[(C₅H₆N₂)₂H](Sb₄F₁₃): a polyfluoroantimonite with a strong second harmonic generation effect

Abstract

It is of great difficulty to create a new antimonite with second-harmonic-generation (SHG) intensity larger than 6 times that of KDP. In this study, a polyfluoroantimonite strategy has been proposed to explore fluoroantimonites with large nonlinear optical (NLO) coefficients. Under the cooperation of chemical (highly asymmetric π-conjugated organic amine) and physical (viscous reaction medium ethylene glycol) methods, two novel polyfluoroantimonites, namely, (3PC)₂(Sb₄F₁₄) and (3AP)₂(Sb₄F₁₃), have been achieved. Interestingly, these two structures contain two new polyfluoroantimonite groups respectively, an isolated (Sb₄F₁₄)²⁻ four-member polyhedral ring and an infinite [Sb₄F₁₃]_∞⁻ helical chain. More importantly, the polar (3AP)₂(Sb₄F₁₃) displays a strong SHG intensity of 8.1 × KDP, a large birefringence of 0.258@546 nm and a high laser-induced damage threshold (LIDT) value of 149.7 MW cm⁻². Theoretical calculations indicated that its strong SHG effect stems from the synergistic effect of the helical [Sb₄F₁₃]_∞⁻ polyfluoroantimonite chain and π-conjugated 3AP⁺ cation, with a contribution ratio of 48.93% and 50.77% respectively. This work provides a new approach for the design and synthesis of high-performance fluoroantimonites.

---

Introduction

Nonlinear optical crystals capable of widening the wavelength range of output radiations have attracted widespread attention. SHG crystals, as the most representative NLO materials, have been applied in fields such as optical communication, laser technology, biomedicine, photonics, and sensor technology.^1–9 Fluoroantimonites are considered as a potential candidate for excellent SHG materials. On the one hand, the Sb³⁺ cation with stereochemically active-lone-pair (SCALP) electrons can undergo second-order Jahn–Teller (SOJT) distortion, resulting in high polarizability.¹⁰ On the other hand, fluorine, being the most electronegative, can enlarge the band gaps of target compounds due to the deeper F-2p orbit than O-2p.^11–14 Recently, scientists have developed various design strategies to create new SHG fluoroantimonites, mainly including the following: (a) introduction of tetrahedral groups (SO₄²⁻ and PO₄³⁻), which can widen the band gaps of fluoroantimonites, such as CsSbF₂SO₄ (3.0 × KDP, 4.76 eV) and Rb₂Sb(P₂O₇)F (5.1 × KDP, 4.74 eV);^15,16 (b) combination of Sb³⁺ with delocalized π-conjugated groups such as oxalate and nitrate as in RbSb₂(C₂O₄)F₅ (1.3 × KDP) and Rb₂SbF₃(NO₃)₂ (2.7 × KDP);^17,18 (c) introduction of SOJT distorted d⁰ transition metal cations (Mo⁶⁺, W⁶⁺, Nb⁵⁺, etc.) such as Cs₆Sb₄Mo₃O₅F₂₆ (0.7 × KDP).¹⁹ However, the development of new fluoroantimonites with high SHG intensity is still a challenge. The SHG intensities of most reported fluoroantimonites are less than 6 × KDP. Consequently, it is necessary to develop a new strategy to enhance the SHG effects of fluoroantimonites further.

It is believed that the condensation of functional building blocks can enhance the density of SHG active groups, thereby impacting the structure and SHG capability of target compound.^20–25 This approach has facilitated the development of numerous high-performance polyiodates, such as GdI₅O₁₄ (15 × KDP) and [o-C₅H₄NHOH]₂[I₇O₁₈(OH)]·3H₂O (8.5 × KDP).^26,27 If fluoroantimonate groups can be condensed into polyfluoroantimonite groups, it is expected to obtain new NLO materials with strong SHG effects in polyfluoroantimonite system. However, research on polyfluoroantimonites is very rare, especially for the NLO material, which can be attributed to the following two reasons: (i) the synthetic conditions for polyfluoroantimonites are rather complicated and still unclear. (ii) The bond energy of the Sb–F bond is lower than that of the Sb–O bond, which makes the polymerization of fluoroantimonite groups difficult compared to oxysalts.^28,29

In order to explore new fluoroantimonites with strong SHG effects, we have dedicated our efforts to polyfluoroantimonite system, and found a universal synthesis method for polyfluoroantimonites, which involves using organic amine groups (chemical method) and enhancing the viscosity of the reaction media (physical method). Highly asymmetric organic amine groups may not only facilitate the polymerization of fluoroantimonite groups during their protonation process, but also trigger the formation of noncentrosymmetric (NCS) structures. Furthermore, the viscous reaction medium can reduce the migration rate of fluoroantimonite groups, which can also promote the combination of fluoroantimonate groups to form new polyfluoroantimonite anions. Two organic amine cations, namely, 3-pyridinecarboxamide [3PC⁺, (C₆H₇N₂O)⁺] and 3-aminopyridine [3AP⁺, (C₅H₇N₂)⁺], were identified as the highly asymmetric NLO active groups through theoretical calculations.^30–32 The 3PC⁺ and 3AP⁺ cations feature significant polarizability anisotropy (δ) and hyperpolarizability (β), surpassing most of the inorganic or organic π-conjugated groups, such as B₃O₆³⁻, (C₃N₃O₃)³⁻, (C₅H₆NO)⁺, and (C₄H₆N₃)⁺ (Fig. 1). The selection of viscous solvents is also very important. For instance, phosphoric acid exhibits a propensity to selectively bind with organic ligands, leading to the formation of by-products during the reaction process, and highly viscous glycerol is not conducive to crystallization.^33–35 Finally, ethylene glycol, a viscous and neutral organic solvent, was selected as the reaction medium.

Fig. 1 Geometry and key NLO parameters of the conventional functional groups, the 3PC⁺ cation and the 3AP⁺ cation.

Guided by the above method, an appropriate amount of hydrofluoric acid was added dropwise into a mixture of organic amines (3PC and 3AP) and SbF₃ to protonate the amines, and ethylene glycol was used to enhance the viscosity of the reaction medium. Two novel organic–inorganic hybrid polyfluoroantimonites, namely, (3PC)₂(Sb₄F₁₄) and (3AP)₂(Sb₄F₁₃), have been successfully synthesized by a facile solvent evaporation method. (3AP)₂(Sb₄F₁₃) crystallizes in a NCS space group because the 3AP⁺ cation features a larger hyperpolarizability than the 3PC⁺ cation. Interestingly, (3AP)₂(Sb₄F₁₃) displays a unique [Sb₄F₁₃]_∞⁻ helical chain, a strong SHG effect (8.1 × KDP) under 1064 nm laser radiation, a large birefringence (0.258 @ 546 nm) and a high LIDT value (149.7 MW cm⁻²). Herein, we report the syntheses, structural analyses, optical properties and first-principles studies of (3PC)₂(Sb₄F₁₄) and (3AP)₂(Sb₄F₁₃).

Results and discussion

Single crystals of (3PC)₂(Sb₄F₁₄) and (3AP)₂(Sb₄F₁₃) were synthesized using a facile solvent evaporation method (Fig. S1†). The detailed crystallographic data for both structures are given in Table S1.† The atomic coordinates and bond lengths are listed in Tables S2 and S3.† The presence of Sb, F, C and N elements was confirmed by field-emission scanning electron microscopy (FESEM) analyses (Fig. S2†). Powder XRD analyses confirmed the purity of the products (Fig. S3†).

(3PC)₂(Sb₄F₁₄) crystallizes in the triclinic crystal system with the CS space group P1̄ (No. 2). There are one (Sb₂F₇)⁻ anion and one 3PC⁺ cation in its asymmetric unit (Table S2†). Sb(1) and Sb(2) are coordinated to four and five fluorine atoms, respectively, forming the SbF₄ seesaw and SbF₅ square pyramid, with lone-pair electrons located in the open sites. The Sb–F bond lengths range from 1.927(3) to 2.521(3) Å (Table S3†). Two Sb(1)F₄ and two Sb(2)F₅ groups further linked to a (Sb₄F₁₄)²⁻ four-member polyhedral ring (4-MPR) via corner-sharing of F(1) and F(4) atoms (Fig. 2a). The bond-valence-sum results show that the bond valences of Sb(1) and Sb(2) are 3.093 and 3.181, respectively, suggesting their oxidation states of +3. As shown in Fig. 2b, the (Sb₄F₁₄)²⁻ anionic clusters, separated by the 3PC⁺ cations, were stacking linearly along the c-axis to form the structure of (3PC)₂(Sb₄F₁₄). The 3PC⁺ cations were connected with (Sb₄F₁₄)²⁻ anion clusters through hydrogen bonds with hydrogen bond lengths in the range of 3.108(6)–3.229(5) Å (Table S4†). The adjacent 3PC⁺ cationic pyrimidine rings are parallel to each other, with an inter-ring distance of 3.323 Å (Fig. S4a†).

Fig. 2 The configuration of the (Sb₄F₁₄)²⁻ 4-MPR and the Sb–F bond lengths (Å) (a) and the crystal structure of (3PC)₂(Sb₄F₁₄) (b).

When the 3PC⁺ cation was replaced by the 3AP⁺ cation, a NCS polyfluoroantimonite, namely, (3AP)₂(Sb₄F₁₃), was obtained. (3AP)₂(Sb₄F₁₃) crystallizes in a chiral triclinic space group P1 (No. 1) and its asymmetric unit is composed of one 3AP⁺ cation, one 3AP molecule and one (Sb₄F₁₃)^– anion (Table S2†). Sb(1), Sb(3) and Sb(4) adopt the SbF₄ seesaw configuration, whereas Sb(2) is in the SbF₅ square pyramid configuration. The Sb–F bond distances are in the range of 1.915(5)–2.522(6) Å (Table S3†). The calculated total bond valences for Sb(1), Sb(2), Sb(3) and Sb(4) are 3.045, 3.104, 2.857 and 2.818, respectively, indicating that their oxidation states are all +3. Its structure features a unique [Sb₄F₁₃]_∞⁻ helical polyfluoroantimonite chain along the b axis with 3AP⁺ cations and 3AP molecules located at the inter-chain space to keep charge balance (Fig. 3c). Sb(1)F₄, Sb(2)F₅, Sb(3)F₄ and Sb(4)F₄ groups are interconnected by corner-sharing [F(4), F(8) and F(11)] to form a (Sb₄F₁₄)²⁻ tetramer (Fig. 3a). Neighbouring (Sb₄F₁₄)²⁻ tetramers are further interconnected into a unique [Sb₄F₁₃]_∞⁻ anionic chain along the b axis via Sb(1)–F(10)–Sb(4) bridges (Fig. 3b). The 3AP⁺ cations are linked with the [Sb₄F₁₃]_∞⁻ helical polyfluoroantimonite chains through hydrogen bonds with the hydrogen bond lengths in the range of 2.697(12)–3.080(15) Å (Table S4†). There is also π–π interaction between neighbouring pyridine rings with a dihedral angle of 2.257° and inter-ring distances ranging from 3.837 to 4.001 Å (Fig. S4b†). The dipole moment of the Sb₄F₁₄ tetramer in (3AP)₂(Sb₄F₁₃) was calculated. The local dipole moments have the values of 8.988–19.160 D for SbF₄ units, and 8.719 D for SbF₅ units. The local dipole moment of the Sb₄F₁₄ tetramer is 18.558 D (Table S5†), implying its polar nature.

Fig. 3 The configuration of the (Sb₄F₁₄)²⁻ tetramer and the Sb–F bond lengths (Å) (a), the [Sb₄F₁₃]_∞⁻ helical polyfluoroantimonite chain (b) and the crystal structure of (3AP)₂(Sb₄F₁₃) (c).

Benefitting from the viscous reaction media and the organic amine groups, both (3PC)₂(Sb₄F₁₄) and (3AP)₂(Sb₄F₁₃) exhibit novel polyfluoroantimonite structures formed by the condensation of the SbF₄ seesaw and SbF₅ square pyramid (Fig. S5†). These two types of polyfluoroantimonite groups have not been reported previously. For (3PC)₂(Sb₄F₁₄) and (3AP)₂(Sb₄F₁₃), the Sb–F bonds of the Sb–F–Sb bridges are 2.074(3)–2.521(3) and 1.940(6)–2.522(6) Å, respectively, which are longer than their terminal Sb–F bonds [1.900(3)–1.940(3) and 1.915(5)–1.964(6) Å]. These values are comparable with those of polyfluoroantimonite crystals reported before.²⁹

The crystal structures of (3PC)₂(Sb₄F₁₄) and (3AP)₂(Sb₄F₁₃) undergo a transition from CS to NCS arrangements. To comprehend the reasons behind this transition, a detailed structural comparison has been conducted between the two compounds. Specifically, the basic building units in these two compounds are similar, including the condensed fluoroantimonite groups and highly asymmetric π-conjugated organic amine cations. The difference lies in the arrangement of organic amine cations and the polymerization modes of fluoroantimonite groups. In comparison to the antiparallel arrangement of 3PC⁺ cations, the 3AP⁺ cations and 3AP molecules exhibit a parallel pairing pattern (Fig. 4a and b), favouring the formation of NCS structures. It is worth noting that this parallel pairing pattern is exceptionally rare in organic amines. Furthermore, there have been significant changes in the polymerization modes of fluoroantimonite groups. The (Sb₄F₁₄)²⁻ tetramer in (3PC)₂(Sb₄F₁₄) appears as a 4-MPR configuration, while the tetramer in (3AP)₂(Sb₄F₁₃) features an open semi-circular configuration (Fig. 4c and d). We calculated the hyperpolarizability of these two condensed Sb₄F₁₄ groups. The hyperpolarizability of the semi-circular Sb₄F₁₄ tetramer (β = 69 047) is 38 times more than that of the Sb₄F₁₄ 4-MPR (β = 1778).³⁶ Therefore, the polymerization mode of the fluoroantimonite groups is also crucial. The hyperpolarizabilities of closed cyclic clusters are often small due to the opposite arrangements of the functional groups within the rings.

Fig. 4 The arrangement modes of 3PC⁺ (a) and 3AP⁺ (b) cations, and the Sb₄F₁₄ tetramers in (3PC)₂(Sb₄F₁₄) (c) and (3AP)₂(Sb₄F₁₃) (d).

Thermogravimetric analyses of (3PC)₂(Sb₄F₁₄) and (3AP)₂(Sb₄F₁₃) were performed in the temperature range of 20–800 °C under a nitrogen atmosphere. (3PC)₂(Sb₄F₁₄) and (3AP)₂(Sb₄F₁₃) can stabilize to 134 and 153 °C, respectively (Fig. S6†). The NCS polyfluoroantimonite, (3AP)₂(Sb₄F₁₃), underwent complete decomposition in the temperature range of 153 to 600 °C (Fig. S6†). Compared to other inorganic–organic hybrid NLO crystals, such as α- and β-(C₄H₅N₂O)(IO₃)·HIO₃ (120 °C), (C₉H₁₄N)SbCl₄ (122 °C) and KLi(HC₃N₃O₃)·2H₂O (125 °C), (3AP)₂(S₄F₁₃) exhibits better thermal stability.^37–39

The infrared (IR) spectra of (3PC)₂(Sb₄F₁₄) and (3AP)₂(Sb₄F₁₃) were analysed at room temperature in the wavelength range of 4000–400 cm⁻¹. Fig. S7† displays the IR spectra of the two compounds, and the corresponding assignments of the IR absorption peaks can be found in Table S6.† The absorption peaks around 3460 and 3250 cm⁻¹ can be attributed to the N–H vibrations. The peaks observed at 3070–2850 cm⁻¹ and 1050–650 cm⁻¹ correspond to the C–H vibrations. The bands at 1690–1380 cm⁻¹ and 1350–1110 cm⁻¹ are assigned to C–C vibrations and C–N vibrations, respectively. The sharp absorption peaks in the range of 600–450 cm⁻¹ can be attributed to the Sb–F vibrations. The above assignments are consistent with previously reported compounds.^40–43

The UV-vis-NIR diffuse reflectance spectra reveal that the cutoff edge of (3PC)₂(Sb₄F₁₄) and (3AP)₂(Sb₄F₁₃) is 272 and 313 nm, respectively. Their experimental optical band gaps were estimated to be 3.71 and 3.05 eV (Fig. S8†). Notably, the NCS (3AP)₂(Sb₄F₁₃) possesses a larger bandgap in comparison with the 1.97 eV of the 3AP molecule (Fig. 5). This phenomenon can be attributed to the highly electronegative fluorine anions. Compared with C-2p or N-2p states, the F-2p orbitals are usually located at deeper energy levels, which induces the high level valence band to shift toward lower energy, resulting in a wide band gap.^44,45 Under the excitation at 330 nm, (3AP)₂(Sb₄F₁₃) exhibits a blue emission band with a peak at 400 nm (Fig. S9†), but its photoluminescence quantum yield is less than 1%. A large red shift is observed compared to the neutral 3AP, which is likely because of the protonation of the organic molecule and the influence of [Sb₄F₁₃]_∞⁻ on its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO).^46,47

Fig. 5 The experimental bandgap spectra of 3AP and (3AP)₂(Sb₄F₁₃).

The birefringence of (3PC)₂(Sb₄F₁₄) and (3AP)₂(Sb₄F₁₃) was determined at λ = 546 nm using a polarized microscope (ZEISS AxioScope. A1).⁴⁸ Both crystals can achieve complete extinction under ortho-polarized light (Fig. S10 and S11†). The optical path differences of the two samples were measured to be 9.591 and 7.323 μm with the thicknesses of 64.88 and 28.39 μm (Fig. S10c and S11c†), respectively. According to the formula R = |n_e – n_o| × d = Δn × d, the experimental birefringences of (3PC)₂(Sb₄F₁₄) and (3AP)₂(Sb₄F₁₃) at 546 nm were measured to be 0.148 and 0.258, respectively. Due to the SCALP electrons of Sb³⁺ and the large anisotropy of organic amine cations, they both exhibit high birefringence. In particular, the birefringence of (3AP)₂(Sb₄F₁₃) exceeds those of all inorganic fluoroantimonites, such as K₂SbMoO₂F₇ (0.220@550 nm), (NH₄)₃SbF₃(NO₃)₃ (0.164@546 nm) and K₂SbP₂O₇F (0.157@546 nm).^49–51

Under 1064 nm laser irradiation, powder SHG measurement was performed on (3AP)₂(Sb₄F₁₃) using KDP (particle size 210–300 μm) as a reference. As shown in Fig. 6a, (3AP)₂(Sb₄F₁₃) exhibits a strong SHG intensity of 8.1 × KDP. Furthermore, it can realize phase matching at 1064 nm (Fig. 6b). In terms of structure–property relationships, the excellent SHG property of (3AP)₂(Sb₄F₁₃) can be ascribed to its polar structure, characterized by the unique [Sb₄F₁₃]_∞⁻ polyfluoroantimonite chain and the highly asymmetric organic amine groups 3AP. Note that the SHG intensity of (3AP)₂(Sb₄F₁₃) is larger than those of most of the reported fluoroantimonites (Table S7†). To evaluate the comprehensive performance of (3AP)₂(Sb₄F₁₃), we further compared (3AP)₂Sb₄F₁₃ with other inorganic–organic hybrid Sb(III)-based compounds in terms of SHG intensity, band gap, and thermal stability (Table S8†). As shown in Fig. 7, compared with representative compounds such as (C₉H₁₄N)SbCl₄, (C₉H₂₆N₃)SbCl₆ and L-H₂his·SbBr₅·H₂O, (3AP)₂(Sb₄F₁₃) shows great comprehensive performance.^38,52,53

Fig. 6 Oscilloscope traces of the SHG signals for (3AP)₂(Sb₄F₁₃) (150–210 μm) (a) and the SHG intensity vs. particle size of (3AP)₂(Sb₄F₁₃) under 1064 nm laser irradiation (b).

Fig. 7 The comparison of the SHG effect, band gap and thermal stability between (3AP)₂(Sb₄F₁₃) (a) and other representative inorganic–organic hybrid Sb(III)-based compounds: (b) (C₉H₁₄N)SbCl₄, (c) (C₉H₂₆N₃)SbCl₆ and (d) L-H₂his·SbBr₅·H₂O.

Based on the reported powder method, we conducted a preliminary estimation of the LIDT value for (3AP)₂(Sb₄F₁₃) using sieved crystals.⁵⁴ The powder LIDT of (3AP)₂(Sb₄F₁₃) and AGS was assessed with a Q-switched pulse laser featuring a flat-top laser beam distribution, yielding values of 149.7 and 4.0 MW cm⁻², respectively. It suggests that the LIDT of (3AP)₂(Sb₄F₁₃) is approximately 37.5 times that of AGS. The powder LIDT of (3AP)₂(Sb₄F₁₃) is comparable with that of NaLu(SeO₃)₂, despite the substantial difference in their band gaps (3.05 vs. 5.30 eV).⁵⁵

To gain further insights into the structure–property relationships of (3AP)₂(Sb₄F₁₃) and (3PC)₂(Sb₄F₁₄), theoretical calculations were carried out using density functional theory (DFT). The calculation results along the high symmetric points in the first Brillouin region revealed that (3AP)₂(Sb₄F₁₃) is a direct bandgap compound with a bandgap of 2.28 eV (Fig. 8a), whereas (3PC)₂(Sb₄F₁₄) is an indirect bandgap compound with a bandgap of 2.47 eV (Fig. S12a†). The calculated band gaps of (3AP)₂(Sb₄F₁₃) and (3PC)₂(Sb₄F₁₄) are smaller than the experimental values (3.05 and 3.71 eV, respectively), which can be attributed to the limitation of the GGA method. Accordingly, in order to accurately analyse the optical properties of (3AP)₂(Sb₄F₁₃) and (3PC)₂(Sb₄F₁₄), the scissor operators of 0.77 and 1.24 eV were adopted, respectively.

Fig. 8 Calculated band structure (a), total and partial densities of states (b), birefringence (c), electron density difference maps (d) and SHG density of d₂₂ in the VB (e) and the CB (f) for (3AP)₂(Sb₄F₁₃).

The total and partial density of states of (3AP)₂(Sb₄F₁₃) and (3PC)₂(Sb₄F₁₄) are shown in Fig. 8b and S12b.† Since the optical properties of the compounds were mainly determined by the states near the Fermi level, we analyzed the top of the valence band (VB) and the bottom of the conduction band (CB). The VB near the Fermi level of (3AP)₂(Sb₄F₁₃) is primarily composed of C-2p, and N-2p states, while the CB is mainly attributed to C-2p and N-2p states (Fig. 8b). Therefore, the band gap of (3AP)₂(Sb₄F₁₃) is predominantly determined by C and N atoms. For (3PC)₂(Sb₄F₁₄), the top of the VB is mainly contributed by O-2p orbitals, whereas the bottom of the CB is occupied by C-2p and N-2p orbitals (Fig. S12b†), indicating that the bandgap of (3PC)₂(Sb₄F₁₄) is primarily dominated by O, C, and N atoms.

The refractive index dispersion curves depicted in Fig. 8c and S12c† demonstrate that (3AP)₂(Sb₄F₁₃) and (3PC)₂(Sb₄F₁₄) exhibit strong anisotropy. For (3AP)₂(Sb₄F₁₃), the trend of optical anisotropy is n₀₁₀ > n₀₀₁ > n₁₀₀, and the derived birefringence (Δn_cal.) is 0.267@546 nm (Fig. 8b), in good agreement with the experimental value (0.258@546 nm). The calculated birefringence of (3PC)₂(Sb₄F₁₄) at 546 nm is 0.154, which also closely matches the experimental value (0.149@546 nm). The electron differential density (EDD) maps of (3AP)₂(Sb₄F₁₃) and (3PC)₂(Sb₄F₁₄) indicate the strong anisotropy of π-conjugated organic amine cations and the highly stereochemically active Sb³⁺ (Fig. 8d and S12d†), which is beneficial for generating the large birefringence. In addition, the π–π stacking of organic amine cations in these two compounds also facilitates the enhancement of their birefringence.

The second-order nonlinear optical susceptibilities of (3AP)₂(Sb₄F₁₃) were also evaluated. With the constraints of Kleinman's symmetry and the space group (P1), its SHG tensor d₂₂ gives the largest value (3.69 pm V⁻¹), which agrees well with the experimental value. Furthermore, to precisely reveal the nonlinear origin of (3AP)₂(S₄F₁₃), the SHG-weighted electron density (SHG density for short) of d₂₂ was analyzed (Fig. 8e and f). Specifically, the SHG effect in the VB originates primarily from Sb-5s5p orbitals, the nonbonding N-2p orbitals, π_p bonding orbitals between C–C bonds and a small number of F-2p orbitals, and in the CB, the dominating SHG sources are the antibonding orbitals of on 3AP⁺ groups, the unoccupied Sb-5p orbitals and some unoccupied F-2p orbitals. Numerically speaking, the SHG contribution percentages of helical [Sb₄F₁₃]_∞⁻ polyfluoroantimonite chain and π-conjugated 3AP⁺ cations were calculated to be 48.93% and 50.77%, respectively, indicating that the strong SHG effect of (3AP)₂(S₄F₁₃) originates from the synergistic effect of the [Sb₄F₁₃]_∞⁻ polyfluoroantimonite chain and π-conjugated 3AP⁺ cations.

Conclusions

In summary, a universal synthesis method for polyfluoroantimonites has been developed for the first time, affording two novel polyfluoroantimonites, namely, (3PC)₂(Sb₄F₁₄) and (3AP)₂(Sb₄F₁₃). They exhibit totally different polymerization modes, which can be regulated by the organic amines. The 3AP⁺ cation with stronger hyperpolarizability results in a [Sb₄F₁₃]_∞⁻ helical polyfluoroantimonite chain, while the 3PC⁺ cation brings about a unique (Sb₄F₁₄)²⁻ 4-MPR. The NCS and polar (3AP)₂(Sb₄F₁₃) features a strong SHG effect of 8.1 × KDP, a large birefringence of 0.258@546 nm and a high LIDT value of 149.7 MW cm⁻². Detailed structural analysis and theoretical calculations reveal that its strong SHG response originates from the synergistic effect of the helical [Sb₄F₁₃]_∞⁻ polyfluoroantimonite chain and π-conjugated 3AP⁺ cations. Our work demonstrates that the polyfluoroantimonite strategy is an effective approach for designing and synthesizing high-performance SHG materials. Further studies on NCS polyfluoroantimonites are in progress.

Data availability

The data that support the findings of this study are available in the ESI† of this article.

Author contributions

Jia-Hang Wu: conceptualization, investigation, data curation, and writing – original draft; Chun-Li Hu: software and formal analysis; Ya-Feng Li: data curation and visualization; Jiang-Gao Mao: project administration and funding acquisition; Fang Kong: methodology, writing – review & editing, and supervision.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 22031009, 22375201, 21921001 and 91963105) and the Natural Science Foundation of Fujian Province (Grant No. 2023J01216).

Notes and references

1. K. M. Ok, Toward the Rational Design of Novel Noncentrosymmetric Materials: Factors Influencing the Framework Structures, Acc. Chem. Res., 2016, 49(12), 2774–2785.
2. S. J. Han, A. Tudi, W. B. Zhang, X. L. Hou, Z. H. Yang and S. L. Pan, Recent Development of Sn(II), Sb(III)-based Birefringent Material: Crystal Chemistry and Investigation of Birefringence, Angew. Chem., Int. Ed., 2023, 62(26), e202302025.
3. X. Liu, Y. C. Yang, M. Y. Li, L. Chen and L. M. Wu, Anisotropic structure building unit involving diverse chemical bonds: a new opportunity for high-performance second-order NLO materials, Chem. Soc. Rev., 2023, 25, 8699–8720.
4. X. M. Liu, P. F. Gong, Y. Yang, G. M. Song and Z. S. Lin, Nitrate nonlinear optical crystals: A survey on structure-performance relationships, Coord. Chem. Rev., 2019, 400, 213045.
5. P. S. Halasyamani, New nonlinear opportunities in the ultraviolet, Nat. Photonics, 2023, 17(8), 639–640.
6. Y. W. Kang and Q. Wu, A review of the relationship between the structure and nonlinear optical properties of organic-inorganic hybrid materials, Coord. Chem. Rev., 2024, 498, 215458.
7. P. F. Li, J. G. Mao and F. Kong, A survey of stereoactive oxysalts for linear and nonlinear optical applications, Mater. Today Phys., 2023, 37, 101197.
8. Z. X. Chen, W. Liu and S. P. Guo, A review of structures and physical properties of rare earth chalcophosphates, Coord. Chem. Rev., 2023, 474, 214870.
9. M. Mutailipu, J. Han, Z. Li, F. Li, J. Li, F. Zhang, X. Long, Z. Yang and S. Pan, Achieving the full-wavelength phase-matching for efficient nonlinear optical frequency conversion in C(NH₂)₃BF₄, Nat. Photonics, 2023, 17(8), 694–701.
10. Q. Lu, X. X. Jiang, K. N. Duanmu, C. Wu, Z. S. Lin, Z. P. Huang, M. G. Humphrey and C. Zhang, Record Second-Harmonic Generation and Birefringence in an Ultraviolet Antimonate by Bond Engineering, J. Am. Chem. Soc., 2024, 146(14), 9975–9983.
11. Y. L. Hu, C. Wu, X. X. Jiang, K. N. Duanmu, Z. P. Huang, Z. S. Lin, M. G. Humphrey and C. Zhang, Ultrashort Phase-Matching Wavelength and Strong Second-Harmonic Generation in Deep-UV-Transparent Oxyfluorides by Covalency Reduction, Angew. Chem., Int. Ed., 2023, e202315133.
12. J. B. Wang, M. M. Zhu, Y. Q. Chu, J. D. Tian, L. L. Liu, B. B. Zhang and P. S. Halasyamani, Rational Design of the Alkali Metal Sn-Based Mixed Halides with Large Birefringence and Wide Transparent Range, Small, 2023, 2308884.
13. M. Yan, R. L. Tang, W. L. Liu and S. P. Guo, From Ba₃Nb₂O₂F₁₂(H₂O)₂ to Ba_0.5NbO₂F₂(H₂O): Achieving Balanced Nonlinear Optical Performance by O/F Ratio Regulation, Inorg. Chem., 2022, 61(50), 20709–20715.
14. P. F. Gong, Y. Yang, F. G. You, X. Y. Zhang, G. M. Song, S. Z. Zhang, Q. Huang and Z. S. Lin, ASbF₃Cl (A = Rb, Cs): Structural Evolution from Centrosymmetry to Noncentrosymmetry, Cryst. Growth Des., 2019, 19(3), 1874–1879.
15. X. H. Dong, L. Huang, C. F. Hu, H. M. Zeng, Z. E. Lin, X. Wang, K. M. Ok and G. H. Zou, CsSbF₂SO₄: An Excellent Ultraviolet Nonlinear Optical Sulfate with a KTiOPO₄ (KTP)-type Structure, Angew. Chem., Int. Ed., 2019, 58(20), 6528–6534.
16. X. H. Dong, H. B. Huang, L. Huang, Y. Q. Zhou, B. B. Zhang, H. M. Zeng, Z. E. Lin and G. H. Zou, Unearthing Superior Inorganic UV Second-Order Nonlinear Optical Materials: A Mineral-Inspired Method Integrating First-Principles High-Throughput Screening and Crystal Engineering, Angew. Chem., Int. Ed., 2024, e202318976.
17. W. Y. Wang, X. Y. Wang, L. Xu, D. Zhang, J. L. Xue, S. Y. Wang, X. H. Dong, L. L. Cao, L. Huang and G. H. Zou, Centrosymmetric Rb₂Sb(C₂O₄)_2.5(H₂O)₃ and Noncentrosymmetric RbSb₂(C₂O₄)F₅: Two Antimony (III) Oxalates as UV Optical Materials, Inorg. Chem., 2023, 62(32), 13148–13155.
18. L. Wang, H. M. Wang, D. Zhang, D. J. Gao, J. Bi, L. Huang and G. H. Zou, Centrosymmetric RbSnF₂NO₃ vs. noncentrosymmetric Rb₂SbF₃(NO₃)₂, Inorg. Chem. Front., 2021, 8(13), 3317–3324.
19. J. H. Wu, B. Zhang, T. K. Jiang, F. Kong and J. G. Mao, From Cs₈Sb₄Nb₅O₅F₃₅ to Cs₆Sb₄Mo₃O₅F₂₆: the first noncentrosymmetric fluoroantimonite with d⁰ transition metal, Chin. J. Struct. Chem., 2023, 42(1), 100016.
20. K. M. Ok and P. S. Halasyamani, The Lone-Pair Cation I⁵⁺ in a Hexagonal Tungsten Oxide-Like Framework: Synthesis, Structure, and Second-Harmonic Generating Properties of Cs₂I₄O₁₁, Angew. Chem., Int. Ed., 2004, 116(41), 5605–5607.
21. F. Ding, K. J. Griffith, W. Zhang, S. Cui, C. Zhang, Y. Wang, K. Kamp, H. Yu, P. S. Halasyamani, Z. Yang, S. Pan and K. R. Poeppelmeier, NaRb₆(B₄O₅(OH)₄)₃(BO₂) Featuring Noncentrosymmetry, Chirality, and the Linear Anionic Group BO₂, J. Am. Chem. Soc., 2023, 145(9), 4928–4933.
22. W. J. Xie, R. L. Tang, S. N. Yan, N. Ma, C. L. Hu and J. G. Mao, Ba₄B₁₄O₂₅: A Deep Ultraviolet Transparent Nonlinear Optical Crystal with Strong Second Harmonic Generation Response Achieved by a Boron-Rich Closed-Loop Strategy, Small, 2023, 2307072.
23. H. T. Qiu, F. M. Li, Z. Li, Z. H. Yang, S. L. Pan and M. Mutailipu, Breaking the Inherent Interarrangement of [B₃O₆] Clusters for Nonlinear Optics with Orbital Hybridization Enhancement, J. Am. Chem. Soc., 2023, 145(44), 24401–24407.
24. Y. C. Yan, J. H. Jiao, C. C. Tu, M. Zhang, Z. H. Yang and S. L. Pan, CsAB₈O₁₂F₂·CsI (A = K+, NH4+): design of two fluorooxoborates with benign layered structures via a salt-inclusion strategy, J. Mater. Chem. C, 2022, 10(22), 8584–8588.
25. H. N. Liu, H. P. Wu, Z. G. Hu, J. Y. Wang, Y. C. Wu and H. W. Yu, Cs₃[(BOP)₃(B₃O₇)₃]: A Deep-Ultraviolet Nonlinear Optical Crystal Designed by Optimizing Matching of Cation and Anion Groups, J. Am. Chem. Soc., 2023, 145(23), 12691–12700.
26. Q. Q. Chen, C. L. Hu, J. Chen, Y. L. Li, B. X. Li and J. G. Mao, [o-C₅H₄NHOH]₂[I₇O₁₈(OH)·3H₂O: An Organic-inorganic Hybrid SHG Material Featuring an [I₇O₁₈(OH)_∞^2- Branched Polyiodate Chain, Angew. Chem., Int. Ed., 2021, 60(32), 17426–17429.
27. J. Chen, C. L. Hu, F. F. Mao, B. P. Yang, X. H. Zhang and J. G. Mao, REI₅O₁₄ (RE=Y and Gd): Promising SHG Materials Featuring the Semicircle-Shaped I₅O₁₄^3- Polyiodate Anion, Angew. Chem., Int. Ed., 2019, 58(34), 11666–11669.
28. G. Zhang, J. G. Qin, T. Liu, Y. J. Li, Y. C. Wu and C. T. Chen, NaSb₃F₁₀: A new second-order nonlinear optical crystal to be used in the IR region with very high laser damage threshod, Appl. Phys. Lett., 2009, 95(26), 261104.
29. L. A. Zemnukhova, A. A. Udovenko, N. V. Makarenko, S. I. Kuznetsov and T. A. Babushkina, Crystal structure and Sb NQR parameters of ammonium tridecafluorotetraantimonate(III) NH₄Sb₄F₁₃, J. Struct. Chem., 2017, 58(4), 694–699.
30. H. T. Tian, C. S. Lin, X. Zhao, F. Xu, C. Wang, N. Ye and M. Luo, Ba(SO₃CH₃)₂: A Deep-Ultraviolet Transparent Crystal with Excellent Optical Nonlinearity Based on a New Polar Non-π-Conjugated NLO Building Unit SO₃CH₃⁻, CCS Chem., 2023, 5, 2497–2505.
31. X. Y. Song, Z. P. Du, A. Belal, Y. Q. Li, Y. Zhou, Y. P. Song, W. Q. Huang, J. Y. Zheng, J. H. Luo and S. G. Zhao, A UV Solar-Blind Nonlinear Optical Crystal with Confined π-Conjugated Groups, Inorg. Chem. Front., 2023, 10(18), 5462–5467.
32. X. H. Meng, X. Y. Zhang, Q. X. Liu, Z. Y. Zhou, X. X. Jiang, Y. G. Wang, Z. S. Lin and M. J. Xia, Perfectly Encoding π-Conjugated Anions in the RE₅(C₃N₃O₃)(OH)₁₂(RE=Y, Yb, Lu) Family with Strong Second Harmonic Generation Response and Balanced Birefringence, Angew. Chem., Int. Ed., 2023, 62(1), e202214848.
33. J. Lu, X. Liu, M. Zhao, X. B. Deng, K. X. Shi, Q. R. Wu, L. Chen and L. M. Wu, Discovery of NLO Semiorganic (C₅H₆ON)⁺(H₂PO₄)⁻: Dipole Moment Modulation and Superior Synergy in Solar-Blind UV Region, J. Am. Chem. Soc., 2021, 143(9), 3647–3654.
34. Z. P. Zhang, X. Liu, X. M. Liu, Z. W. Lu, X. Sui, B. Y. Zhen, Z. S. Lin, L. Chen and L. M. Wu, Driving Nonlinear Optical Activity with Dipolar 2-Aminopyrimidinium Cations in (C₄H₆N₃)⁺(H₂PO₃)⁻, Chem. Mater., 2022, 34(4), 1976–1984.
35. F. F. Mao, C. L. Hu, J. Chen, B. L. Wu and J. G. Mao, HBa_2.5(IO₃)₆(I₂O₅) and HBa(IO₃)(I₄O₁₁): Explorations of Second-Order Nonlinear Optical Materials in the Alkali-Earth Polyiodate System, Inorg. Chem., 2019, 58, 3982–3989.
36. H. T. Tian, N. Ye and M. Luo, Sulfamide: A Promising Deep-Ultraviolet Nonlinear Optical Crystal Assembled from Polar Covalent [SO₂(NH₂)₂] Tetrahedra, Angew. Chem., Int. Ed., 2022, 61(17), e202200395.
37. Q. Q. Chen, C. L. Hu, M. Z. Zhang, B. X. Li and J. G. Mao, α- and β-(C₄H₅N₂O)(IO₃)·HIO₃: Two SHG Materials Based on Organic–Inorganic Hybrid Iodates, Inorg. Chem., 2023, 62(32), 12613–12619.
38. F. F. Wu, Q. Y. Wei, X. Q. Li, Y. Liu, W. Q. Huang, Q. Chen, B. X. Li, J. H. Luo and X. T. Liu, Cooperative Enhancement of Second Harmonic Generation in an Organic-Inorganic Hybrid Antimony Halide, Cryst. Growth Des., 2022, 22(6), 3875–3881.
39. D. H. Lin, M. Luo, C. S. Lin, F. Xu and N. Ye, KLi(HC₃N₃O₃).2H₂O: Solvent-drop Grinding Method toward the Hydro-isocyanurate Nonlinear Optical Crystal, J. Am. Chem. Soc., 2019, 141(8), 3390–3394.
40. M. Zhang, B. B. Zhang, D. Q. Yang and Y. Wang, Synergistic Effect of π-Conjugated [C(NH₂)₃] Cation and Sb(III) Lone Pair Stereoactivity on Structural Transformation and Second Harmonic Generation, Inorg. Chem., 2021, 60(23), 18483–18489.
41. M. Yang, W. Liu and S. P. Guo, Sb₅O₇I: Exploration of Ternary Antimony-Based Oxyhalide as a Nonlinear-Optical Material, Inorg. Chem., 2022, 61(37), 14517–14522.
42. J. Kee and K. M. OK, Hydrogen-Bond-Driven Synergistically Enhanced Hyperpolarizability: Chiral Coordination Polymers with Nonpolar Structures Exhibiting Unusually Strong Second-Harmonic Generation, Angew. Chem., Int. Ed., 2021, 60(38), 20656–20660.
43. X. Hao, M. Luo, C. S. Lin, D. H. Lin, L. L. Cao and N. Ye, RE(H₂C₃N₃O₃)₂·(OH).xH₂O (RE = La, Y and Gd): potential UV birefringent materials with strong optical anisotropy originating from the (H₂C₃N₃O₃)^- group, Dalton Trans., 2019, 48(32), 12296–12302.
44. T. H. Wu, X. X. Jiang, C. Wu, Y. L. Hu, Z. S. Lin, Z. P. Huang, M. G. Humphrey and C. Zhang, Ultrawide Bandgap and Outstanding Second-Harmonic Generation Response by a Fluorine-Enrichment Strategy at a Transition-Metal Oxyfluoride Nonlinear Optical Material, Angew. Chem., Int. Ed., 2022, 61(26), e202203104.
45. C. B. Jiang, X. X. Jiang, C. Wu, Z. P. Huang, Z. S. Lin, M. G. Humphrey and C. Zhang, Isoreticular Design of KTiOPO4-Like Deep-Ultraviolet Transparent Materials Exhibiting Strong Second-Harmonic Generation, J. Am. Chem. Soc., 2022, 144(44), 20394–20399.
46. J. J. Wu, J. L. Qi, Y. Guo, S. F. Yan, W. L. Liu and S. P. Guo, Reversible Tristate Structural Transition within Hybrid Copper (I) Bromides toward Tunable Multiple Emissions, Inorg. Chem. Front., 2023, 59, 16465–16469.
47. C. Sun, J. P. Zhang, Y. Q. Liu, Q. Q. Zhong, X. X. Xing, J. P. Li, C. Y. Yue and X. W. Lei, Lead-Free Hybrid Indium Perovskites with Highly Efficient and Stable Green Light Emissions, CCS Chem., 2022, 4, 3106–3121.
48. L. L. Cao, G. Peng, W. B. Liao, T. Yan, X. F. Long and N. Ye, A microcrystal method for the measurement of birefringence, CrystEngComm, 2020, 22(11), 1956–1961.
49. J. H. Wu, C. L. Hu, T. K. Jiang, J. G. Mao and F. Kong, Highly Birefringent d⁰ Transition Metal Fluoroantimonite in the Mid Infrared Band: Order-Disorder Regulation by Cationic Size, J. Am. Chem. Soc., 2023, 145(44), 24416–24424.
50. Q. Wang, J. X. Ren, D. Wang, L. L. Cao, X. H. Dong, L. Huang, D. J. Gao and G. H. Zou, Low Temperature Molten Salt Synthesis of Noncentrosymmetric (NH₄)₃SbF₃(NO₃)₃ and Centrosymmetric (NH₄)₃SbF₄(NO₃)₂, Inorg. Chem. Front., 2023, 10(7), 2107–2114.
51. Y. L. Deng, L. Huang, X. H. Dong, L. Wang, K. M. Ok, H. M. Zeng, Z. E. Lin and G. Zou, K₂Sb(P₂O₇)F: Cairo Pentagonal Layer with Bifunctional Genes Reveal Optical Performance, Angew. Chem., Int. Ed., 2020, 59(47), 21151–21156.
52. X. M. Wen, J. Cheng, P. Q. Qian, Z. Z. Zhang, H. M. Zeng, L. Huang, G. H. Zou and Z. E. Lin, Synthesis and structure-dependent optical properties of two new organic–inorganic hybrid antimony(III) chlorides, Dalton Trans., 2024, 53(1), 260–266.
53. J. Cheng, P. Q. Qian, M. Yang, L. Huang, H. M. Zeng, G. H. Zou and Z. E. Lin, Second-Harmonic Generation in Homochiral Antimony Halides Directed by l-Histidine, Inorg. Chem., 2023, 62(41), 16673–16676.
54. M. J. Zhang, B. X. Li, B. W. Liu, Y. H. Fan, X.-G. Li, H. Y. Zeng and G. C. Guo, Ln₃GaS₆ (Ln = Dy, Y): new infrared nonlinear optical materials with high laser induced damage thresholds, Dalton Trans., 2013, 42(39), 14223–14229.
55. P. F. Li, C. L. Hu, J. G. Mao and F. Kong, A UV non-hydrogen pure selenite nonlinear optical material for achieving balanced properties through framework-optimized structural transformation, Mater. Horiz., 2024, 11, 1704–1709.

---

Footnote

† Electronic supplementary information (ESI) available: Experimental procedures, theoretical calculations, crystallographic data, and measurements of physical properties. CCDC 2311987 and 2311988 for (3PC)₂Sb₄F₁₄ and (3AP)₂Sb₄F₁₃. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4sc01716a

---