Large optical anisotropy-oriented construction of a carbonate-nitrate chloride compound as a potential ultraviolet birefringent material

The design of new birefringent materials is very significant owing to their indispensable role in modulating the polarization of light and is vital in laser technology. Herein, by applying a large optical anisotropy-oriented construction induced by a synergy effect of multiple anionic groups, a promising carbonate-nitrate chloride, Na3Rb6(CO3)3(NO3)2Cl·(H2O)6, has been designed and synthesized successfully by the solvent evaporation method and single crystals of centimeter size were obtained by the recrystallization method in aqueous solution. It crystallizes in the hexagonal P63/mcm space group, the RbO9Cl polyhedra and the NaO7 polyhedra construct a three-dimensional (3D) framework by sharing O or Cl atoms and trigonal plane units (CO3 and NO3). The transmittance spectrum based on a 1 mm thick single-crystal plate shows that its short UV cut-off edge is about 231 nm. And the refractive index differences (0.14 @ 546 nm) measured by using a polarizing microscope on the (101) crystal plane, proves that Na3Rb6(CO3)3(NO3)2Cl·(H2O)6 has a large birefringence, which has potential application in the solar blind ultraviolet region. The theoretical calculations reveal that the π-conjugated CO3 and NO3 groups are the main cause of the birefringence. It demonstrates that combining π-conjugated CO3 and NO3 groups in one structure is an extremely effective strategy to explore new UV birefringent crystals.


Introduction
Exploring new birefringent materials is a current academic and technological hotspot due to their applications in polarimetry, optical communication, and scientic instrumentation. Hitherto, several birefringent crystals have been commercialized, such as MgF 2 , 1 a-BaB 2 O 4 , 2 CaCO 3 , 3 and so on, but some drawbacks hinder their practical application in the ultraviolet (UV) including the solar blind ultraviolet region (200-280 nm) or the deep-ultraviolet (DUV, #200 nm) region. For example, although a natural calcite crystal (CaCO 3 ) with large birefringence (0.17 @ 633 nm) is the most widely used birefringent material for a prism polarizer in the UV to the near-infrared spectral range, its poor optical homogeneity and cleavability limit its application. Thus, designing new UV birefringent crystals is important for scientic and technological research.
Birefringence is the key parameter for birefringent crystals with non-cubic crystal structures, which is generally quanti-ed as the maximum difference between refractive indices exhibited by the material. 4 Generally, the metal cations and the types of anionic groups play an important role in the magnitude of birefringence. A metal cation centered polyhedron (Pb, Sn, Bi, etc.) [5][6][7][8][9][10][11][12] with a stereochemically active lone pair is benecial to the generation of high-polarizability anisotropy, which would enhance the optical anisotropy and birefringence of crystals, but they are oen impracticable in designing new UV/DUV birefringent crystals since the cut-off edge may be redshied with these cations. Alkali metal or alkaline-earth metal cations are usually chosen to ensure wide transparency in the UV region, but they generally contribute less to the polarizability anisotropy. [13][14][15][16][17][18][19][20][21][22] Thus, the contribution of anionic groups that are active for birefringence would be crucial in exploring new UV birefringent crystals. Non-pconjugated anionic groups, i.e., PO 4 or BO 4 , exhibit a large bandgap, while their relatively small polarizability anisotropy imparts a small birefringence. And it is found that the introduction of uorine in the tetrahedron can enhance the birefringence or optimize the dispersion of refractive indices, TO 4−x F x (T = B, x = 1-4; T = P, x = 1-2; T = S, and x = 1). [23][24][25][26][27][28][29][30][31][32][33][34][35] Planar p-conjugated anionic groups, such as BO 2 , BO 3 , B 3 O 6 , C 3 N 3 O 3 , CO 3 , NO 3 , etc., 35-44 exhibit strong polarizability anisotropy, which are regarded as the preferred functional units to induce large birefringence. Borates with relatively wide transparency and large birefringence have been studied systematically in recent decades. Among borate birefringent materials, the commercial birefringent material of a-BaB 2 O 4 shows relatively large birefringence (Dn = 0.12 @ 546 nm) 45 due to large polarizability anisotropy of B 3 O 6 anionic groups. Simultaneously, as birefringent crystals, carbonates have emerged in recent years, but nitrates have rarely been reported because they always show water solubility and deliquescence. However, in comparison with BO 3 and CO 3 anionic groups, NO 3 has higher microscopic polarizability anisotropy (Table S5 †), and it is easier to obtain large size single crystals, which are useful to further study other properties.
Based on these aspects, our strategy is to introduce multi anionic groups such as CO 3 51 In this work, we combine two types of p-conjugated anionic groups and a halogen anion in one compound. And Na 3 Rb 6 (CO 3 6 , the rst carbonate-nitrate chloride has been successfully synthesized by the solvent evaporation method. And the crystal growth was carried out by the recrystallization method, and the single crystals of centimeter size were obtained. Herein, the details of synthesis and crystal growth, the IR spectrum, thermal behavior, the UV-vis-NIR diffuse-reectance spectrum, the transmittance spectrum and the refractive index difference of Na 3 -Rb 6 (CO 3 ) 3 (NO 3 ) 2 Cl$(H 2 O) 6 are reported. Simultaneously, we analyzed the inuence mechanism of multiple anionic units by rst-principles calculations. The real space atom-cutting method and electron density difference map were employed to analyze the origin of the large birefringence. The experimental and theoretical results show that Na 3 Rb 6 (CO 3 ) 3 (NO 3 ) 2 Cl$(H 2 O) 6 is a potential UV birefringent crystal with a large birefringence and short UV cut-off edge.

Experimental
Synthesis of the single crystal Na 3 Rb 6 (CO 3 ) 3 (NO 3 ) 2 Cl$(H 2 O) 6 was synthesized by the solvent evaporation method through reacting NaHCO 3 (10 mmol, 0.8401 g), RbNO 3 (10 mmol, 1.4747 g), RbCl (10 mmol, 1.2092 g) and deionized water. The mixture was stirred at room temperature and the solution turned colorless and transparent, and then the solution was evaporated at room temperature. Finally, transparent and block crystals grew out in 3 days. As shown in Fig. S1, † single crystals of centimeter size were obtained by the recrystallization method in aqueous solution in 14 days.

Characterization
A coreless single crystal with suitable size of the title compound was selected for single crystal X-ray diffraction. All diffraction data were collected on a Bruker D8 Venture with Mo Ka (l = 0.71073Å) at 298(2) K. The intensity, reduction, and cell renement investigations were carried out on a Bruker SAINT. 52 All the structures were solved by a direct method and rened through the full-matrix least-squares tting on F 2 with SHELX 53 and OLEX2 54 soware. PLATON 55 was used to conrm the higher symmetry. Crystallographic data and further details for structural analyses are listed in Table 1, and the selected bond distances are summarized in Table S2. † Powder X-ray diffraction (PXRD) data were collected by putting the powder sample onto at sample holders utilizing a Bruker D2 Phaser X-ray diffractometer equipped with Cu-Ka radiation (l = 1.54056Å) and the diffraction patterns were taken in the range from 10 to 70°(2 theta). The powder XRD patterns of pure polycrystalline samples exhibit good consistency with the calculated XRD ones. The calculated XRD patterns were produced by the Mercury v3.8 program and their single-crystal structure data. Polycrystalline samples used for thermogravimetric (TG) analysis and differential scanning calorimetry (DSC) were ground from bulk crystals directly. Infrared spectroscopy was carried out on a Shimadzu IR Affinity-1 Fourier transform IR spectrometer with a resolution of 2 cm −1 in the 500 to 4000 cm −1 range. The polycrystalline samples were mixed thoroughly with dried KBr with a mass ratio of about 1 (polycrystalline sample) : 100 (KBr). A Shimadzu Solid Spec-3700 DUV spectrophotometer was used to collect the UV-vis-NIR diffuse-reectance data for the title compounds. The spectrophotometer worked ranging from 175 to 2600 nm at room temperature. The transmittance spectrum was measured using a transparent crystal plate with 1 mm thickness in the 175-1600 nm wavelength range. Absorption data (K/S) were worked out from the following Kubelka-Munk 56 function: in which K is the absorption, R is the reectance, and S is the scattering. Extrapolating the linear part of the sloping upward curve to zero in the (K/S)-versus-E plot yields the appearance of absorption. The refractive index difference of the title compound was characterized by using a polarization microscope equipped (ZEISS Axio Scope. 5 pol) with a Berek compensator. The average wavelength of the light source was 546 nm. The formula for calculating the birefringence is listed below, Here, R represents the optical path difference; N g , N p and Dn mean the refractive index of fast light, slow light, and the difference value of the refractive index, respectively; d denotes the thickness of the crystal.

Theoretical calculations
The electronic structure was calculated using density functional theory (DFT) performed using the plane wave pseudopotential implemented in the CASTEP package. 57 Using the normconserving pseudopotential (NCP), 58 the explicitly treated valence electrons for each atom were calculated as follows: Na, 3s 1 ; Rb, 5s 1 ; C, 2s 2 2p 2 ; N, 2s 2 2p 3 ; H, 1s 1 , O, 2s 2 2p 4 , and Cl, 3s 2 3p 5 . The exchange-correlation functional was the Perdew-Burke-Ernzerhof (PBE) functional within the generalized gradient approximation (GGA). 59,60 The TS method was used for the DFT-D correction. 61 The plane-wave energy cutoff was set at 750.0 eV. The separation of the k-point was set as 0.03Å in the Brillouin zone. The number of empty bands were set as 3 times of valence bands for the calculation of the optical properties. Because the GGA method always underestimates the bandgap, the scissors operators were utilized to shi the conduction bands so that they agree with the experimental bandgap values, and then the refractive indices were obtained by the real part of the dielectric function on the base of the Kramers-Kronig transform. The HOMO-LUMO energy gap and polarizability anisotropy (d) of anionic groups were calculated using DFT implemented by the Gaussian09 package. 62 The B3LYP (Becke, three-parameter, Lee-Yang-Parr) exchange-correlation functional with the Lee-Yang-Parr correlation functional at the 6-31G basis set in Gaussian was employed.

Results and discussion
Crystal structure The crystallographic analysis reveals that Na 3 Rb 6 (CO 3 ) 3 (NO 3 ) 2 -Cl$(H 2 O) 6 (CCDC 2181425) crystallizes in the space group P6 3 / mcm (no. 193) of the hexagonal crystal system. There is one independent Na (6g), one independent Rb (12k), two independent C (4d and 2a), one independent N (4c), four independent O (12i, 12j, 12k, and 6g), one independent Cl (2b) and one independent H (24l) wyckoff sites (Table S1 † pentagonal bipyramid and the Rb atoms are coordinated by nine O atoms and one Cl atom to form an RO 9 Cl polyhedron. The Rb-centered polyhedron and the NaO 7 polyhedron construct a three-dimensional (3D) framework by sharing O or Cl atoms with each other. As shown in Fig. 1c, there are two types (9-membered ring channel (type A) and 6-membered ring channel (type B)) of channels in the 3D framework. In the framework, C(2)O 3 Fig. 1d shows the total structure of title compounds in the bc plane. As shown in Fig. 1e, the photograph of an as-grown Na 3 -Rb 6 (CO 3 ) 3 (NO 3 ) 2 Cl$(H 2 O) 6 crystal is consistent with the theoretical crystal morphology (Fig. S6 †), and the crystal orientation is determined by using an X-ray crystal orientation instrument.

Optical properties and birefringence
In order to further conrm the coordination environments of anionic groups in the structure, IR spectroscopic measurements were carried out (see Fig. S2b Fig. S2a. † Its UV cut-off absorption edge is at about 220 nm, which demonstrates that the title compound is a UV optical material. To further conrm the UV cut-off edge, the crystal was grown for the UV-vis-NIR transmittance spectrum measurement, and the result based on a 1 mm thick preliminary polishing single-crystal plate shows that it has a UV cut-off edge of 231 nm. It is well-known that the planar triangular groups of CO 3 and NO 3 have large polarizability anisotropy. Therefore, the crystal may show a large birefringence when the CO 3 and NO 3 groups are stacked parallelly to form a perfect layered structure. The calculated linear optical results of Na 3 Rb 6 (CO 3 ) 3 (NO 3 ) 2 -Cl$(H 2 O) 6 show that n x = n y > n z , that is, n o > n e , indicating that the title compound is a negative uniaxial optical crystal. The interference pattern of polarized light indicates that Na 3 Rb 6 (-CO 3 ) 3 (NO 3 ) 2 Cl$(H 2 O) 6 is a uniaxial crystal (Fig. 2a). According to the calculated refractive index dispersion curves, its birefringence is 0.12 at 546 nm (Fig. 2d), which is comparable to that of a-BBO. The refractive index difference was measured under the polarizing microscope method, using (101) crystal planes. The crystal thickness of the title compound is 25.655 mm. And the optical path differences at 546 nm are 12.75 and 12.95 mm. According to the formula, the refractive index differences on the (101) crystal planes can be calculated to be 0.14 @ 546 nm, which indicates that Na 3 Rb 6 (CO 3 ) 3 (NO 3 ) 2 Cl$(H 2 O) 6 may have a birefringence larger than 0.14 @ 546 nm.

Electronic structure and mechanism of birefringence
In order to get more insight into the interaction between the microstructure and optical properties for Na 3 Rb 6 (CO 3 (Fig. 3b), which is similar to the hydrogen bond in paracetamol. 63 At the bottom of the CB, several separate electronic states of N-2p, O-2p (specically, O(4)-2p, Fig. 3b) and Na-p appear, among which the interaction between N-2p and O(4)-2p means a NO 3 group. These separate electronic structures at the bottom of the CB were also reported in other nitrates. 64,65 Therefore, the bandgap is mainly determined by the hydrogen bond, Na-p, N-p and O-p.
In order to identify the origin of the large birefringence, the real space atom-cutting (RSAC) 66 method and electron density difference map 67 are employed. From the calculated electron density difference map of CO 3 and NO 3 (Fig. 3c and d), we can see that there are obvious covalent characteristics between the C-O or N-O bonds. As shown in Table S6, † the main contribution comes from the CO 3 and NO 3 anionic units. To evaluate the contribution of hydrogen bonds to birefringence, H 2 O was cut off from the structure, and the calculated birefringence is also unchanged (Table S6 †). In this case, the calculated birefringence is 0.12 @ 546 nm. Thus, we speculate that hydrogen bonds do not contribute signicantly to the birefringence of the title compound. Based on the calculated results which are listed in Table S6, † the planar p-conjugated CO 3 and NO 3 anionic units play a major role in the large birefringence.

Conclusions
In conclusion, the rst carbonate-nitrate chloride, Na 3 Rb 6 (-CO 3 ) 3 (NO 3 ) 2 Cl$(H 2 O) 6 , with two types of p-conjugated anionic groups has been characterized as a new UV birefringent crystal with large birefringence and a short UV cut-off edge. For Na 3 -Rb 6 (CO 3 ) 3 (NO 3 ) 2 Cl$(H 2 O) 6 , the RbO 9 Cl and NaO 7 polyhedra construct a three-dimensional framework by sharing O or Cl atoms, and the planar p-conjugated groups (CO 3 and NO 3 ) reside in a 9-membered ring channel and 6-membered ring channel, respectively in the 3D framework. And the hydrogen bond interaction between CO 3 and H 2 O will further stabilize the structure. We have obtained single crystals of centimeter size by a simple and environmentally friendly aqueous solution method. Simultaneously, we analyze the inuence mechanism of multiple anionic units by rst-principles calculations. The result shows that the planar p-conjugated CO 3 and NO 3 anionic units play a major role in the large birefringence. The experimental refractive index difference is 0.14 @ 546 nm on the (101) crystal plane, which is comparable to that of the a-BBO (0.12 @ 546 nm) crystal. The synthesis and investigation of other new alkaline-metal carbonate-nitrate compounds with excellent properties are still in progress.

Data availability
All of the related experimental and computational data are provided in the ESI. †

Author contributions
M. C. and S. L. P. designed the research study; M. C. synthesized the compound and performed the experiments. W. Q. J. and Z. H. Y. performed the optical theoretical calculations. All authors wrote and revised the manuscript. All the authors contributed to the nal manuscript preparation.