Low temperature molten salt synthesis of noncentrosymmetric (NH₄)₃SbF₃(NO₃)₃ and centrosymmetric (NH₄)₃SbF₄(NO₃)₂

Abstract

Two fluorine antimony nitrates named (NH₄)₃SbF₃(NO₃)₃ and (NH₄)₃SbF₄(NO₃)₂ have been successfully synthesized by a low temperature molten salt method. Although these two title compounds possess NO₃⁻ groups and similar Sb-polyhedra, they exhibit completely different macroscopic centricities, (NH₄)₃SbF₃(NO₃)₃ being noncentrosymmetric while (NH₄)₃SbF₄(NO₃)₂ being centrosymmetric, which is induced by the discrepant coordination environments of Sb³⁺. Interestingly, the noncentrosymmetric (NH₄)₃SbF₃(NO₃)₃ exhibits a strong second harmonic generation response (3.3 × KDP), a large birefringence (0.098@546 nm) and a large band gap (3.77 eV), indicating its potential as a UV nonlinear optical material. Detailed structure–property analysis confirms that the superior optical property is due to the synergistic effect of the planar π-conjugated NO₃⁻ groups and the stereochemically active lone pair containing Sb³⁺ cations.

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Introduction

At present, nonlinear optical (NLO) materials, which are the pivotal component of the solid-state laser, have developed to an important branch of modern optics and are widely used in information, medical, scientific research, military and industrial manufacturing fields.^1–6 In general, an ideal NLO crystal should meet the following conditions: large nonlinear optical coefficient, appropriate birefringence, wide transmission range, high laser damage threshold, stable physicochemical properties, and so on.^7–10 In fact, few compounds could satisfy all of the above conditions, for some of the criteria are contradictory. Therefore, the design and synthesis of NLO materials with excellent overall properties has become a great challenge.

It is well known that the primary characteristic of a NLO crystalline material is that it has a noncentrosymmetric (NCS) structure; that is, the compound must crystallize in a non-centrosymmetric space group.^11,12 It has been found that the reasonable combination of different asymmetric chromophores is an effective tactic for the synthesis of NLO materials, which is conducive to the generation of large second harmonic generation (SHG) response.^13,14 Common asymmetric chromophores include (1) planar π-conjugated anions, such as BO₃³⁻, CO₃²⁻, and NO₃⁻;^15–17 (2) distorted polyhedra of d⁰ and d¹⁰ transition metal ions, such as Ti⁴⁺, Mo⁶⁺, Zn²⁺, and Hg²⁺;^18–22 (3) lone pair electron cations with stereochemical activity, such as Pb²⁺ and Bi³⁺.^23–25 In comparison to planar π-conjugated anionic ions, the BO₃³⁻ system is relatively mature at present, and numerous splendid NLO crystals have been obtained, such as β-BaB₂O₄ (β-BBO),²⁶ LiB₃O₅ (LBO),²⁷ and KBe₂BO₃F₂ (KBBF).²⁸ The CO₃²⁻ anion is unstable and easily decomposed at high temperatures, and the crystal growth of carbonates is difficult. Moreover, among them, NO₃⁻ has been proved to be a splendid functional anion to constitute NLO materials because of its large microscopic second-order polarization and mild synthesis conditions conducive to the growth of large-sized single crystals.^29,30 As for the cationic ions, compounds containing metal ions (Pb²⁺, Bi³⁺, Sn²⁺, and Sb³⁺) with lone pair electrons always exhibit excellent NLO properties, such as Pb₃Mg₃TeP₂O₁₄ (13.5 × KDP),³¹ K₂Bi₂(SO₄)₂Cl₄ (5.5 × KDP),³² [Sn₂(H₂PO₂)₃]Br (3.0 × KDP)³³ and K₂Sb(P₂O₇)F (4.0 × KDP).³⁴ Among them, the development of antimony(III)-based nonlinear optical crystal materials is proceeding slowly due to the easy oxidation and hydrolysis of Sb³⁺. In the mid- and far-infrared region, some Sb(III)-chalcogenides have been reported,^35,36 whereas in the ultraviolet region, few antimony(III)-based oxide or oxyfluoride nonlinear optical materials have been reported due to the difficulty in the crystal growth.

Compared with the aqueous phase reactions like hydrothermal and aqueous solution methods, molten salt synthesis could provide an anhydrous environment for the reaction, which would effectively solve the oxidation and hydrolysis problems. Conventional molten salts tend to have a high melting point, which is not suitable for the system containing Sb³⁺ cations; the exploration of a suitable molten salt is required.³⁷ Ionic liquids (ILs) are a novel type of low temperature molten salt; they show advantages in material synthesis in some specific fields.^38,39 However, they are organic salts composed of entirely ionic species, and the products of these reactions are extremely sensitive to air and moisture; a proper inorganic molten salt with a low melting point would be a better choice.⁴⁰ Nitrates possess the ability to dissolve many inorganic and organic compounds, for that nitrates exhibit low melting point temperature; especially, NH₄NO₃ has a melting point as low as 170 °C, and it could be used as a good medium for the reaction. In addition, molten salt nitrates can also provide NLO functional units in the reaction to construct NLO materials.

Based on the above views, SbF₃ with active lone pair electrons was combined with a low temperature molten salt nitrate NH₄NO₃ to explore novel NLO materials, and two ammonium antimony nitrates named (NH₄)₃SbF₃(NO₃)₃⁴¹ and (NH₄)₃SbF₄(NO₃)₂ were successfully obtained. The two title compounds possess the same chemical composition but different atomic ratios, which induce their discrepant coordination modes of Sb³⁺ and opposite symmetries. In particular, the NCS compound (NH₄)₃SbF₃(NO₃)₃ reveals a large SHG efficiency of about 3.3 × KDP and large birefringence of 0.098@546 nm.

Experimental section

Synthesis of (NH₄)₃SbF₃(NO₃)₃ and (NH₄)₃SbF₄(NO₃)₂

The reaction reagents NH₄F (≥96.0%) and SbF₃ (≥99.8%) were purchased from Aladdin, and NH₄NO₃ (≥99.7%) was purchased from Xi Long Chemical without further purification.

Nitrate with a low melting point is a good reactant for the synthesis of molten salt method. We first mixed NH₄NO₃ and SbF₃ in a ratio of 3 : 1 for (NH₄)₃SbF₃(NO₃)₃ and 2 : 1 for (NH₄)₃SbF₄(NO₃)₂. Then, no further reagent was added for the synthesis of (NH₄)₃SbF₃(NO₃)₃, and 1 mmol NH₄F (0.037 g) was added for (NH₄)₃SbF₄(NO₃)₂. After the mixture was fully ground, it was placed in a Teflon autoclave and heated at 100 °C for 4 days. Then the mixture was cooled to room temperature at a rate of 5 °C h⁻¹. Colorless bulk crystals of (NH₄)₃SbF₃(NO₃)₃ and (NH₄)₃SbF₄(NO₃)₂ were obtained, respectively (Fig. S1†). Notably, the atomic ratio of the synthesized compounds is exactly the same as that of the raw materials. Therefore, the molten salt of nitrates could not only provide the anhydrous environment to inhibit the hydrolysis of Sb³⁺, but also participate in the reaction as reactants. By adjusting the ratio of raw materials, the products change accordingly; hence, the molten salt method could make the synthesis of novel compounds more effective.

Single crystal structure determination

The crystal structures of (NH₄)₃SbF₃(NO₃)₃ and (NH₄)₃SbF₄(NO₃)₂ were measured using a Rigaku XtaLAB Synergy R diffractometer, and determined by the direct method through SHELX-2014.⁴² The coordinates of all the atoms were solved by the least square plane convergence algorithm based on F² full matrix. Finally, PLATON was used to check the space group, and there was no higher space group.⁴³ The crystallographic data and other relevant data of the two compounds are presented in Table 1 and Tables S1–S4.†

Table 1 Crystal data and structure refinement for (NH₄)₃SbF₃(NO₃)₃ and (NH₄)₃SbF₄(NO₃)₂

a R 1(F) = ∑||Fo| − |Fc||/∑|Fo|. wR2(Fo^2) = [∑w(Fo^2 − Fc^2)^2/∑w(Fo^2)^2]^1/2.
Formula	(NH4)3SbF3(NO3)3	(NH4)3SbF4(NO3)2
Formula weight	418.91	374.07
Crystal system	Monoclinic	Orthorhombic
Space group	P21	Pnma
a (Å)	9.0362(4)	12.6805(4)
b (Å)	7.4301(3)	12.3608(3)
c (Å)	9.5591(4)	7.0737(2)
α (°)	90	90
β (°)	101.118(4)	90
γ (°)	90	90
V (Å^3)	629.75(5)	1108.74(5)
Z	2	4
ρ(calcd) (g cm^−3)	2.209	2.241
Temperature (K)	297.07(10)	295.94(10)
λ (Å)	0.71073	0.71073
F (000)	408.0	725.0
μ (mm^−1)	2.284	2.535
R 1, wR2 (I > 2σ(I))^a	0.0389/0.1046	0.0189/0.0401
R 1, wR2 (all data)	0.0422/0.1082	0.0234/0.0415
GOF on F^2	1.042	1.067
Flack parameter	0.04(2)	—

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Powder X-ray Diffraction

A Smart Lab powder X-ray diffractometer with Cu-Kα radiation was adopted to measure the powder X-ray diffraction spectrum of the target compounds, with a rate of 8° min⁻¹, a 2θ range of 5–70° and a scanning step width of 0.02°.

Thermal analysis

Thermogravimetric analysis (TGA) measurements were carried out on a Discovery-TGA thermal analyzer. The crystalline material was loaded into a platinum crucible and heated from 0 °C to 800 °C at 10 °C min⁻¹ under a N₂ atmosphere.

Infrared spectroscopy

The infrared spectra of the two title compounds were measured using a Vertex 70 Fourier transform infrared spectrometer (FTIR) with KBr as the background. KBr (100 mg) and powder samples (1 mg) of the title compounds were thoroughly mixed in an agate mortar and then pressed into transparent plates with a diameter of 13 mm and a thickness of about 1 mm. The wavelength range was 400 cm⁻¹ to 4000 cm⁻¹.

UV−Vis diffuse reflectance spectroscopy

(NH₄)₃SbF₃(NO₃)₃ and (NH₄)₃SbF₄(NO₃)₂ were tested using a Shimadzu UV-2600 spectrophotometer at room temperature with BaSO₄ as a standard reference, and the test range was 185–800 nm.

Birefringence measurements

The birefringence characteristics of (NH₄)₃SbF₃(NO₃)₃ and (NH₄)₃SbF₄(NO₃)₂ were measured using a polarizing microscope (Carl Zeiss Axioscope 5). The wavelength of the light source is 546 nm.

Frequency-doubling effect

The powder SHG signal of (NH₄)₃SbF₃(NO₃)₃ was measured by the Kurtz and Perry method under a Q-switched Nd:YAG laser with 1064 nm radiation.⁴⁴ (NH₄)₃SbF₃(NO₃)₃ samples were divided into six different sizes (25–45, 45–58, 58–75, 75–106, 106–150 and 150–212 μm) for testing, and the particle size of KH₂PO₄ (KDP) references was consistent with that of (NH₄)₃SbF₃(NO₃)₃ samples.

Theoretical calculations

In order to understand the electronic structures of the two compounds more clearly, their band structures, density of states (DOS) and partial density of states (PDOS) were calculated by density functional theory (DFT).⁴⁵ The Perdew–Burke–Ernzerhof (PAW-PBE) functional was used in the calculation,⁴⁶ as implemented in the Vienna Ab initio Simulation Package (VASP).^47–50 The k-points in the Brillouin band were set to be 4 × 7 × 4 for (NH₄)₃SbF₃(NO₃)₃ and 3 × 3 × 5 for (NH₄)₃SbF₄(NO₃)₂, respectively, and the cutoff kinetic energy was set to 550 eV for both of them.

Results and discussion

Crystal structure description

(NH₄)₃SbF₃(NO₃)₃ crystallizes in a NCS space group of P2₁, consisting of one Sb atom, three F atoms, three N atoms, nine O atoms, and three NH₄⁺ cations. In this compound, each Sb atom forms the [SbF₃] complex with three F atoms (Fig. 1a), and the distance between the Sb–F bond is 1.921–1.968 Å. The [SbF₃] complex and the NO₃⁻ unit are arranged separately in the crystal. NH₄⁺ is distributed in the cavity of the entire crystal as a charge compensation unit, which is represented by a grey tetrahedron. The [SbF₃] complex and NO₃⁻ moiety are further linked with NH₄⁺ through N–H⋯O and N–H⋯F hydrogen bonds, with the hydrogen bond length in the range of 2.033–2.159 Å and 1.892–2.614 Å, respectively (Fig. 1b).

Fig. 1 (a and c) The representations of different coordination modes of Sb³⁺ and a unit cell. (b and d) The arrangement mode of (NH₄)₃SbF₃(NO₃)₃ and (NH₄)₃SbF₄(NO₃)₂.

(NH₄)₃SbF₄(NO₃)₂ crystallizes in a centrosymmetric (CS) space group of Pnma, consisting of one Sb atom, four F atoms, six O atoms, two N atoms and three NH₄⁺. In (NH₄)₃SbF₄(NO₃)₂, the Sb atom forms the [SbF₄]⁻ anion group with four F atoms (Fig. 1c), and the Sb–F bond distance is in the range of 1.941–2.073 Å. NO₃⁻ is independently arranged in the crystal with the N–O bond distance in the range of 1.240–1.247 Å. NH₄⁺, as a charge balancing unit, connects the [SbF₄]⁻ and NO₃⁻ units into a 3D network framework through N–H⋯F and N–H⋯O hydrogen bonds (Fig. 1d). The lengths of N–H⋯F and N–H⋯O hydrogen bonds are in the ranges of 1.843–2.060 Å and 2.028–2.171 Å, respectively.

It is generally accepted that the crystal structure of a compound is determined more by the basic structural unit than by the chemical composition. As can be seen from the above descriptions, although (NH₄)₃SbF₃(NO₃)₃ and (NH₄)₃SbF₄(NO₃)₂ possess the same chemical composition, the different atomic ratio induces the coordination discrepancy of Sb³⁺. Therefore, the different basic building units with different degrees of distortion may ultimately lead to their structural discrepancy. NCS (NH₄)₃SbF₃(NO₃)₃ contains a tri-coordinated [SbF₃] complex that is scattered, and the lone pair electrons of Sb³⁺ show two directions that are left and right symmetric along the b axis (Fig. 1b). Sb³⁺ with different directions are alternately aligned, and the compound (NH₄)₃SbF₃(NO₃)₃ exhibits a final NCS structure. In (NH₄)₃SbF₄(NO₃)₂, the Sb atom bridges four F atoms to form a twisted seesaw [SbF₄]⁻ complex, and the lone pair electrons of Sb atoms also exhibit two orientations that are anti-parallel along the a-axis. NO₃⁻ in (NH₄)₃SbF₄(NO₃)₂ also has anti-parallel orientations along the c axis. For that anti-parallel arrangement of basic units usually leads to a centrosymmetric structure, and hence (NH₄)₃SbF₄(NO₃)₂ ultimately presents the CS structure as shown in Fig. 1d. Understanding the key factors dominating the symmetries of crystals and the structure–property relationship is helpful in the exploration of novel NLO materials.

Powder X-ray diffraction

Powder X-ray diffraction was carried out on (NH₄)₃SbF₃(NO₃)₃ and (NH₄)₃SbF₄(NO₃)₂, respectively. The phase purity of the two compounds was verified for that the experimental results were consistent with the fitting results of single crystal X-ray diffraction (Fig. S2†).

Thermal properties

Thermogravimetric analysis was performed for (NH₄)₃SbF₃(NO₃)₃ and (NH₄)₃SbF₄(NO₃)₂, respectively. As shown in Fig. S3,† (NH₄)₃SbF₃(NO₃)₃ could stabilize to 100 °C, while (NH₄)₃SbF₄(NO₃)₂ started to decompose at 220 °C. After heating from 100 °C to 800 °C under a nitrogen atmosphere, the total weight loss of the former is about 70%, while that of the latter is about 50%. X-ray powder diffraction was used to determine the heating products of the two compounds, and the final residue was confirmed to be Sb₂O₄ in both cases, as shown in Fig. S4.† (NH₄)₃SbF₃(NO₃)₃ and (NH₄)₃SbF₄(NO₃)₂ possess the same chemical composition, and the difference in their thermal stability may be caused by their different hydrogen bonds.⁵¹ Compounds with higher hydrogen bond density and shorter hydrogen bond length always exhibit better thermal stability. Comparing the two title compounds, their hydrogen bond densities are 0.03335 and 0.03788 Å⁻³, respectively, and the shortest hydrogen bond of 1.843 Å exists in (NH₄)₃SbF₄(NO₃)₂; therefore, (NH₄)₃SbF₄(NO₃)₂ should have better thermal stability, which is consistent with the experimental value.

Optical properties

The IR spectra for (NH₄)₃SbF₃(NO₃)₃ and (NH₄)₃SbF₄(NO₃)₂ have been measured and are shown in Fig. S5.† The strong bands around 3200 and 1400 cm⁻¹ in (NH₄)₃SbF₃(NO₃)₃ and (NH₄)₃SbF₄(NO₃)₂ can be attributed to anti-symmetric stretching and bending of the N–H bond. The absorption peaks at 1338, 1042 cm⁻¹ in (NH₄)₃SbF₃(NO₃)₃ and 1378, 1052 cm⁻¹ in (NH₄)₃SbF₄(NO₃)₂ are mainly attributed to the symmetric/asymmetric stretching vibration of the NO₃⁻ group, and the medium peaks at 832/720 cm⁻¹ and 837/737 cm⁻¹ could be attributed to the bending vibrations of the NO₃⁻ group. The characteristic absorption peaks at 561/506/466 cm⁻¹ and 556/518/452 cm⁻¹ originate from the asymmetric stretching and bending vibrations of the Sb–F bond. All the assignments are consistent with previous reports.^52,53

UV-Vis diffuse reflectance spectroscopy was performed on the compounds (NH₄)₃SbF₃(NO₃)₃ and (NH₄)₃SbF₄(NO₃)₂. It can be seen that their band gaps are 3.77 eV and 3.64 eV, with cutoff edges of 330 nm and 340 nm, respectively (Fig. 2a and b), indicating that the two title compounds are potential UV crystalline materials.

Fig. 2 (a and b) The UV optical diffuse reflectance spectra of (NH₄)₃SbF₃(NO₃)₃ and (NH₄)₃SbF₄(NO₃)₂. Inset: the Tauc plot of (F(R)hν)²versus hν for the band-gap energy. (c) The birefringence measurement results of (NH₄)₃SbF₃(NO₃)₃ and (NH₄)₃SbF₄(NO₃)₂. (d) The phase-matching curve and SHG intensity for (NH₄)₃SbF₃(NO₃)₃ with KDP as the reference (150–212 μm).

The birefringence of (NH₄)₃SbF₃(NO₃)₃ and (NH₄)₃SbF₄(NO₃)₂ was determined using a Carl Zeiss Axioscope 5 polarizing microscope (Fig. 2c). (NH₄)₃SbF₃(NO₃)₃ exhibits a moderate birefringence of 0.098@546 nm, while (NH₄)₃SbF₄(NO₃)₂ exhibits a large birefringence of 0.164@546 nm, indicating that both compounds are potential birefringent materials. As studied by Lin et al., the introduction of highly polar anisotropic building units with ordered arrangement would induce large birefringence.⁵⁴ Comparing the orientations of lone pair electrons of Sb³⁺ and π-conjugated NO₃⁻ groups, in (NH₄)₃SbF₃(NO₃)₃, the lone pair electrons with two directions show a certain angle, and NO₃⁻ anions also represent a chaotic arrangement. However, in (NH₄)₃SbF₄(NO₃)₂, the lone pair electrons are completely parallel to the a axis; meanwhile, NO₃⁻ anions are parallel to the c axis. The parallel arrangement of polar functional units is conducive to the generation of large birefringence; therefore, (NH₄)₃SbF₄(NO₃)₂ with a CS structure exhibits much larger birefringence than NCS (NH₄)₃SbF₃(NO₃)₃.

NLO properties

Since (NH₄)₃SbF₃(NO₃)₃ belongs to the NCS P2₁ space group, KH₂PO₄ (KDP) was used as the reference sample for the SHG test. As shown in Fig. 2d, with the increase of grain size of (NH₄)₃SbF₃(NO₃)₃, the SHG response increases first and tends to be constant gradually, indicating that (NH₄)₃SbF₃(NO₃)₃ is type I phase-matchable. Meanwhile, the SHG signal intensity map of (NH₄)₃SbF₃(NO₃)₃ shows a strong frequency doubling signal value of about 3.3 × KDP, which could be attributed to the lone pair electrons of Sb³⁺ and the high density of NO₃⁻ anions.

Theoretical calculations

The refractive index dispersion curves of (NH₄)₃SbF₃(NO₃)₃ and (NH₄)₃SbF₄(NO₃)₂ have been calculated, which show appropriate anisotropy (Fig. 3a and b). Taking (NH₄)₃SbF₃(NO₃)₃ as an example, n_z > n_x > n_y, where n_x, n_y and n_z represent the refractive index in the x, y and z directions, respectively. According to the formula Δn = n_z − n_y, the birefringence index Δn is calculated to be 0.111@546 nm for (NH₄)₃SbF₃(NO₃)₃ and 0.183@546 nm for (NH₄)₃SbF₄(NO₃)₂, which are in good agreement with the experimental values.

Fig. 3 (a and b) Calculated refractive indexes for compounds (NH₄)₃SbF₃(NO₃)₃ and (NH₄)₃SbF₄(NO₃)₂. (c and d) The total DOS and partial DOS of (NH₄)₃SbF₃(NO₃)₃ and (NH₄)₃SbF₄(NO₃)₂. The Fermi level is set to 0 eV. (e and f) Electron-density difference maps of (NH₄)₃SbF₃(NO₃)₃ and (NH₄)₃SbF₄(NO₃)₂.

In order to explore the intrinsic relationship between the structure and optical properties for the two title compounds, systematic theoretical calculations were performed based on density functional theory. The direct band gaps of (NH₄)₃SbF₃(NO₃)₃ and (NH₄)₃SbF₄(NO₃)₂ calculated by PBE were 2.95 eV and 3.20 eV, respectively (Fig. S6†), which were 0.82 eV and 0.44 eV lower than the experimental values.⁵⁵ The total and partial state densities (TDOS/PDOS) were also calculated for the two title compounds (Fig. 3c and d), which are similar in terms of structural similarity. The results show that in the range of −25 to 0 eV, the valence bands of these two compounds are mainly from H-1s, O-2p, N-2p, and F-2p orbitals, with a small amount from Sb-5p and O-2s orbitals. From 0 eV to 15 eV, the N-2p, O-2p, and Sb-5p orbitals make the major contribution to both compounds. It is well known that the optical properties of compounds are mainly caused by electron transitions near the Fermi level. That is, the [SbF_x] polyhedra and NO₃⁻ groups are primarily responsible for the linear and nonlinear optics of the title compounds. The results affirm the strategy that the synergistic effect of Sb³⁺ cations containing lone pair electrons and planar π-conjugated NO₃⁻ groups should be helpful in generating excellent optical properties, as confirmed by the electron density difference map. As shown in Fig. 3e and f, highly asymmetric lobes can be observed for the Sb³⁺ cations in (NH₄)₃SbF₃(NO₃)₃ and (NH₄)₃SbF₄(NO₃)₂, indicating the stereoscopic activity of lone pair electrons on Sb atoms. Meanwhile, a deepened N–O bond in the red electron cloud can be observed, indicating the presence of electron transfer in the π-conjugated NO₃⁻ unit.

Conclusions

Two antimony nitrates, (NH₄)₃SbF₃(NO₃)₃ and (NH₄)₃SbF₄(NO₃)₂, were successfully synthesized by adopting low temperature molten salt synthesis. The discrepancy in stoichiometric ratios induces the diverse coordination environments of Sb³⁺, which further results in the different macroscopic centricities of the two compounds. Owing to the synergistic effect of the π-conjugated NO₃⁻ group and the lone pair containing Sb³⁺ cations, both the compounds (NH₄)₃SbF₃(NO₃)₃ and (NH₄)₃SbF₄(NO₃)₂ reveal a large birefringence of 0.098@546 nm and 0.164@546 nm. NCS (NH₄)₃SbF₃(NO₃)₃ in particular has a strong SHG coefficient of about 3.3 × KDP, indicating that antimony nitrates are promising superior optical materials. The low temperature molten salt method provides a new way to overcome the difficulty in crystal growth which is easy hydrolysis and oxidation.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank Dr Dingguo Xu at the College of Chemistry, Sichuan University, for help in the Vienna Ab initio Simulation Package calculations. This work was supported by the National Natural Science Foundation of China (no. 22122106, 22071158, 21971171).

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Footnote

† Electronic supplementary information (ESI) available: Additional crystallographic data, crystal photographs, XRD patterns, TGA curves, XRD patterns after melting, IR spectra and band structures calculation. CCDC 2222170 and 2222171. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2qi02637f

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