Hg₃O₂(NO₃)F: a mercury nitrate oxyfluoride with an unprecedented [(Hg₃O₂F)⁺]_∞ cationic framework and excellent optical anisotropy

Abstract

Combining multiple anions to design compounds with novel structures and excellent optical properties has become a hot research field. In this paper, a novel nitrate oxyfluoride, Hg₃O₂(NO₃)F, has been obtained. Hg₃O₂(NO₃)F features an unprecedented [(Hg₃O₂F)⁺]_∞ cationic framework constructed by V-shaped HgO₂ units and original HgO₂F₂ tetrahedra and with isolated NO₃⁻ anions balancing the charge. Hg₃O₂(NO₃)F is the first nitrate oxyfluoride containing a d¹⁰ metal. Importantly, Hg₃O₂(NO₃)F exhibits superior optical anisotropy with the calculated birefringence of Δn = 0.23 at 1064 nm. Based on the theoretical calculation analyses, the good optical anisotropy is mainly derived from the well-arranged V-shaped HgO₂ units. This work proves that the strategy of introducing heteranions is effective for exploring high-performance optical materials.

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Introduction

Developing inorganic compounds with novel crystal structures has aroused widespread concern. The combination of two or more kinds of anion groups to design compounds with interesting structure types is a favorable and fruitful route.^1–32 These new crystals may synergize the properties of multiple anions and can be potential candidates in some application fields including birefringence, nonlinear optics, fluorescence, and catalysis. Based on the structure performance relationship, many research systems focusing on optical performance have been developed in recent years.³³ It is well known that deep investigations have been performed on borate, phosphate, and chalcogenide compounds for nonlinear optics. However, in the last few years, some heteranions including halogen anions and O²⁻ anions, especially the F⁻ anion, have been widely introduced into oxate systems forming some promising research topics in nonlinear optics and birefringent materials, such as fluorooxoborates or borate fluorides, phosphate halides, and oxysulfides.³⁴ Some compounds derived from the above mixed anion systems exhibit excellent linear and nonlinear optical properties, such as AB₄O₆F (A = NH₄, Na, Rb, and Cs), Pb₂(BO₃)(NO₃), Sr₆Cd₂Sb₆O₇S₁₀ and Sn₂PO₄I.^35–39

Nitrate compounds, with a π-conjugated system, also have received intensive attention for their diverse optical properties. For instance, RE(OH)₂NO₃ (RE = La, Y, and Gd), Rb₂Na(NO₃)₃, Sr₂(OH)₃NO₃ and Pb₁₆(OH)₁₆(NO₃)₁₆ are good nonlinear optical crystals.^40–43 During the past few years, nitrates containing halogen atoms have aroused the enthusiasm of researchers due to their multifunctional optical performances. Until now, about forty-five inorganic nitrates containing halogen atoms have been reported (Table S3†). In the class of nitrates containing fluoride atoms, Pb₂(NO₃)₂(H₂O)F₂ shows the largest second harmonic generation (SHG) effect (12 × KH₂PO₄) and a very large birefringence (0.23 @1064 nm).⁴⁴ Besides, Rb₃SbF₃(NO₃)₃, (NH₄)₃SbF₃(NO₃)₃ and Rb₂SbF₃(NO₃)₂, also exhibit good SHG effects.^45–47 Moreover, nitrate halides including Cs₂Pb(NO₃)₂Br₂ and CsHgNO₃Cl₂ show good optical anisotropy.^48,49 It follows that the incorporation of halogen anions in nitrates enriches the structure diversity and can provide more promising optical materials. While, based on the survey of nitrates containing halogen atoms, intensive investigations have been performed on compounds comprising metal cations (Pb²⁺, Sn²⁺, Sb³⁺) with lone pair electrons, which is beneficial for achieving favorable SHG effects and optical anisotropy,^28,39,45 other metal cations including d⁰ and d¹⁰ metals which can also produce large polarizability have been less explored. Hence, further explorations are necessary for nitrate halides.

Besides the metal cations with lone pair electrons, Hg²⁺ has received widespread attention. It can form diverse coordination configurations including linear, trigonal-planar, or tetrahedral units, which are widely used to construct excellent NLO and birefringent materials, such as HgBr₂, β-HgBrCl, LiHgPO₄, Ag₂HgI₄, Ba₂HgTe₅ and trigonal HgS.^50–57 As mentioned, CsHgNO₃Cl₂ is a good birefringent crystal.⁵⁰ However, Hg-based nitrates with halogen anions are very rare: only CsHgNO₃Cl₂, HgINO₃ and Ag₂HgI₂(NO₃)₂·H₂O have been reported.^49,55,58 Therefore, we mainly focused on the research of Hg-based nitrate halides for developing compounds with novel structures and promising optical performances.

Here, a new Hg-based nitrate fluoride, Hg₃O₂(NO₃)F, has been obtained through a simple hydrothermal reaction method. In this work, we discuss the synthesis, crystal structure and comparison, optical performances, and the structure–property relationship based on theoretical calculations of Hg₃O₂(NO₃)F.

Experimental section

Synthesis

Caution! HF solution is highly corrosive! Proper protective equipment is essential for safety. LiF (Damas, 99.9%), Hg(NO₃)₂·H₂O (Damas, 99%), GeO₂ (Damas, 99%), and HF (Aladdin, 40% aqueous solution) without any further purification, were used to synthesize Hg₃O₂(NO₃)F via a hydrothermal reaction. A mixture of 1 mmol of LiF (5.939 mg), 0.5 mmol of GeO₂ (52.32 mg) and 1 mmol of Hg(NO₃)₂·H₂O (281.62 mg) was weighed and poured into 20 mL Teflon liners, with 0.2 mL of HF and 3 mL of deionized water as a solvent for the reaction. The reaction temperature was set at 200 °C with a heating rate of 1 °C per minute from room temperature, which was maintained for three days, and then cooled to room temperature at the rate of 2 °C per hour. The yield of this compound is about 70% based on LiF.

Crystal structure determination

The single crystal X-ray diffraction (SXRD) of Hg₃O₂(NO₃)F was performed using a Bruker D8 QUEST X-ray diffractometer, Mo Kα radiation (λ = 0.71073 Å). The direct method was used to record data, and then F² was performed with SHELX-2014 software and Olex2.⁵⁹ for the full-matrix least squares fitting process, and the correctness of the structure was checked using the PLATON program, and no problems were found.⁶⁰ The crystallographic data and refinement parameters of Hg₃O₂(NO₃)F are shown in Table 1. Atomic coordinates, equivalent isotropic parameters, and selected bonds and angles are shown in Tables S1 and S2 (ESI†). The CIF document for Hg₃O₂(NO₃)F is stored in the CCDC at number 2268674.†

Table 1 Crystal data and structure refinement parameters for Hg₃O₂(NO₃)F

a R 1 = ||Fo| − |Fc||/|Fo|. b wR2 = [w(Fo^2 − Fc^2)^2]/[w(Fo^2)^2]^1/2.
Empirical formula	Hg3O2(NO3)F
Formula weight	714.78
Temperature/K	296(2)
Crystal system	Orthorhombic
Space group	Pnma
a/Å	7.5474(10)
b/Å	10.99033(14)
c/Å	6.9906(9)
Volume/Å^3	579.86(13)
Z	4
ρ calc/g cm^−3	8.188
μ/mm^−1	79.215
F(000)	1184.0
Crystal size/mm^3	0.15 × 0.13 × 0.1
Radiation	Mo Kα (λ = 0.71073)
2Θ range for data collection/°	6.908 to 59.308
Index ranges	−10 ≤ h ≤ 10, −15 ≤ k ≤ 13, −9 ≤ l ≤ 9
Reflections collected	5163
Independent reflections	857 [Rint = 0.0536, Rsigma = 0.0371]
Data/restraints/parameters	857/46/68
Goodness-of-fit on F^2	1.086
Final R indexes [I ≥ 2σ(I)]^a^,^b	R 1 = 0.0237, wR2 = 0.0494
Final R indexes [all data]^a^,^b	R 1 = 0.0290, wR2 = 0.0510
Largest diff. peak/hole/e Å^−3	1.62/−1.61

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Energy-dispersive X-ray spectroscopy (EDS)

EDS analysis was performed on several selected crystals using a Bruker quantum dispersive X-ray spectroscope. The data has proved the presence of elements Hg, N, O and F in the crystal, and the ratio is close to that from crystal structure determination (Fig. S4†).

Powder X-ray diffraction (PXRD)

A Bruker D8 Advance diffractometer with Cu-Kα radiation (λ = 1.5406 Å) was used for PXRD experiments on Hg₃O₂(NO₃)F powder samples. The 2θ range is 10–70°, the step size is 0.02°, and the scanning rate of each step is 1s. The Mercury v3.8 program was used to obtain a simulated PXRD map of the single crystal structure data of Hg₃O₂(NO₃)F. The purity of the powder sample was confirmed by PXRD analysis (Fig. 1).

Fig. 1 Experimental and simulated powder XRD patterns of Hg₃O₂(NO₃)F.

Infrared (IR) and UV-vis-NIR diffuse reflectance spectra

The infrared spectra of the powder samples were characterized in the range of 400–4000 cm⁻¹ using a Magna 750 FI-IR spectrometer and using KBr pure powder samples as a reference. With BaSO₄ powder as the background, Hg₃O₂(NO₃)F UV-vis-NIR diffuse reflection data in the range of 200–1200 nm was recorded on a Carry 5000 spectrometer. The Kubelka–Munk function was used to obtain diffuse reflection, and direct extrapolation methods were used to derive the band gap.

Thermogravimetric analysis

The thermal properties in flowing N₂ gas were determined on the Netzsch STA 449 F3 thermal analyzer. The powder samples were placed in an alumina crucible and heated from 20 to 1000 °C at a rate of 15 °C min⁻¹.

Theoretical calculation

The CASTP model based on the density functional theory (DFT) method was used to analyze the electronic structure and optical properties.^61–63 The Perdew–Burke–Ernzerhof (PBE) functional and generalized gradient approximation (GGA) were used for exchange correlation as valence electrons, considering the following orbital electrons: Hg: 5d¹⁰6s², N: 2s²2p³, O: 2s²2p⁴, F: 2s²2p⁵. The cut-off energies of Hg₃O₂(NO₃)F were set to 850 eV, and the Monkhorst–Pack k-point grids were 3 × 2 × 4. The cut-off energies of Hg₃O₂(NO₃)₂ were set to 340 eV, and the Monkhorst–Pack k-point grids were 4 × 2 × 2.^64–66

Results and discussion

Crystal structure

Hg₃O₂(NO₃)F belongs to the orthorhombic system with the space group Pnma (No. 62). The asymmetrical unit contained two unique Hg, one unique F, one N, and four O atoms (Fig. 2a). In particular, the π-conjugated NO₃ unit is semi-occupied at the crystallographic site. The crystal structure of Hg₃O₂(NO₃)F consists of planar π-conjugated NO₃⁻ anion units and a [(Hg₃O₂F)⁺]_∞ cationic framework. In the crystal structure of Hg₃O₂(NO₃)F, one N atom is coordinated with three O atoms forming an isolated NO₃ plane triangle (Fig. S3†) with N–O bond distances ranging from 1.24 to 1.26 Å and bond angles within the range of 119.6–120.2°. A Hg(1) atom is connected with two O(1) and two F(1) atoms to build a Hg(1)O₂F₂ tetrahedron (Fig. 2a). However, Hg(2) is surrounded by two O(1) atoms to form a V-shaped Hg(2)O₂ unit. The Hg(1)–O(1), Hg(2)–O(1) and Hg(1)–F(1) bond distances are 2.12, 2.067 and 2.38 Å. The O–Hg–O angles are 116.7 and 177.6°, the angles of F–Hg–O are 88.9 and 90.46°, and the F–Hg–F angles are 133.47° (Table S1†). Hg(1)O₂F₂ tetrahedra and V-shaped Hg(2)O₂ units are interconnected via a corner-shared O(1) atom to construct (Hg₃O₂F₂)_∞ layers with a honeycomb feature in the bc plane (Fig. 2c). Furthermore, these (Hg₃O₂F₂)_∞ layers are linked together via sharing of F atoms along the a axis to form the whole [(Hg₃O₂F)⁺]_∞ cationic framework, and the NO₃⁻ anions act as the counter ions to balance the charge (Fig. 2b). All the bond distances and bond angles are close to some reported compounds.^48,49,67

Fig. 2 (a) Coordination geometry of Hg₃O₂(NO₃)F; (b) the whole structure of Hg₃O₂(NO₃)F; (c) the (Hg₃O₂F₂)_∞ layer.

To date, there are less than fifty inorganic nitrate halides, among which twenty-five nitrate fluorides have been studied (Table S3†). About the mercury-based nitrate halides, only CsHgNO₃Cl₂, HgINO₃ and Ag₂HgI₂(NO₃)₂·H₂O have been reported.^49,55,58 Hence Hg₃O₂(NO₃)F is the first mercury-based nitrate containing F. In the crystal structure of CsHgNO₃Cl₂, the Hg atom adopts a high coordination configuration of a HgO₆Cl₂ polyhedron and further connects with NO₃ groups through the shared O atoms to form a [HgNO₃Cl₂]⁻ anionic layer.⁴⁹ HgINO₃ features a neutral 2D framework with interconnected HgO₄I₂ and NO₃ units.⁵⁵ However, Ag₂HgI₂(NO₃)₂·H₂O shows a 3D network with Hg atoms connected to O atoms from NO₃ units and I atoms to form HgO₆I₂ polyhedra.⁶⁰ The coordination mode of Hg atoms is similar to that in CsHgNO₃Cl₂ and the NO₃ units are also disordered.⁴⁹ Moreover, Hg₃O₂(NO₃)F can be regarded as the equivalent anion substitution from the compound Hg₃O₂(NO₃)₂.⁶⁹ With NO₃⁻ anion in Hg₃O₂(NO₃)₂ being replaced by one F⁻ anion, the symmetry has been changed from orthorhombic Pbca to Pnma of Hg₃O₂(NO₃)F. The coordination modes of the Hg atoms in Hg₃O₂(NO₃)₂ change from the original three kinds of V-shaped HgO₂ units to two kinds of different Hg-based units including tetrahedral Hg(1)O₂F₂ and V-shaped Hg(2)O₂ units. The unit cell parameters are a = 6.98; b = 13.56; c = 15.43; V = 1463.17; and Z = 8 for Hg₃O₂(NO₃)₂. It is evident that the unit cell parameters of b, c, Z, and V decreased compared with that of Hg₃O₂(NO₃)F, which may be induced by the smaller space occupancy of F⁻ than NO₃⁻ anions. In the crystal structure of Hg₃O₂(NO₃)₂ (Fig. S1†), the HgO₂ units are interconnected with each other to build two corrugated Hg₃O₂ honeycomb nets with isolated NO₃ units to balance the charge. The difference is that although both compounds exhibit a cellular framework and the NO₃ unit is separated, the cellular network is different and connected by a shared F atom in Hg₃O₂(NO₃)F. The introduction of the F atom causes the repeating unit to change from Hg₃O₂ to Hg₃O₂F₂, and the original two-dimensional structure to a three-dimensional structure, with F atoms participating in the connection of the (Hg₃O₂F₂)_∞ layers. The distance between the layers decreases from 7.03 in Hg₃O₂(NO₃)₂ to 4.11 Å in Hg₃O₂(NO₃)F. The arrangement of NO₃⁻ has also transformed, from one half of them being located between the nets and the other half almost in the interstices of the nets, to all of it being near the nets. The honeycomb pattern on the layer changes from being arranged along the ac plane to being arranged along the bc plane. The introduction of F atom reorganizes the structure and produces new chemical structures. The O–Hg–O angles of Hg₃O₂(NO₃)₂ are in the range from 167 (2) to 177.6 (2)° which are less than the O–Hg–O angles in Hg₃O₂(NO₃)F and the honeycomb nets are not completely spread out on a plane (Fig. S1†).⁶⁹ All Hg-based nitrates are centric compounds, possibly because the arrangements of structural units are very symmetrical in three dimensions. Specifically, the orientations of NO₃ groups are antiparallel, leading to the cancellation of polarities, which is more likely to form compounds with centric crystal structures. The crystal structures of HgINO₃ and CsHgNO₃Cl can prove this statement.^49,55 To sum up, Hg₃O₂(NO₃)F exhibits a novel structure in nitrates and presents the first nitrate oxyfluoride containing d¹⁰ metal. Moreover, the tetrahedral HgO₂F₂ unit is reported for the first time in the title compound. In addition, it is very rare for a nitrate system to contain MO_xF_y fluoro-oxygen units, such as Pb₂(NO₃)₂(H₂O)F₂ and PbCdF(SeO₃)(NO₃).^44,46

Optical measurements

Based on the UV-vis-NIR spectrum (Fig. 3) of Hg₃O₂(NO₃)F and the Kubelka–Munk function, the practical band gap of Hg₃O₂(NO₃)F is 2.19 eV. The band gap of Hg₃O₂(NO₃)F is relatively smaller compared with other nitrate halides including Cs₂Pb(NO₃)₂Br₂ (3.01 eV) and CsHgNO₃Cl₂ (3.1 eV).^48,49 There are no obvious vibration peaks at the range of 1500–4000 cm⁻¹ in the IR spectra (Fig. 4). The intense band at 1315 cm⁻¹ is attributable to the N–O stretching vibrations in the NO₃ triangles and the band at 804 cm⁻¹ is ascribed to the nonplanar bending vibrations of the NO₃ planar groups. The peaks at 705 cm⁻¹ and 673 cm⁻¹ are attributed to the symmetric and asymmetric stretching of Hg–F bonds according to some previous literature. The peaks at 588 cm⁻¹ and 522 cm⁻¹ are attributed to the symmetric and asymmetric stretching of Hg–O bonds according to some previous literature.^48,49,68

Fig. 3 UV-vis-NIR diffuse reflectance spectrum of Hg₃O₂(NO₃)F.

Fig. 4 IR spectrum of Hg₃O₂(NO₃)F.

Thermal stability

Fig. 5 shows the DTA curves of Hg₃O₂(NO₃)F. We can see that Hg₃O₂(NO₃)F can be stable below 236 °C, and then in the range of 236–1000 °C, weight loss can be divided into several steps.^48,49

Fig. 5 TG–DSC curves of Hg₃O₂(NO₃)F.

Theoretical studies

In order to better elaborate the structure–performance relationship, first-principles calculations are carried out. The calculated band structure of Hg₃O₂(NO₃)F indicates that the compound has a direct band gap of 1.12 eV (Fig. 6a). Due to the limitation of the exchange and correlation functions of GGA-PBE, the calculated band gap value is less than the experimental value, so a scissor operator of 1.07 eV is used to calculate the optical properties of Hg₃O₂(NO₃)F. For Hg₃O₂(NO₃)F, the top of valence bands (VBs) are mainly contributed by O-2p and parts of F-2p and Hg-5d (Fig. 6b). The bottom of conduction bands (CBs) are mainly occupied by the Hg-6s and O-2p orbitals. From the DOS diagram of this study, it can be seen that F contributes very little to the optical properties of the compound, which may be the reason for its small band gap. We can improve the band gap by introducing alkali metal, alkaline earth or alkaline elements, or increasing the proportion of F atoms, as well as adjusting the proportions of F⁻ and NO₃⁻ anions. In summary, we conclude that the charge transfer between valence and conduction bands is mainly determined by Hg, O and F atoms. The calculated band gap of Hg₃O₂(NO₃)₂ is 1.16 eV (Fig. S2a†), which is close to the calculated value of Hg₃O₂(NO₃)F. According to the total and partial densities of states, the optical properties of Hg₃O₂(NO₃)₂ are mainly determined by Hg, and O atoms (Fig. S2b†).⁶⁹

Fig. 6 (a) Calculated band gap; (b) density of states (DOS). The Fermi level is set at 0 eV; and (c) calculated refractive index dispersion curves of Hg₃O₂(NO₃)F.

Hg₃O₂(NO₃)F crystallizes in an orthogonal crystal system, which belongs to a biaxial crystal. The refractive index curves are calculated in Fig. 6c, showing a trend of n_y > n_x > n_z in the wavelength range. The birefringence of Hg₃O₂(NO₃)F at 1064 nm is calculated to be 0.23, which is the maximum among the nitrates which have been investigated on birefringence. The birefringence of Hg₃O₂(NO₃)F is significantly enhanced compared with that of the mercury-based nitrate halide CsHg(NO₃)Cl₂ (0.145@1064 nm), and is larger than that of other nitrate halides (Table 2), including Cs₂Pb(NO₃)₂Br₂ (0.147@546 nm), (NH₄)₃SbF₄(NO₃)₂ (0.164@546 nm), (NH₄)₃SbF₃(NO₃)₃ (0.098@546 nm), PbCdF(SeO₃)(NO₃) (0.055@1064 nm), and Hg₃O₂(NO₃)₂ (0.123@1064 nm).^46–49,69 In addition, the birefringence of Hg₃O₂(NO₃)F is equal to that of Pb₂(NO₃)(H₂O)F. Pb₂(NO₃)(H₂O)F shows excellent optical anisotropy, which is induced by the synergistic effect of the NO₃ groups and lone pair electrons, combined with the superimposed enhanced polarization of PbO₉F₂ polyhedrons.^46,48,49 It is well-known that the anisotropic polarizability of the NO₃ anion is the largest in the planar triangular anion groups including BO₃, CO₃ and NO₃.^47,48,70,71 However, for Hg₃O₂(NO₃)F, the NO₃ units are not ideally arranged. Hence, the main contribution for optical anisotropy may be from the Hg-based units.

Table 2 Birefringence comparison of inorganic nitrates

Compounds	Birefringence	Ref.
Hg3O2(NO3)F	0.23@1064 nm	This Work
Pb2(NO3)2(H2O)F2	0.23@1064 nm	44
(NH4)3SbF4(NO3)2	0.164@546 nm	47
Cs2Pb(NO3)2Br2	0.147@546 nm	48
CsHgNO3Cl2	0.145@546 nm	49
Na3Rb6(CO3)3(NO3)2Cl·(H2O)6	0.14@546 nm	73
Bi3TeO6OH(NO3)2	0.115@1064 nm	10
Gd(NO3)(Se2O5)·3H2O	0.109@1064 nm	71
(NH4)3SbF3(NO3)3	0.098@546 nm	47
Ba2NO3(OH)3	0.082@532 nm	72
Rb2SbF3(NO3)2	0.06@1064 nm	46
PbCdF(SeO3)(NO3)	0.055@1064 nm	47
RbSnF2NO3	0.05@1064 nm	45
Pb16(OH)16(NO3)16	0.0365@700 nm	44

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In order to further comprehend the contribution of each group to the favourable optical anisotropy of Hg₃O₂(NO₃)F, calculations of the electronic density difference map of Hg₃O₂(NO₃)F have been performed. As exhibited in Fig. 7, even though the NO₃ units are not non-parallelly arranged, which has less contribution to the excellent linear optical properties, the electron cloud of Hg²⁺ shows nice interactions with O²⁻ and the polarizabilities of Hg–O bonds in the bc plane are stronger than that of the Hg–F bonds along the a axis, resulting the large optical anisotropy of Hg₃O₂(NO₃)F. Therefore, the electron density difference map of Hg₃O₂(NO₃)F further confirms that the excellent optical anisotropy mainly come from the Hg-based units (Fig. 7).²⁹ The enhanced birefringence of Hg₃O₂(NO₃)F compared with that of Hg₃O₂(NO₃)₂ may come from the more ideal arrangements of HgO₂ units in plane and additional polarizability of the Hg–F bonds. It can be seen from Table S4† that many Hg-based compounds exhibit large birefringence, especially most of them built with Hg-based units with low coordination numbers. The large birefringence may be derived from the larger polarizabilities and suitable arrangements of these units.

Fig. 7 Electron–density difference map of Hg₃O₂(NO₃)F. The Fermi level is set to 0 eV.

Conclusions

The first mercury nitrate oxyfluoride, Hg₃O₂(NO₃)F, was discovered via a simple hydrothermal reaction. Hg₃O₂(NO₃)F is the first nitrate oxyfluoride containing a d¹⁰ metal and shows a novel crystal structure. Besides, Hg₃O₂(NO₃)F exhibits outstanding optical anisotropy among nitrates mainly induced by V-shaped HgO₂ units with high polarizabilities based on the analyses of theoretical calculations. The discovery of Hg₃O₂(NO₃)F greatly enriches the family of nitrate compounds and may pave new avenues for the synthesis of mixed anion compounds. Further research will be carried out for investigating nitrate halides with diverse crystal structures and large birefringence.

Conflicts of interest

The authors declare that they have no conflict of interest.

Acknowledgements

The authors acknowledge the financial support from the National Natural Science Foundation of China (22101248), the National Key Laboratory Development Fund (20210002), and the Lvyangjinfeng Talent Program of Yangzhou (YZLYJFJH2021YXBS083). We thank Doctor Chun-Li Hu of the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences for the help with theoretical calculations.

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Footnote

† Electronic supplementary information (ESI) available: Additional tables and figures. CCDC 2268674. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3qi01146a

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