Rb3SbF3(NO3)3: an excellent antimony nitrate nonlinear optical material with a strong second harmonic generation response fabricated by a rational multi-component design

Lei Wang a, Fei Yang a, Xiaoyu Zhao b, Ling Huang a, Daojiang Gao a, Jian Bi *a, Xin Wang *b and Guohong Zou *b
aCollege of Chemistry and Materials Science, Sichuan Normal University, Chengdu, 610068, P. R. China. E-mail: bijian686@126.com
bCollege of Chemistry, Sichuan University, Chengdu, 610064, P. R. China. E-mail: wangxin@scu.edu.cn; zough@scu.edu.cn

Received 8th August 2019 , Accepted 12th September 2019

First published on 12th September 2019


A novel antimony fluoride nitrate, Rb3SbF3(NO3)3, fabricated by a rational multi-component design, has been successfully synthesized by a rapid evaporation concentration method. This title compound crystallizes in a polar noncentrosymmetric (NCS) space group (P21) and presents a complicated three-dimensional (3D) network composed of SbO3F3, Rb-centered distorted polyhedra and π-conjugated planar triangular NO3 groups. Rb3SbF3(NO3)3 is a potential UV nonlinear optical material due to its large phase-matchable second harmonic generation (SHG) response of approximately 2.2 times that of KH2PO4 (KDP) and a wide band gap of 3.75 eV. Theoretical calculation analysis was used to rationalize the large SHG effect, which originates from the multi-component synergistic effect including moderately polarizable Rb+ cations, a stereochemically active lone pair effect of Sb3+ and NO3 groups with a π-conjugated planar triangular structure.


Introduction

Ultraviolet (UV) nonlinear optical (NLO) materials have drawn great attention due to their potential uses in industrial applications, such as semiconductor inspection and laser direct imaging.1–5 Among all-solid-state UV lasers, there are not many lasers that can meet industrial needs due to the limit of their output wavelength which could be extended through frequency conversion using NLO materials. Hence, the exploration of superior performing UV NLO materials, the critical factor for UV solid state lasers, is urgently needed. On account of rigorous prerequisites for applications, an excellent UV NLO material should have a large phase-matching second harmonic generation (SHG) coefficient, a wide transmission region, and stable physical and chemical properties.6–10 Particularly, the NLO coefficient directly determines the conversion efficiency of lasers. Hence, it is essential to develop a universal strategy to design new UV NLO materials with strong SHG responses.

After continuous efforts made by chemists and materials scientists, several strategies for developing novel NLO materials with strong SHG effect have been proposed.11,12 Among them, introducing the NLO-active transition metal centered polyhedra resulting from the Second-Order Jahn–Teller (SOJT) distortion has been proved to be effective, for instance, TiO6, VO6, NbO6, etc.13–15 Quite a few representative compounds such as KTiOPO4 (KTP),16 BaNbO(IO3)5 (ref. 17) and A3VO(O2)2CO3 (A = K, Rb, and Cs)18–20 exhibiting strong SHG responses have been synthesized. However, the band gaps for the above compounds have been ineluctably narrowed due to the presence of d–d transitions caused by the introduction of transition metal cations, thus limiting their practical application in the UV region. Therefore, the rational selection of basic building blocks is crucial for designing new UV NLO materials with superior performance.

So far, the reported UV NLO crystals have mainly concentrated on borates such as β-BaB2O4 (BBO)21 with [B3O6]3− groups, LiB3O5 (LBO)22 with [B3O7]5− groups, SrB4O7 (ref. 23) with [BO4]5− groups and Sr2Be2B2O7 (SBBO)24 with [BO3]3− groups, and the SHG coefficients of these UV NLO borates mainly originate from the boron–oxygen anionic units.25 Particularly, the [BO3]3− anionic groups with a π-conjugated planar triangular structure have been proved to be the optimal NLO basic structural units in designing UV NLO materials because they possess a large microscopic second-order susceptibility.26 Apart from the (BO3)3− group, the (NO3) and (CO3)2− groups also possess an analogous π-conjugated planar triangular structure.27–31 As is well-known, borates have been systematically investigated and carbonates are difficult to grow. Though the [NO3] group possesses a large microscopic second-order susceptibility and mild synthetic conditions, nitrates have rarely been reported so far as NLO materials.32 Hence, there is a great chance that new UV NLO materials will be obtained in a nitrate system.

In order to obtain enhanced SHG responses, more UV NLO-active units should be introduced into the nitrate system. A recent study has indicated that apart from anion groups, some cations can not only balance the charge to stabilize the crystal framework, but also affect the overall SHG effect. Alkali and alkaline-earth metal cations with larger radii easily exhibit strong polarization which is beneficial to enhance the SHG response, for example, Cs+ in Cs3VO(O2)2CO3 (ref. 20) and Ba2+ in Ba4B11O20F.33 Also there are no d–d and f–f electron transitions in alkali or alkaline earth metals which is helpful for high transmittance in the UV region. Simultaneously, the main group element cations possessing a large ionic radius and a stereochemically active lone pair (SCALP), for instance, Sb3+, Pb2+ and Bi3+, usually exhibit multiple and asymmetric coordination modes, which are conducive to producing a large NLO coefficient response, suggesting that these cations are good candidates for UV NLO micro-structural units to design new materials.34–39 In our previous work, especially Sb3+ exhibited much superiority in producing an excellent UV NLO material CsSbF2SO4 possessing a large phase-matching NLO coefficient and a wide transparency window.40

Based on the above-mentioned ideas, a novel NCS polar rubidium antimony fluoride nitrate, Rb3SbF3(NO3)3, was fabricated by a rational multi-component design by simultaneously introducing two asymmetric UV NLO-active cation functional groups, Rb+ cations with moderate polarization and SbO3F3 distorted octahedra produced by Sb3+ cations with the SCALP effect, into the nitrate system and was successfully synthesized by a rapid evaporation concentration method. Rb3SbF3(NO3)3 exhibited a strong phase-matching SHG response of ca. 2.2 × KDP and a wide band gap (3.75 eV), suggesting that the title compound is a potential UV NLO material. Detailed theoretical calculations along with structural analysis confirmed that the synergistic effect of the three coexisting components constituting UV NLO-active units mainly contributes to the large SHG effect.

Experimental section

Synthesis

RbNO3 (99.0%, AR), and SbF3 (99.0%, AR) were used as original starting reagents. For Rb3SbF3(NO3)3, 1.00 mmol (0.179 g) SbF3 and 4.00 mmol (0.588 g) RbNO3 were added to 5 mL of deionized water with a few drops of concentrated nitrate acid inhibiting the hydrolysis of SbF3, and the mixture was stirred for 20 minutes while heating at 80 °C. The mixture was then cooled to room temperature and placed inside the refrigerator at a temperature of 4 °C until transparent colourless block crystals of Rb3SbF3(NO3)3 were obtained in one day (Fig. S1).

Single crystal X-ray diffraction

Single crystal X-ray diffraction was performed with a fully transparent block crystal picked under an optical microscope. The crystal data were collected under the graphite monochromated Mo Kα radiation (λ = 0.71073 Å) at 150 K, using a Bruker D8 Venture diffractometer. The analysis of the crystal structure was performed using SHELX-2014.41

The positions of heavy atoms were determined by the heavy atom method and the remaining atomic coordinates were obtained from Fourier difference maps. The structure of the crystal was solved by the direct method and the F2-based full-matrix least-squares plane refinement to convergence was performed on the coordinates of all atoms cast. Finally, the space group was checked by the PLATON42 and no higher space group was found. The crystallographic data of the compound and the other related data are listed in Table 1 and Table S1, respectively.

Table 1 Crystal data and structure refinement for Rb3SbF3(NO3)3
Compound Rb3SbF3(NO3)3
a R 1(F) = Σ||Fo| − |Fc||/Σ|Fo|. b wR2(Fo2) = [Σw(Fo2Fc2)2/Σw(Fo2)2]1/2.
Formula mass 621.19
Crystal system Monoclinic
Space group P21
a (Å) 8.985(1)
b (Å) 7.340(1)
c (Å) 9.530(1)
V3) 616.66(12)
Z 2
ρ(calcd) (g cm−3) 3.345
Temperature (K) 150(2)
λ (Å) 0.71073
F(000) 564
μ (mm−1) 14.08
R 1, wR2 (I > 2σ(I))a 0.0274/0.0632
R 1, wR2 (all data)b 0.0314/0.0674
GOF on F2 1.023


Powder X-ray diffraction

Powder XRD of the polycrystalline Rb3SbF3(NO3)3 was carried out using an automated Smart lab powder X-ray diffractometer, and a Cu-Kα radiation source at room temperature. The samples of Rb3SbF3(NO3)3 were ground, placed in a glass tank and compressed and then the measurements were performed in the 2θ angular range of 5°–70°, at a fixed counting time of 0.2 s per step and a scan step width of 0.08°.

Infrared spectroscopy

A Vertex 70 Fourier transform infrared (FT-IR) spectrometer was used to measure the IR spectrum of Rb3SbF3(NO3)3 at room temperature, and the range of wavenumbers was from 400 to 4000 cm−1. The ground polycrystalline compound was mixed with KBr in the ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]100.

Thermal analysis

The thermal properties of the compound Rb3SbF3(NO3)3 were measured with a Discovery TGA thermal instrument under nitrogen gas flow. The samples (about 10 mg) were heated in the range of 30–700 °C in a platinum crucible and the rate of heating and cooling was 5 °C min−1.

UV-vis diffuse reflectance spectroscopy

A UV-2600, Shimadzu spectrophotometer was used to measure the UV-vis-NIR diffuse reflectance of the grounded Rb3SbF3(NO3)3 samples, at a wavelength range of 185–800 nm at room temperature, and a BaSO4 plate was used as the standard reference.

Second-harmonic generation (SHG)

Through the Kurtz and Perry method, the powder frequency doubling experiment of the Rb3SbF3(NO3)3 samples was performed.43 A Q-switched Nd:YAG laser was used to measure the polycrystalline second-harmonic generation, and the output laser wavelength was 1064 nm, and the pulse width was 10 ns. The polycrystalline Rb3SbF3(NO3)3 samples were ground and sieved into distinct particle sizes (25–45, 45–58, 58–75, 75–106, 106–150, and 150–212 μm), and were subjected to frequency doubling tests. All experiments were completed using KH2PO4 (KDP) with the same particle sizes as the references.

Theoretical calculations

The band structures and density of states (DOS)/partial DOS (PDOS) of Rb3SbF3(NO3)3 were calculated using the CASTEP software package.44 The calculations were performed using Perdew–Burke–Ernzerhof (PBE) under generalized gradient approximation (GGA),45 Ultrasoftpseudopotentials (USP) were employed for all the atoms.46 Moreover, the kinetic energy cut-off was 500 eV, and in the Brillouin zone the k-point sampling was performed using 2 × 2 × 2. The default values of the CASTEP code were used to set the rest of the calculation parameters.

Results and discussion

Single-crystal structure

Crystallographic analysis reveals that Rb3SbF3(NO3)3 crystallizes in the noncentrosymmetric space group P21 (monoclinic) and consists of one crystallographically independent Sb atom, three Rb atoms, nine O atoms, three F atoms and three N atoms. In this compound, every Sb atom is six coordinated with three fluorine atoms and three oxygen atoms from three different NO3 anions with the Sb–O and Sb–F distances of 2.582–2.899 Å and 1.957–1.976 Å, respectively and forms the [SbF3(NO3)3]3− anion group (Fig. 1a). All the [SbF3(NO3)3]3− complexes arrange systematically in the ac plane and present two directions with seemingly centrosymmetric directions (Fig. 1c), but it can be clearly seen that the anion groups with different directions distort an angle by nearly 90° when viewed down the a axis. Three kinds of Rb+ cations locate around the [SbF3(NO3)3]3− groups and connect the dispersive anions to a three dimensional network (Fig. 1d). Rb1 and Rb3 atoms bridge with the oxygen and fluorine atoms to form the layers first, and then connect with the Rb2 atoms to form the 3-D framework, and in the meantime form the [Rb1O9F2]19−, [Rb2O8F3]18−, [Rb3O7F3]16− distorted polyhedra with the Rb–O and Rb–F bond lengths in the range of 2.916–3.541 Å and 2.785–3.001 Å, respectively (Fig. 1b).
image file: c9dt03233a-f1.tif
Fig. 1 (a) and (b) The coordination mode of the N, Sb and three Rb atoms; (c) the 3D network viewed down the b-axis; (d) the 3D framework formed by the linkage of Rb atoms.

The bond valence sums (BVS) of each atom in Rb3SbF3(NO3)3 were calculated using the following formula,

image file: c9dt03233a-t1.tif
where Sij represents the bond valence related to the bond length rij, r0 and B are empirically determined parameters, and B is usually set as 0.37.47,48 BVS for Sb3+, Rb+, F, N5+, and O2− in Rb3SbF3(NO3)3 are 2.802, 1.095–1.249, 1.043–1.250, 4.784–4.927 and 1.687–2.129, respectively (Table S1).

Powder X-ray diffraction

The purity of the synthesized compound has been confirmed by powder X-ray diffraction. It is observed from the XRD patterns that the sample has no impurities and the experimental pattern matches well with the calculated pattern (Fig. 2).
image file: c9dt03233a-f2.tif
Fig. 2 Experimental and calculated XRD patterns for compound Rb3SbF3(NO3)3.

Thermal properties

The thermogravimetric analysis of Rb3SbF3(NO3)3 was performed (Fig. 3). It can be seen from the diagram that the title compound can be stable up to 320 °C and undergo a weight loss between 320 °C and 400 °C. The remaining compounds after the thermogravimetric measurement were confirmed to be RbSbO3 and Sb2O3 through powder X-ray diffraction (Fig. S3).
image file: c9dt03233a-f3.tif
Fig. 3 TGA curve for Rb3SbF3(NO3)3.

Optical properties

The IR spectrum of Rb3SbF3(NO3)3 is shown in Fig. S2. The bands observed at 1384, 1046, 834 and 723 cm−1 are attributed to the asymmetric, symmetric and bending stretching vibrations of the NO3 group. The asymmetric and bending stretching of the Sb–O band show the peaks at 547 and 492 cm−1. All the vibrations are consistent with the reported compounds.29,35,40

The diffuse-reflectance spectra of Rb3SbF3(NO3)3 were recorded in the region from 185 nm to 800 nm (Fig. 4). The absorption (K/S) data were induced from the Kubelka–Munk function.49,50 It is obvious that the title compound can be considered a wide band-gap semiconductor because it exhibits a band gap of about 3.75 eV which may be attributed to fluoride anions with large electronegativity and alkali metal rubidium cations without d–d and f–f transitions. Hence, Rb3SbF3(NO3)3 has the potential to be employed in the UV region.


image file: c9dt03233a-f4.tif
Fig. 4 The UV absorption and optical diffuse reflectance spectra of Rb3SbF3(NO3)3.

Nonlinear optical properties

The SHG response was measured with a Q-switched 1064 nm laser using the Kurtz–Perry method since Rb3SbF3(NO3)3 crystallizes in the polar space group. The results indicate that the SHG response of Rb3SbF3(NO3)3 was approximately 2.2 times that of the KDP standard with the same particle size (Fig. 5b). The SHG measurement as a function of particle size of the crystals reveals that Rb3SbF3(NO3)3 is a type I phase-matchable material (Fig. 5a), which suggests that the title compound is a potentially valuable UV NLO material and the nitrates are good candidates for UV application.
image file: c9dt03233a-f5.tif
Fig. 5 (a) Phase-matching curves for Rb3SbF3(NO3)3 and KDP samples; (b) SHG intensity of Rb3SbF3(NO3)3 with KDP as the reference with the particle size in the range of 150–212 μm.

Theoretical calculations

To investigate the origin of the SHG response of Rb3SbF3(NO3)3, density functional theory (DFT) was used to calculate its band structures and density of states (DOS). As shown in Fig. 6a, Rb3SbF3(NO3)3 is a direct band gap semiconductor because both the valence-band maximum and the conduction-band minimum are localized at the same Y point. The calculated band gap of Rb3SbF3(NO3)3 was found to be 3.08 eV, which is slightly smaller than the experimental value of 3.75 eV, and this slight change in theoretical prediction may result from the fact that the band gap computed by the DFT-GGA method is generally underestimated.51,52 The partial and total density of states for individual elements were calculated to show the detailed contributions of atomic orbitals to bands (Fig. 6b). The states lower than −10 eV are mainly composed of F-2s, Rb-5s, N-2s, N-2p, and O-2s. Clearly, as regards the upper part of the valence bands (VB), the contributions mainly originate from the F-2p, N-2p, Rb-5p and O-2p states, and slightly from the Sb-5s, Sb-5p, N-2s and O-2s states from −10 to 0 eV. Moreover, regarding the bottom of the conduction band (CB), the Sb-5p orbital is the major contributor. From the PDOS, the overlap states can be found from −30.0 to 0.00 eV. For instance, the N-2s, N-2p and the Sb-5s, Sb-5p states overlap with the O-2s, and O-2p states, indicating strong covalent bonds between N, O and Sb, O. The F-2p states overlap with the Sb-5s and Sb-5p states, which reveal covalent interactions between the Sb and F atoms. As is known, the linear and nonlinear optical properties (e.g., SHG response) are mainly determined by the electronic transitions among the states nearby the Fermi level like Sb-5p, Rb-5p, N-2s and O-2p orbits, indicating that three kinds of asymmetric chromophores, namely, Rb-centered distorted polyhedra (Rb1O9F2, Rb2O8F3, and Rb3O7F3), SbO3F3 distorted octahedra and NO3 groups result in the large SHG response of Rb3SbF3(NO3)3.
image file: c9dt03233a-f6.tif
Fig. 6 (a) Calculated band structure of Rb3SbF3(NO3)3; (b) the total and partial density of states for Rb3SbF3(NO3)3, the Fermi level is normalized to 0 eV; (c) and (d) the electron-density difference maps of Rb3SbF3(NO3)3.

To further prove the contributions of asymmetric chromophores to the SHG response, dipole moment calculations for Rb3SbF3(NO3)3 were performed. The local dipole moments for Rb1O9F2, Rb2O8F3, Rb3O7F3, SbO3F3 and NO3 groups were calculated to be 3.26 D (Debyes), 4.14 D, 4.55 D, 16.82 D, and 0.08–0.47 D (Table S3), respectively, showing that the SbO3F3 octahedra and Rb based polyhedra and NO3 groups make the major contribution to the SHG response. Moreover, electron-density difference maps for Rb3SbF3(NO3)3 were obtained in order to demonstrate charge transfer and polarization. As is shown in Fig. 6d, highly asymmetric lobes can be clearly found around the Sb3+ cations which may be thought as a SCALP. Besides, the charge transfer is presented in the Sb–O, Sb–F and N–O linker zones.

Conclusions

In summary, a novel polar NCS UV NLO material, Rb3SbF3(NO3)3, has been successfully produced through a rational multi-component design by simultaneously introducing polarizable Rb+ cations and Sb3+ cations with stereochemically active lone pair electrons into the nitrate system. This title compound has been synthesized through a rapid evaporation concentration method and features a complicated 3D crystal structure composed of [Rb1O9F2]19−, [Rb2O8F3]18−, [Rb3O7F3]16− and [SbF3(NO3)3]3− complexes. SHG tests reveal that Rb3SbF3(NO3)3 exhibits a large phase-matching NLO efficiency of approximately 2.2 × KDP compared with the reported nitrate NLO materials, suggesting that the title compound is a potentially useful UV NLO material. The large SHG effect was confirmed by theoretical calculations along with structural analysis and derived from the multicomponent synergistic effect i.e., the moderately polarizable Rb+ cations, the SCALP effect of Sb3+, and the NO3 planar triangular groups. This synthetic strategy will allow further advances in the design of novel UV NLO materials with excellent comprehensive performance and may have a vital impact on the further exploration of new NLO materials.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank Dr Daichuan Ma at the Analytical and Testing Center, Sichuan University for technical help with the Material Studio calculations. This work was supported by the National Natural Science Foundation of China (no. 21875146) and the Fundamental Research Funds for the Central Universities (no. YJ201921).

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Footnote

Electronic supplementary information (ESI) available: Photograph of crystals, additional crystallographic data, IR spectrum, XRD. CCDC 1914422. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/C9DT03233A

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