R.; Yao, W. J.; Zhai, N. X.; Chen, C. T. NaSr3Be3B3O9F4: A Promising Deep-Ultraviolet Nonlinear Optical Material Resulting from the Cooperative Alignment of the [Be3B3O12F]10‑ Anionic Group. Angew. Chem., Int. Ed. 2011, 50, 9141−9144. (4) (a) Huang, H. W.; Li, X. W.; Wang, J. J.; Dong, F.; Chu, P. K.; Zhang, T. R.; Zhang, Y. H. Anionic Group Self-Doping as a Promising Strategy: Band-Gap Engineering and Multi-Functional Applications of High-Performance CO32− Doped Bi2O2CO3. ACS Catal. 2015, 5, 4094−4103. (b) Huang, H. W.; Cao, R. R.; Yu, S. X.; Xu, K.; Hao, W. C.; Wang, Y. G.; Dong, F.; Zhang, T. R.; Zhang, Y. H. Single-unit-cell layer established Bi2WO6 3D hierarchical architectures: Efficient adsorption, photocatalysis and dye-sensitized photoelectrochemical performance. Appl. Catal., B 2017, 219, 526−537. (5) Chen, C. T.; Wu, B. C.; Jiang, A. D.; You, G. M. A New-Type Ultraviolet SHG Crystal - Beta-BaB2O4. Sci. Chin. B 1985, 28, 235−243. (6) Chen, C. T.; Wu, Y. C.; Jiang, A. D.; Wu, B. C.; You, G. M.; Li, R. K.; Lin, S. J. New Nonlinear-Optical Crystal - LiB3O5. J. Opt. Soc. Am. B 1989, 6, 616−621. (7) (a) Mori, Y.; Kuroda, I.; Nakajima, S.; Sasaki, T.; Nakai, S. New Nonlinear-Optical Crystal - Cesium Lithium Borate. Appl. Phys. Lett. 1995, 67, 1818−1820. (b) Tu, J. M.; Keszler, D. A. CsLiB6O10 - a Noncentrosymmetric Polyborate. Mater. Res. Bull. 1995, 30, 209−215. (8) (a) Chen, C. T.; Xu, Z. Y.; Deng, D. Q.; Zhang, J.; Wong, G. K. L.; Wu, B. C.; Ye, N.; Tang, D. Y. The vacuum ultraviolet phase-matching characteristics of nonlinear optical KBe2BO3F2 crystal. Appl. Phys. Lett. 1996, 68, 2930−2932. (b) Wu, B. C.; Tang, D. Y.; Ye, N.; Chen, C. T. Linear and nonlinear optical properties of the KBe2BO3F2 (KBBF) crystal. Opt. Mater. 1996, 5, 105−109. (9) Chen, C. T.; Sasaki, T.; Li, R. K.; Wu, Y.; Lin, Z.; Mori, Y.; Hu, Z.; Wang, J.; Uda, S., Yoshimura, M.; Kaneda, Y. Nonlinear Optical Borate Crystals: Principles and Applications; Wiley-VCH, 2012. (10) Zhang, B. B.; Shi, G. Q.; Yang, Z. H.; Zhang, F. F.; Pan, S. L. Fluorooxoborates: Beryllium-Free Deep-Ultraviolet Nonlinear Optical Materials without Layered Growth. Angew. Chem., Int. Ed. 2017, 56, 3916−3919. (11) Shi, G.; Wang, Y.; Zhang, F.; Zhang, B.; Yang, Z.; Hou, X.; Pan, S.; Poeppelmeier, K. R. Finding the Next Deep-Ultraviolet Nonlinear Optical Material: NH4B4O6F. J. Am. Chem. Soc. 2017, 139, 10645− 10648. (12) Liang, F.; Kang, L.; Gong, P.; Lin, Z.; Wu, Y. Rational Design of Deep-Ultraviolet Nonlinear Optical Materials in Fluorooxoborates:

calculated as 1.17 pm/V, which is about 3 times that of the commercial KH2PO4 crystal (0.39 pm/V). The NLO coefficient is smaller than those of Ca3(C3N3O3)2 and β-Sr3(C3N3O3)2 because of the unfavorable alignment of the (C3N3O3)3− groups in the structure (Figure S6). In summary, the novel cubic cyanurate 1 was grown from CdCl2 and KOCN. In the structure, π-conjugated (C3N3O3)3− 6MR groups exist, which is the first time that they have been observed in the cubic system to our knowledge. First-principles analysis reveals that the structure is kinetically stable according to the phonon vibration, and the NLO response can be mainly attributed to the π-conjugated (C3N3O3)3− group. The growth of larger single crystals and measurements of NLO and other physical properties are underway.

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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.7b02860. Crystallographic data, computational methods, and additional experimental data (PDF) Accession Codes

CCDC 1585968 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Mingjun Xia: 0000-0001-8092-6150 Fei Liang: 0000-0002-4932-1329 Jiyong Yao: 0000-0002-4802-5093 Zheshuai Lin: 0000-0002-9829-9893 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grants 51502307 and 51772304), the National Key R&D Program of China (Grant 2016YFB0402103), the China “863” Project (Grant 2015AA034203), and the Foundation of the Director of Technical Institute of Physics and Chemistry, Chinese Academy of Sciences.

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DOI: 10.1021/acs.inorgchem.7b02860 Inorg. Chem. XXXX, XXX, XXX−XXX