Ba3(C3N3O3)2: A New Phase of Barium Cyanurate Containing

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Ba3(C3N3O3)2: A New Phase of Barium Cyanurate Containing Parallel #-Conjugated Groups as Birefringent Material Replacement for Calcite Jian Tang, Fei Liang, Xianghe Meng, Kaijin Kang, Wenlong Yin, Tixian Zeng, Mingjun Xia, Zheshuai Lin, Jiyong Yao, Guochun Zhang, and Bin Kang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01782 • Publication Date (Web): 28 Dec 2018 Downloaded from http://pubs.acs.org on January 1, 2019

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Crystal Growth & Design

Ba3(C3N3O3)2: A New Phase of Barium Cyanurate Containing Parallel

π-Conjugated

Groups

as

Birefringent

Material

Replacement for Calcite Jian Tang†,#, Fei Liang‡,#, Xianghe Meng‡, Kaijin Kang†, Wenlong Yin,†,*, Tixian Zeng§, Mingjun Xia‡,*, Zheshuai Lin‡, Jiyong Yao‡, Guochun Zhang‡, and Bin Kang† †Institute

of Chemical Materials, China Academy of Engineering Physics, Mianyang 621900, China 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 §Physics and Space Science College, China West Normal University, Nanchong 637002, China Supporting Information Placeholder ‡Beijing

ABSTRACT: A new phase of Ba3(C3N3O3)2 (I) was grown from RbCNO and BaCl2 by a solid state cyclotrimerization reaction. It crystallizes in space group R3 with the cell parameters of a = 7.0934(2) Å, c = 20.1130(9) Å, featuring a layered structure 2 2― stacked from the ∞[𝐵𝑎(𝐶3𝑁3𝑂3)2] double layers along c direction. I exhibits wide ultraviolet transparency and large optical anisotropy of Δn = 0.32 at 800 nm, two times than that of the benchmark birefringent crystal calcite. The discovery of I shows that replacement of calcite is conceivable and opens the novel pathway for designing novel ultraviolet birefringent crystals in metal cyanurates.

Optical anisotropy, especially in crystal optics, means the phenomenon of double refraction.1-10 And the birefringence, the maximum difference between refractive indices with different polarization, is related to the optical anisotropic polarizability in the medium.11 Birefringent materials as fundamental optical parameter for linear and nonlinear optical elements, can be used in various polarization devices such as optical polarizers, beam splitters, wave plates, circulators and phase matching devices.12, 13 Up to date, several birefringent crystals were commercially available in the wavelength region from infrared (IR) to deep ultraviolet (UV) range, such as YVO4,14 calcite CaCO3,11 α-BaB2O4 (αBBO),15 MgF2,16 etc. Especially, the widely utilized birefringent crystals are all uniaxial beneficial for practical applications. YVO4 crystal has a large birefringence, but it is not transparent at the wavelength below 400 nm and is commonly applicable in the near-IR range.14 MgF2 is a unique deep ultraviolet birefringent crystal used below 200 nm because of its very short UV transparency cutoff of 110 nm, but its birefringence (Δn = 0.0128 at 253.7 nm) is very small, resulting in that the fabricated prism has a large volume causing an inconvenience in the application.16 Among them, calcite, as a long history birefringent crystal, usually used in the visible or near UV wavelength region.11 However, calcite is a kind of natural mineral and it is difficult to obtain large size crystals for fabricating polarized prism with high optical quality especially in the ultraviolet range due to their different geological impurities. On the other hand, calcite experiences the troublesome mechanical process because of its complete cleavage. Since α-BBO is proposed as birefringent crystal, borates are explored to find ultraviolet birefringent materials in recent years.15 According to the empirical method to calculate the anisotropic refractive indices of borate compounds, the planar (B3O6)3- six-membered-ring (6-MR) group shows the largest anisotropic polarizability in the all basic borate

anionic groups. Accordingly, several borate birefringent crystals containing the (B3O6)3- 6-MR groups, including CsBaB3O6,17 Ba2M(B3O6)2 (M = Mg, Ca, Sr, Pb),18, 19 Ba2Na3(B3O6)2F,20, 21 Ba3Y(B3O6)322 and ABaRE(B3O6)2 (A = K, Rb, Cs and RE = Y, Gd, Tb),23, 24 were reported. Some of them were proposed as UV or deep UV birefringent materials,19, 20 in which (B3O6)3- groups play the dominate role in determining their birefringence. More recently, cyanurate compounds as nonlinear optical (NLO) or birefringent materials are received much attention for their excellent properties resulting from planar πconjugated (C3N3O3)3- anionic groups.25, 26 For example, noncentrosymmetric Ca3(C3N3O3)227 and β-Sr3(C3N3O3)228 compounds, isostructural to the famous β-BBO NLO crystal, show very strong second harmonic generation responses.29, 30 Theoretical calculations31 demonstrated that (C N O )33 3 3 anionic group enables shorter interatomic distance, more uniform p electrons distribution and stronger pπ−pπ conjugated interaction than that of the isoelectronic (B3O6)3group owing to the additional one electron-populated pz orbital from C atoms. In view of large anisotropy, planar (C3N3O3)3- group is a kind of excellent constructing blocks for birefringent materials. For instance, centrosymmetric Ba3(C3N3O3)2,32 Ba2Mg(C3N3O3)233 and Li3Sr2(C3N3O3)2F25 are isomorphic to α-BBO, Ba2Mg(B3O6)2 and Na3Ba2(B3O6)2F birefringent crystals, respectively, and exhibit larger birefringence than that of isostructural borates. It is worth mentioning that Ba3(C3N3O3)2 (II) was first reported by Meyer group,32 and it crystallizes in space group R3𝑐 with cell parameters a = 7.070(2) Å, c = 39.95(2) Å. In this communication, we grew a new phase of Ba3(C3N3O3)2 (I), crystallizing in space group R3 with the halved unit cell of a = 7.0934(2) Å and c = 20.1130(9) Å. Its crystal growth, structure, and optical properties are studied for the first time. The structural stability of the two phases is also investigated by the first principles calculations.

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Single crystals of I with plate habit were grown from the mixture of RbCNO and BaCl2 with a molar ratio of 3:1 in a fused-silica tube under a high vacuum of 10-3 Pa. The tube was heated to 723 K and held for 72 h and then slowly cooled to 373 K at a rate of 3 K/h, and finally cooled to room temperature by switching off the furnace. The polycrystalline samples of I were synthesized at 590 K with excess amount of RbCNO used in the reaction. The above reaction is obvious solid-state polycondensation reaction by cyclotrimerization of three linear cyanate (CNO)− units to construct planar cyanurate (C3N3O3)3− 6-MR groups.34 As shown in Figure 1, the pure phase of I was successfully obtained after the remaining reagents of RbCNO and RbCl were washed away with dry ethanol. The reagents and the synthesized crystals are nontoxic. In addition, by carefully checking the PXRD patterns between I, II and the synthesized samples, the peak located at 32.4o, only belonging to the phase II, is violated in I and the synthesized samples, elucidating that the solved single crystal structure of R3 is exact.

Figure 1. The experimental and calculated powder X-ray diffraction patterns of I. I crystallizes in uniaxial centrosymmetric space group R3 with the cell parameters of a = 7.0934(2) Å, c = 20.1130(9) Å and Z=3, featuring two dimensional layered structure 2 2― stacked from the ∞[𝐵𝑎(𝐶3𝑁3𝑂3)2] double layers along c axis (Figure 2a and Table S1 - 3). In the asymmetric unit of I, there are five independent crystallographic positions with two Ba, one C, N and O atoms. The most important anionic unit is the (C3N3O3)3- 6-MR group composed of condensation by three planar (CN2O)3- triangles via cornersharing N atoms in the structure. The bond lengths of C-O and C-N are 1.306(11) Å and 1.322(14) - 1.365(14) Å, respectively, that is in accordance with the values in the reported cyanurates. The interatomic distances in the cyanurate rings of I are shorter than the corresponding values of oxoborate (B3O6)3- in α-BBO,15 which may be caused by the larger overlapping between C 2p, N 2p, and O 2p orbitals, thus inducing stronger pπ-pπ interaction in the cyanurate rings.31 Ba1 atoms are surrounded by six terminal O atoms of the (C3N3O3)3- 6-MR group to form a regular Ba1O6 octahedron with the equal bond length of 2.651(8) Å. And the Ba1 are linked with the six neighbor (C3N3O3)32 2― groups to form the ∞[𝐵𝑎(𝐶3𝑁3𝑂3)2] double layers (Figure 2b and c), giving that the three above and three beneath (C3N3O3)3- groups are arranged in the antiparallel alignment manner (Figure 2d). The double layers are further connected with Ba2O6N3 unit along c direction to construct three dimensional framework (Figure S1). The Ba2 atoms coordinate to six O and three N atoms, and the bond lengths of Ba-O and Ba-N vary from 2.781(8) Å to 2.884(8) Å and 3.018(9) Å. Owing to the planar (C3N3O3)3- groups are

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arranged parallel to each other in the structure, I are expected to display large optical anisotropy.

Figure

2.

(a)

Crystal structure of I, (b) the double layer, (c) the double layer viewed along c direction, and (d) Ba1 atom coordinating to six (C3N3O3)3− 6-MR groups.

2 [𝐵𝑎(𝐶3𝑁3𝑂3)2]2 ― ∞

The fingerprints of the IR spectroscopy of I reveals that the absorption band ranging from 1500 – 1101 cm-1 and the peaks around 831 cm−1, 598 cm−1 and 472 cm−1 are assigned to the stretching vibrations and the bending vibrations of the cyanurate ring (Figure S2).32 Notably, the experimental IR spectra are in good agreement with the calculated spectra, also confirming the correctness of the structure determination. All vibrational modes are assigned by theoretical analysis. I exhibits high transmittance in the range of 500 – 2000 nm and transparent in the ultraviolet wavelength region according to the UV−vis−IR reflectance spectra (Figure S3). In addition, the band gap was deduced as 5.14(2) eV from the Kubelka-Munk function (Figure S4),35 corresponding to the UV absorption edge at 241 nm, comparable to the transmission optical property of calcite. This value is also comparable to previous bandgap reports, such as K6Cd3(C3N3O3)4 (5.23 eV),36 Ba2Mg(C3N3O3)2 (5.20 eV)33 and Ba2Ca(C3N3O3)2 (5.10 eV).34 In order to elucidate the structure-property relationship in I, we performed the first principles investigations and analysis. First, the optimized lattice constants of I and II match well with the experimental results (error smaller than 2%, see Table 1), indicating that the bond length and bond angle values in these two compounds are quite reasonable. Notably, the calculated error of I is relatively smaller. In addition, there is no imaginary mode in the phonon dispersion curve of I but an imaginary value is in II, suggesting that I is a stable phase and II is a metastable phase (Figure 3).37 As stated before, the powder XRD spectra are well match with standard spectra of I. Therefore, both theoretical and experimental results demonstrate that our refined crystal structure of I is rational and credible. Table 1. The optimized lattice constants and volume of I and II. Compou nd I

II

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exp cal error exp cal error

a (Å)

c (Å)

V (Å3)

7.093 7.080 -0.18% 7.070 7.082 0.16%

20.113 20.282 0.84% 39.952 40.501 1.3%

876.30 880.58 0.48% 1729.36 1759.57 1.7%

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Crystal Growth & Design

Figure 3. The phonon dispersion spectrum of (a) I and (b) II.

Figure 5. Crystal structure of (a) calcite and (b) I, the stacking plane of π-conjugated group are highlighted by cyan plane; Calculated refractive dispersion curve of (c) calcite and (d) I, the experimental values of calcite are listed as reference.11

Figure 4. (a) Total and partial DOS of I; the visualized electronic orbitals of (b) the lowest unoccupied state and (c) the highest occupied state The electronic band properties of I were also simulated using density functional theory. It is a wide indirect bandgap semiconductor (Figure S5) and the calculated bandgap is comparable to those of Ca3(C3N3O3)2 and Ba2Mg(C3N3O3)2.33 The electronic densities of all constituent elements are displayed in Figure 4a. In deep valence region below -10 eV, Ba 6s, C 2s/2p, N 2s, O 2s electrons make the dominated contribution to electronic configuration. Ba 6s orbitals localized at -25 eV, and exhibit no hybridization to adjacent atoms. In comparison, the valence electrons of C atoms strongly interacted with those of N and O atoms in (C3N3O3)3groups, thus forming the localized σ-bond. In the upper region ranged from -10 to -2.5 eV, the parallel C 2p, N 2p and O 2p electrons construct to delocalized π-bond through pπ-pπ interaction. At the valence band maximum, the electronic states are mainly composed of nonbonding 2p electrons of N and O atoms (Figure 4c). Moreover, the conduction band bottom mainly consists of unoccupied anti π-bonds formed by C 2p, N 2p and O 2p orbitals (Figure 4b). It is clear that the bandgap in metal cyanurates can be furtherly enlarged when the dangling states at N and O atoms are eliminated by forming the strong covalent bonds with Al, Be, and B atoms.38-40

As comparatively shown in Figure 5a and 5b, I is constructed by planar π-conjugated (C3N3O3)3- groups and alkaline-earth Ba2+ cations, which is very similar to the structure of commercial birefringent crystal calcite.11 Therefore, it is awaited to show large optical anisotropy. I is a negative uniaxial crystal with no > ne. The ordinary light principle plane locates in the crystallographic ab plane while the extraordinary light principle axis points along the crystallographic c direction. As we known, the refractive index n of a crystal is determined by first-order polarizability α, which is strongly corresponding to the electronic configuration of building blocks. In calcite and I, the electronic polarizability along the c axis is determined by the Ca-O (Ba-O and Ba-N) ionic bonds, which are inflexible to applied light field. Therefore, the ne values of calcite and I are comparable. In contrast, the electronic polarizability along a axis is determined by the conjugated (CO3)2- and (C3N3O3)3π-bonds, which is very flexible to the extra light field. Notably, there is no evident bond length difference in (CO3)2and (C3N3O3)3- group, suggesting a comparable strength of conjugated π-bond. However, the π-bond configuration of (CO3)2- group is 𝜋44, in which the π-electron number is smaller than that of (C3N3O3)3- group with π-bond configuration 𝜋99. Considering that the a lattice constant of calcite and I is 4.991 Å and 7.093 Å, their average π-electron concentration in ab plane can be estimated to be 0.185 and 0.206 per square angstrom, respectively. Therefore, the no of I is much larger than that of calcite. The birefringence Δn is defined as the difference between no and ne. As a result, the calculated birefringence value of I is twice larger than that of calcite (0.32 v.s. 0.16 at 800 nm) (Figure 5c and 5d). The experimental refractive indices of calcite are also plotted,11 which is well consistent with our calculated values, thus suggesting our calculated methods and parameters are accurate enough. In view of practical applications, I is favorable and convenient because it can reach same optical path difference only in halved crystals. Therefore, I is a promising UV birefringent crystal in polarization devices such as optical splitters, actuator, and electro-optics Q

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switch. The large crystal growth and refractive index measurements are still ongoing. In conclusion, we synthesized a new phase of metal cyanurate Ba3(C3N3O3)2 with R3 space group. Its stability, electronic band structure and anisotropic optical properties were calculated based on the first principles calculations. Moreover, its vibrational and transparency properties were also identified by spectrum measurements. Benefitting from its large birefringence, wide transparency and high symmetry, Ba3(C3N3O3)2 is proposed as a potential UV birefringent crystal. This work provides new structural prototype and synthesis strategy for designing practical UV birefringent materials.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Experimental synthesis, computational methods and additional tables and figures

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]

Author Contributions #These

authors contributed equally.

Notes

The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was financially supported by the Natural Science Foundation of China (51502307, 51772304, 91622118 and 91622124), the Institute of Chemical Materials, China Academy of Engineering Physics (32203), the Beijing Natural Science Foundation (2182080), the National Key R&D Program of China (2016YFB0402103), and the Youth Innovation Promotion Association of the Chinese Academy of Sciences (2018035).

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For Table of Contents Use Only

Ba3(C3N3O3)2: A New Phase of Barium Cyanurate Containing Parallel

π-Conjugated

Groups

as

Birefringent

Material

Replacement for Calcite Jian Tang†,#, Fei Liang‡,#, Xianghe Meng‡, Kaijin Kang†, Wenlong Yin,†,*, Tixian Zeng§, Mingjun Xia‡,*, Zheshuai Lin‡, Jiyong Yao‡, Guochun Zhang‡, and Bin Kang†

2

The cyanurate Ba3(C3N3O3)2 features a layered structure stacked from the ∞[𝐵𝑎(𝐶3𝑁3𝑂3)2]2 ― double layers along c direction. It exhibits large birefringence of Δn = 0.32 at 800 nm, two times than that of the ultraviolet benchmark birefringent crystal calcite.

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