Rb3Na(H2C3N3O3)4·3H2O with Large Birefringence.

A mixed alkali-metal nonlinear optical (NLO) dihydro-cyanurate crystal Rb3Na(H2C3N3O3)4·3H2O has been synthesized via the hydrothermal method. Its calculated birefringence is about 0.368, which is very large, its ultraviolet (UV) cutoff edge is down to 230 nm, and the powder second harmonic generation (SHG) intensity is about 0.2 × KDP. In addition, a first-principles investigation of the electronic properties on Rb3Na(H2C3N3O3)4·3H2O was carried out. The calculated band gap and SHG coefficient values agree well with the experimental ones. These results suggest that it could be applied as a UV birefringent material.

Introduction

Birefringence is one of the standards for the materials in which a beam of light propagating in an anisotropic crystal splits into two beams with different polarization directions and refractive indices.[1−4] The materials with suitable birefringence have been explored continuously.[5−16] Recently, numerous scientific endeavors have been made to find new birefringent materials with good nonlinear optical (NLO) performance,[17−21] and these studies suggest that the molecular construction by using large π-conjugated anionic groups like (BO3)3–, (B3O6)3–, (NO2)−, (CO3)2–, (NO3)−, (HCOO)−, and (C3N3O3)3– would be a very efficient way.[22−28] By exploiting these groups, a number of excellent crystals Ca3(BO3)2,[29] Ba2Ca(B3O6)2,[30] KSrCO3F,[31] Ba2NO3(OH)3,[32] K3(COOH)3[B(OH)3]2,[33] Ba3(C3N3O3)2,[34] etc. have been designed and synthesized.

It is well-known that the large π-conjugated (B3O6)3– group is a source of excellent NLO properties in materials like the famous crystal β-BaB2O4 (BBO).[35,36] Owing to structural similarity, the cyanurate group (C3N3O3)3– is a good candidate for the substation of the (B3O6)3– group. In addition, hydro-isocyanurate (HC3N3O3)2–, dihydro-isocyanurate (H2C3N3O3)− were synthesized via a more easy crystal synthesis method compared to none hydrogen bonding cyanurates.[37,38] Ye’s group reported KLi(HC3N3O3)·2H2O with a large second harmonic generation (SHG) response that was 5.3 times that of KH2PO4 (KDP), and Lin’s group calculated the birefringence of Cs3Na(H2C3N3O3)4·3H2O with the value of 0.29 in ab principle plane at 514 nm.[39,40] During our manuscript preparation, five compounds in the isohydro- and dihydro-cyanurates family including the title compound have been reported.[41]

In our article, the single crystal of <span class="Chemical">n class="Chemical">Rb3Na(H2C3N3O3)4·<spn>an class="Chemical">3H2O was obtained by a mild-temperature hydrothermal method in a short synthesis period. We reported the crystal structure and measured the powder X-ray diffractometry (XRD), IR spectrum, UV–vis–NIR spectrum, thermal analysis, SHG properties, and ab initio theoretical calculations of the title compound.

Experimental and Computational Methods

Synthesis

n class="Chemical">n class="Chemical">Rb3Na(H2C3N3O3)4·<spn>an class="Chemical">3H2O was prepared via hydrothermal reaction by using RbF (1.04 g, 10 mmol), H3C3N3O3 (1.29 g, 10 mmol), and NaOH (0.40 g, 10 mmol). These reactants were transferred into 120 mL autoclave equipped with teflon liners (5 mL of deionized water was added) for 16 h at 190 °C, and the mixtures were cooled slowly down to room temperature. The large quantities of crystalline bulk crystals were obtained, and the yield of Rb3Na(H2C3N3O3)4·3H2O was about 67% based on Rb.

Structure Determination

Single-crystal XRD data were collected on a Bruker SMART APEX II CCD detector equipped with <span class="Chemical">graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) and integrated with the SAINT.[42] The crystal structure was identified with programs from SHELXTL-97.[43] Final least-squares refinement on Fo2 with data having Fo2 ≥ 2σ (Fo2) includes anisotropic displacement parameters for non<span class="Chemical">hydrogen atoms. The structure was inspected for missing symmetry elements with PLATON.[44] During the refinement process, we found that Na(1) and Na(1B) were in positional disorder in the structure of the compound. We restricted the sum of the ratio of Na(1) and Na(1B) as 1 and refined them by the least-square method, which can help to get better R values. The crystal data and structure refinement information are summarized in Table S1. Final atomic coordinates and equivalent isotropic displacement parameters, selected bond lengths, and angles are listed in Tables S2 and S3 in the Supporting Information (SI), respectively.

Powder XRD was performed on a Bruker D2 PHASER X-ray diffractometer. The 2θ range of 10–70° with a scan step width of 0.02° and a fixed counting time of 0.1 s/step. The experimental and calculated powder XRD patterns of the compound are presented in Figure S2.

Thermal Analysis

The thermogravimetry/differential thermal analysis (TG–DSC) curves for the n class="Chemical">n class="Chemical">Rb3Na(H2C3N3O3)4·<spn>an class="Chemical">3H2O crystal were recorded with a NETZSCH STA 449C simultaneous thermal analyzer. The sample was placed in an Al2O3 crucible and heated at a rate of 10 °C/min in the range of 25–800 °C under inflowing nitrogen gas.

IR Spectrum Measurement

The IR spectra of Rb3Na(H2C3N3O3)4·3H2O was measured on a Shimadzu IRAffinity-1 spectrometer, which was mixed with dried KBr. The IR spectra were collected in the range of 400–4000 cm–1 with a resolution of 2 cm–1.

UV–vis–NIR Diffuse Reflectance Measurement

The UV–vis–NIR diffuse reflectance spectra of <span class="Chemical">Rb3Na(H2C3N3O3)4·<span class="Chemical">3H2O were measured using a Shimadzu Solid Spec-3700 DUV spectrophotometer with the measurement range extending from 200 to 1400 nm at room temperature.

Theoretical Calculation Methods

Density functional theory based on electronic structure calculations were carried out with the total energy mode of CASETEP package from the Materials Studio 5.5.[45] The exchange and correlation effects were implemented via the Perdew–Burke–Ernzerhof functional[46] with the generalized gradient approximation (GGA).[47] The relations among the ionic cores and the valence electrons were depicted by norm-conserving pseudopotentials.[48] The valence electrons of title compound were calculated as Rb 4s24p65s1, Na 2s22p63s1, C 2s22p2, N 2s22p3, O 2s22p4, and H 1s1. The number of plane waves included in the basis sets was determined by the cutoff energy of 750 eV for <span class="Chemical">Rb3Na(H2C3N3O3)4·<span class="Chemical">3H2O. The numerical integration of the Brillouin zone was performed using the Monkhorst–Pack 24 k-point grids of 3 × 3 × 3. The other parameters and convergent criteria were the same as the default values of the CASTEP program.

Powder Second Harmonic Generation Measurement

The SHG measurement was investigated via the Kurtz–Perry method.[49] The 1064 nm irradiation was produced via a Q-switched Nd: YAG laser. The sample was ground and sieved into distinct particle size ranges, <38–55, 55–88, 88–105, 105–150, and 150–200 μm. For comparisons, the sieved <span class="Gene">KDP samples were categorized into the same particle size ranges.

Results and Discussion

Single-Crystal Structure

n class="Chemical">n class="Chemical">Rb3Na(H2C3N3O3)4·<spn>an class="Chemical">3H2O crystallizes in non-centrosymmetric space group Pmn21 (No. 31) with unit cell parameters: a = 16.020(7) Å, b = 6.601(3) Å, c = 11.716(5) Å and Z = 2. Rb3Na(H2C3N3O3)4·3H2O features two-dimensional layers consisting of π-conjugated (H2C3N3O3)− 6-membered rings. The Rb atoms are eight-coordinated to O/N atoms with two O atoms from H2O, while the Na atoms are six-coordinated to form the NaO6 polyhedron with two O atoms from H2O. Unfortunately, the Na atoms are split off with the ratio of 0.843:0.157, so we mainly discussed the higher one in the structure of the compound. Interestingly, the two polyhedra form two-dimensional layer resembling “√mark” along the a-axis in Figure a. The isolated (H2C3N3O3)− groups form a ∞2[H2C3N3O3]− layer along the b-axis in Figure b. The Na+ and Rb+ cations and H2O are filled between ∞2[H2C3N3O3]− layers with a certain sequence along the a-axis in Figure c. In addition, the positions of hydrogen bonds and water molecules in the whole structure of Rb3Na(H2C3N3O3)4·3H2O is given in Figure S1 in the SI. The bond lengths and angles for Rb3Na(H2C3N3O3)4·3H2O are listed in the SI.

Figure 1

(a) Coordination of the Rb and Na atoms. (b) ∞2[H2C3N3O3]− layer with H2O molecule. (c) Whole crystal structure of Rb3Na(H2C3N3O3)4·3H2O.

(a) Coordination of the Rb and Na atoms. (b) ∞2[H2C3N3O3]− layer with <spn>an class="Chemical">H2O molecule. (c) Whole crystal structure of Rb3Na(H2C3N3O3)4·3H2O.

The experimental powder XRD pattern of the sample is in accordance with the theoretical one inferred from the single-crystal data (cif), implying that the obtained phase is pure (Figure S2). In addition, the distribution of Rb, Na, C, N, and O elements was certified by energy-dispersive X-ray spectroscopy (Figure S3).

IR Spectrum

The IR spectrum of Rb3Na(H2C3N3O3)4·3H2O is shown in Figure S4 in the SI. The peaks around 2700–3500 cm–1 can confirm the presence of O–H and N–H bonds. The absorption bands observed at 1706, 1579, and 1466 cm–1 are characteristics of the (H2C3N3O3)− group. The C–N, N–H, and C–O bands are located at 1380, 1250, and 1074 cm–1, respectively. The peak at 883 cm–1 may be due to the asymmetrical stretching vibration of the C3N3 groups. The bands at 786 and 692 cm–1 are assigned to the N–C–O and and the peak at 554 cm–1 belongs to the C=O bands.[50,51]

The UV–vis–IR reflectance spectra of <span class="Chemical">Rb3Na(H2C3N3O3)4·<span class="Chemical">3H2O in the range of 200–1400 nm are shown in Figure . Evidently, its cutoff edge is down to 230 nm, with the reflectance of 12% at 240 nm, and the energy band gap is about 4.35 eV.

Figure 2

UV–vis–NIR diffuse reflectance spectrum for Rb3Na(H2C3N3O3)4·3H2O.

UV–vis–NIR diffuse reflectance spectrum for <span class="Chemical">Rb3Na(H2C3N3O3)4·<span class="Chemical">3H2O.

Thermal analysis (Figure S4) indicates that n class="Chemical">n class="Chemical">Rb3Na(H2C3N3O3)4·<spn>an class="Chemical">3H2O has two main steps of mass loss from the TG curve. There are 10% mass losses in the first small stage in the temperature range of 150–200 °C for the loss of three crystal water molecules per formula unit. The second large stage in the temperature interval between 350 and 600 °C lost mass is about 40%, which is similar to the calculated value and can be assigned to the decomposition of the (H2C3N3O3)− groups.[52] DSC curve shows that the smaller peaks at 184 °C and the stronger peaks at 378 °C represent the endothermic peaks of water molecules and (H2C3N3O3)− groups, respectively.

Electronic Structures

To better understand the relationship between the structure and property, the first-principle calculations (density functional theory) were carried out by using the CASTEP package. Rb3Na(H2C3N3O3)4·3H2O shows an indirect band gap of 4.54 eV by using the GGA method (Figure a). The partial density of states are drawn in Figure b. From Figure b, we can note that valence band (VB) is mainly composed of Rb sp states, C sp states, H s states, O sp states, and N sp states. The conduction band (CB) mainly consists of Na sp states, C sp states O sp states, and N sp states. The top of the VB is composed of O p states and N sp states and the bottom of CB mainly consists of C sp states N sp states and O sp states, which manifests that the (H2C3N3O3)− groups determine the electronic structure and optical properties.

Figure 3

(a) Electronic band structure. (b) Partial density of states of Rb3Na(H2C3N3O3)4·3H2O.

(a) Electronic band structure. (b) Partial density of states of <span class="Chemical">n class="Chemical">Rb3Na(H2C3N3O3)4·<spn>an class="Chemical">3H2O.

Linear Optical Properties

n class="Chemical">n class="Chemical">Rb3Na(H2C3N3O3)4·<spn>an class="Chemical">3H2O has a large birefringence of 0.368 at 1064 nm (Figure a). The largest reflective index is in the (0 0 1) direction and the (H2C3N3O3)− groups is in parallel to the light incident direction in the unit cell (Figure b). Thus, the largest contribution to birefringence comes from the (H2C3N3O3)− groups. The birefringence was further investigated by using visualized total electron density and electron localized function map (Figure c). It can be inferred that (H2C3N3O3)− holds high electron densities owing to strong covalent C–N and C–O bond. A nonspherical symmetric density distribution is clearly presented around each N (no hydrogenation) and O atom, which is composed of umbrella-shaped N 2s lone pairs and O nonbonding 2p electrons. Six parallel pπ electrons are uniformly distributed at the C3N3 ring, thus corresponding to a π-conjugated group; the π-conjugated groups produce strong anisotropy to be conducive to birefringence. We also calculated the bond population of Rb3Na(H2C3N3O3)4·3H2O, and the result is presented in Table S4. It can be found that the bond population of the H–O, C–O, C–N, and N–H groups are in the range of 0.98–0.99, 0.6–1.06, 0.78–0.98, and 0.69–0.72, respectively. It indicates that the H–O, C–O, C–N, and N–H groups display strong covalent property. As is known to all, the covalent property is beneficial for optical anisotropy and generating large birefringence for the compounds. In conclusion, the large birefringence of Rb3Na(H2C3N3O3)4·3H2O is mainly triggered by (H2C3N3O3)− π-conjugated groups. In addition, the phase-matching wavelength of Rb3Na(H2C3N3O3)4·3H2O was calculated (Figure d). It is remarkable that the calculated type I shortest PM wavelength is about 402 nm.

Figure 4

(a) Birefringence of Rb3Na(H2C3N3O3)4·3H2O. (b) (0 0 1) Direction of Rb3Na(H2C3N3O3)4·3H2O in the unit cell. (c) Electron localization function map of Rb3Na(H2C3N3O3)4·3H2O. (d) Birefringence and accordingly predicted shortest PM wavelength.

(a) Birefringence of n class="Chemical">n class="Chemical">Rb3Na(H2C3N3O3)4·<spn>an class="Chemical">3H2O. (b) (0 0 1) Direction of Rb3Na(H2C3N3O3)4·3H2O in the unit cell. (c) Electron localization function map of Rb3Na(H2C3N3O3)4·3H2O. (d) Birefringence and accordingly predicted shortest PM wavelength.

NLO Properties

n class="Chemical">n class="Chemical">Rb3Na(H2C3N3O3)4·<spn>an class="Chemical">3H2O crystallizes into a non-centrosymmetric space group (Pmn21, No. 31). The SHG intensity of Rb3Na(H2C3N3O3)4·3H2O goes up with the increasing particle size and exhibits the final SHG intensity of about 0.2 × KDP, which indicate that the title compound is phase matchable.

In addition, the SHG coefficients (dij) of n class="Chemical">n class="Chemical">Rb3Na(H2C3N3O3)4·<spn>an class="Chemical">3H2O were computed to further understand the NLO properties. The space group Pmn21 belongs to the class mm2 point group and possesses 2 nonzero independent SHG coefficients (d15 = −0.0675 pm/V, d33 = 0.136 pm/V) under the restriction of Kleinman symmetry. The theoretically estimated value is in good agreement with the experimental one in Figure S6 in the SI.

To further investigate the origin of the SHG effects in n class="Chemical">n class="Chemical">Rb3Na(H2C3N3O3)4·<spn>an class="Chemical">3H2O, the donation of individual atoms in virtual-electron (VE) transition process was computed using the SHG-density method, and the SHG-density of VE occupied (veocc) and unoccupied (veunocc) states are shown in Figure a,b, respectively. For the occupied state, O p and N p orbitals make a significant contribution to the SHG effect. In unoccupied states, the O p, N p, and C p orbitals make the main contribution. These results indicate that the C–O groups and the (H2C3N3O3)− π-conjugated groups are the main contributors for Rb3Na(H2C3N3O3)4·3H2O with large SHG response.

Figure 5

(a) VE occupied and (b) VE unoccupied orbitals of Rb3Na(H2C3N3O3)4·3H2O.

(a) VE occupied and (b) VE unoccupied orbitals of <span class="Chemical">Rb3Na(H2C3N3O3)4·<span class="Chemical">3H2O.

Conclusions

In summary, the single crystal of n class="Chemical">n class="Chemical">Rb3Na(H2C3N3O3)4·<spn>an class="Chemical">3H2O was obtained with the hydrothermal method in a short synthesis period. The calculated birefringence of Rb3Na(H2C3N3O3)4·3H2O is about 0.368, its ultraviolet (UV) cutoff edge is down to 230 nm, the SHG intensity is about 0.2 × KDP, and phase matchable. We believe that this work could contribute to the development of the cyanurates and hydro-isocyanurates.