Pb₂(SeO₃)(SiF₆): the first selenite fluorosilicate with a wide bandgap and large birefringence achieved through perfluorinated group modification

Abstract

Birefringent crystals serve as the core elements of polarizing optical devices. However, the inherent challenge of balancing bandgap and birefringence poses a significant hurdle in designing crystals with excellent overall performance. In this study, we propose a novel approach, namely modification with perfluorinated groups, to achieve dual enhancement of the bandgap and birefringence of selenite materials. We have successfully synthesized the first selenite fluorosilicate, namely, Pb₂(SeO₃)(SiF₆). This compound exhibits a three-dimensional structure composed of two-dimensional lead selenite layers bridged by SiF₆ octahedrons. Notably, by introducing a perfluorinated SiF₆ group, the bandgap of the lead selenite compound has been expanded to 4.4 eV. Furthermore, Pb₂(SeO₃)(SiF₆) demonstrates a large birefringence (0.161 @ 546 nm), surpassing most of the selenite compounds with a bandgap larger than 4.2 eV. Theoretical calculations suggest that the large birefringence of Pb₂(SeO₃)(SiF₆) can be attributed to the synergistic effects of SeO₃, PbO₄ and PbO₃F₄ polyhedrons. Our research not only pioneers a new system for selenite materials, enriching the diversity of selenite structures, but also provides a design methodology for obtaining wide bandgap birefringent selenite.

---

Introduction

Birefringent crystals play a pivotal role as optical functional components in commercial, agricultural, industrial, and military sectors.^1–4 Some crystals have been commercialized, such as α-BaB₂O₄,⁵ CaCO₃,⁶ YVO₄,⁷ MgF₂,⁸ TiO₂,⁹etc. However, these commercially available birefringent crystals have inherent limitations. For example, the transparency of YVO₄ is notably low below 400 nm, and the birefringence of MgF₂ is comparatively small, which fail to meet the demands of modern industry.¹⁰ In recent years, with the rapid advancement of optoelectronic technology, the requirement for miniaturization of short-wave devices has been escalating. The miniaturization of such devices necessitates crystals with large birefringence and a wide bandgap.^11,12 However, the inherent contradiction between the bandgap and birefringence makes wide bandgap birefringent crystals extremely rare. Consequently, addressing these contradictions and achieving a balance between these properties is an urgent issue that requires immediate attention.^13–17

Metal selenites, containing stereo-chemically active lone pair electrons, have shown promise as potential candidates for birefringent materials due to their significant anisotropy.^18,19 Scientists have primarily explored the following avenues to enhance the birefringent performance of selenite crystals: (i) incorporation of stereo-chemically active lone pair cations, such as Pb₂Bi(SeO₃)₂Cl₃ (0.186 @ 1064 nm, 3.45 eV),²⁰ Rb₂Bi₂(SeO₃)₃F₂ (0.105 @ 546 nm, 3.72 eV),²¹ and Bi₄TeO₄F₂(TeO₃)(SeO₃)₂ (0.20 @ 1064 nm, 2.74 eV);²² (ii) incorporation of d⁰ transition metals susceptible to second-order Jahn–Teller (SOJT) distortion, for example, Pb₂(V₂O₄F)(VO₂)(SeO₃)₃ (0.105 @ 1064 nm, 2.35 eV),²³ Bi₄TiO₂F₄(SeO₃)₄ (0.19 @ 1064 nm, 3.58 eV)²² and Gd₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ (0.143 @ 1064 nm, 3.15 eV);²⁴ (iii) incorporation of highly polarizable and deformable d¹⁰ transition metals, for instance, Rb₂Hg₂(SeO₃)₃ (0.055 @ 546 nm, 3.60 eV),²⁵ Pb₂Cd(SeO₃)₂Cl₂ (0.093 @ 1064 nm, 4.1 eV)²⁶ and Pb₂Cd(SeO₃)₂Br₂ (0.116 @ 1064 nm, 3.9 eV).²⁶ However, the relatively low transparency below 400 nm limits their application in the ultraviolet region. To address this, researchers have introduced ions conducive to widening the bandgap in the selenite system, such as alkali metals, alkaline earth metals, etc.²⁷ The successfully synthesized wide bandgap selenites include NaLu(SeO₃)₂ (0.153 @ 546 nm, 5.3 eV),²⁸ Ga₂(SeO₃)₃(H₂O)₃ (0.082 @ 532 nm, 5.3 eV),²⁹ and Y(HSeO₃)(SeO₃)(H₂O)·(H₂O) (0.044 @ 532 nm, 5.5 eV).³⁰ Although their bandgaps have been improved to 5.0 eV or above, their birefringence decreased obviously, except for NaLu(SeO₃)₂. This is mainly attributed to their limited contribution to the anisotropy of polarizability. Consequently, there is an urgent need to develop a new method to achieve enhancement of the bandgap and birefringence in selenite materials.

Our research group has done many studies for the design and synthesis of selenite materials with a wide bandgap and large birefringence, and we have discovered that the bandgap of these materials can be influenced by the type of anionic group. Currently, we are pursuing two approaches to simultaneously improve the bandgap and birefringence of selenites. The first approach involves introducing SO₄ tetrahedral groups with a wide transmittance window,^31,32 exemplified by compounds like Hg₂(SeO₃)(SO₄) (0.133 @ 532 nm, 3.58 eV)³³ and Hg₃(SeO₃)₂(SO₄) (0.118 @ 546 nm, 4.70 eV).³⁴ Another approach involves the use of partially fluorinated metal oxide polyhedrons.^35,36 For instance, CaYF(SeO₃)₂ exhibits a wide bandgap of 5.06 eV and a large birefringence of 0.138 @ 532 nm.³⁷ It is noteworthy that there is still upside potential in enhancing the bandgap and birefringence of selenite compounds.

Based on our literature research, Si⁴⁺ in the IVA group exhibits a large bandgap and strong ultraviolet transmittance capability.^38,39 Additionally, F⁻ in the VIIA group has high electronegativity, strong ultraviolet transmittance capability, and chemical scissor action.^40–42 Combining these elements to form SiF₆ groups can leverage their individual advantages to positively influence the bandgap of compounds.^43,44 Moreover, SiF₆ groups can act as a chemical scissor, regulating the arrangement of metal cations and selenite ions, thereby enhancing the birefringence performance of the compounds. To achieve compounds with large birefringence, Pb²⁺ was chosen as the balancing cation. Aside from possessing stereo-chemically active lone pair electrons, Pb²⁺ readily forms highly anisotropic PbO_xF_y polyhedron when partially fluorinated.^45–47 The synergistic effect of the SiF₆ group, SeO₃ group, and PbO_xF_y polyhedron holds the potential for developing new selenite materials with a wide bandgap and large birefringence (Fig. 1). Consequently, we have focused our attention on the Pb²⁺–Se⁴⁺–Si⁴⁺–F⁻–O²⁻ system for the first time. As of now, no compounds have been discovered in this system. Through our exploration, a novel compound, Pb₂(SeO₃)(SiF₆), has been obtained. Pb₂(SeO₃)(SiF₆) stands as the initial example of a fluorosilicate selenite system. Theoretical calculations and experimental measurements indicate that the selenite fluorosilicate exhibits a large birefringence and a wide bandgap. The synergistic effect of the SeO₃ group, PbO₄ and PbO₃F₄ polyhedrons is the primary contributor to its large birefringence. This paper will report on the design, synthesis, structure, and optical properties of Pb₂(SeO₃)(SiF₆).

Fig. 1 From low birefringence, small bandgap to high birefringence wide bandgap regulated by SiF₆ groups.

Results and discussion

Pb₂(SeO₃)(SiF₆) was synthesized using SeO₂, H₂SiF₆, and Pb(BF₄)₂ through a low-temperature (105 °C) hydrothermal method. The detailed synthesis procedures are outlined in the ESI† under the section titled “Syntheses”. The elemental distribution map confirmed the presence of fluorine (F) element (Fig. 2), and quantitative elemental analysis results revealed the stoichiometry of F : Pb : Si : Se as 6 : 2 : 1 : 1 in the compound (Fig. S1a†). The crystal purity was verified using X-ray diffraction (XRD), as depicted in Fig. S1b.† Crystallographic details of Pb₂(SeO₃)(SiF₆) can be found in Table S1.†

Fig. 2 SEM images of Pb₂(SeO₃)(SiF₆) and its elemental distribution maps.

The first example of the selenite fluorosilicate compound, Pb₂(SeO₃)(SiF₆), crystallizes in the orthorhombic space group Pnma (no. 62). The lattice parameters are a = 13.8153(4) Å, b = 5.4470(2) Å, c = 9.6098(3) Å, α = β = γ = 90°, and V = 723.16(4) ų. Its asymmetric unit contains two lead atoms, one selenium atom, one silicon atom, four fluorine atoms, and two oxygen atoms, totaling ten atoms. Among them, F(1), F(4), and O(1) are located at general positions, while the remaining atoms occupy special positions. The selenium atom coordinates with three oxygen atoms, forming a trigonal pyramid with Se–O bond lengths ranging from 1.674(7) to 1.727(4) Å. The silicon atom bonds with six fluorine atoms, resulting in an octahedral SiF₆ configuration, with Si–F bond lengths ranging from 1.674(6) to 1.699(6) Å. Within the range of 2.5 to 2.8 Å, Pb(1) coordinates with four oxygen atoms, while Pb(2) coordinates with three oxygen and four fluorine atoms. Bond valence calculations indicate that the oxidation state of Se(1) is 3.958. The oxidation state of Si(1) is 4.513, which is relatively elevated and commonly observed in hexafluoro silicate compounds, such as compounds KLiSiF₆ (Si: 4.496),⁴⁸ Tl₂SiF₆ (Si: 4.506)⁴⁹ and Li₂SiF₆ (Si: 4.518).⁵⁰ Given that it is not an isolated case, we believe the high bond valence may be caused by the overestimated bond valence parameter (1.58) for the Si⁴⁺–F bond. If the parameter was decreased to 1.55, the oxidation state of Si in Pb₂(SeO₃)(SiF₆) will decrease to 4.16 and the same phenomenon is observed in KLiSiF₆ (Si: 4.15), Tl₂SiF₆ (Si: 4.15) and Li₂SiF₆ (Si: 4.16).⁵¹ The calculated oxidation state for Pb(1) and Pb(2) is 1.374 and 1.443 respectively. The calculated oxidation state for Pb(1) and Pb(2) is 1.374 and 1.443 respectively. When considering the longer Pb–O and Pb–F bonds in the range of 2.80 to 2.97 Å, the oxidation states of Pb(1) and Pb(2) can be increased to 1.868 and 1.755, respectively, approaching their standard oxidation states (Table S2†).

Pb₂(SeO₃)(SiF₆) exhibits a novel three-dimensional (3D) structure composed of two-dimensional (2D) lead selenite layers bridged by SiF₆ groups (Fig. 3). Initially, several Pb(1)O₄ polyhedrons are linked through O(1)⋯O(1) edge-sharing, forming one-dimensional (1D) lead oxide chains. These chains are reinforced by Pb(2)O₃F₄ polyhedrons via O(1) atoms (Fig. 3a). The Se(1)O₃ groups serve as hexadentate ligands, coordinating bidentately with one Pb(1) atom and bridging to two Pb(1) and three Pb(2) atoms (Fig. 3b). These Se(1)O₃ groups further connect the 1D lead oxide chains to create a 2D lead selenite layer on the bc plane (Fig. 3d). Ultimately, the SiF₆ groups bridge these 2D layers, resulting in the formation of a novel 3D Pb₂(SeO₃)(SiF₆) network structure (Fig. 3e). Notably, the SiF₆ groups act as tridentate anionic ligands, coordinating bidentately with one Pb(2) atom and linking to two Pb(2) atoms (Fig. 3c). This complex configuration underscores the unique structural characteristics of the compound.

Fig. 3 One-dimensional lead oxide chains enhanced with lead oxyfluoride modification (a), the coordination environments of the SeO₃ group (b) and SiF₆ group (c), two-dimensional lead selenite layer structure (d) and the 3D structure of (e) Pb₂(SeO₃)(SiF₆).

Thermogravimetric analysis (TGA) of the compound Pb₂(SeO₃)(SiF₆) was performed within a temperature range of 20 to 1200 °C. It is noteworthy that the compound remains stable before reaching 220 °C. In the temperature range of 220 to 500 °C, the weight loss of the compound is primarily attributed to the decomposition of one molecule of SeO₂, with an experimental value of 15.8%, closely approximating the theoretical value of 16.2%. Beyond 500 °C, the weight loss of the compound should be associated with the incomplete decomposition of lead and fluorine components (Fig. S2†).⁴³ The incomplete weight loss of lead and fluorine components has been previously observed in the TGA of compounds Pb₂Al₃F₃(Te₆F₂O₁₆) and Pb₂Ga₃F₃(Te₆F₂O₁₆), as reported by our group.⁵² This further strengthens the reliability of our observations. In addition, Pb₂(SeO₃)(SiF₆) also exhibits good environmental stability. The samples can still be stable after being exposed to room temperature for three months.

The infrared spectrum (IR) of Pb₂(SeO₃)(SiF₆) was obtained at room temperature in the wavenumber range of 4000–400 cm⁻¹. The compound exhibits good transparency in the range of 4000–800 cm⁻¹. The characteristic peak observed around 638 cm⁻¹ is attributed to the absorption of fluorine ions. Strong peaks in the regions of 420–510 cm⁻¹ and 700–800 cm⁻¹ are attributed to the bending and stretching vibrations of the Se–O bonds (Fig. S3†).

The UV-vis-NIR diffuse reflectance spectra indicate that the Pb₂(SeO₃)(SiF₆) compound exhibits a UV cutoff edge at 247 nm, accompanied by a bandgap of 4.4 eV (Fig. S4†). Notably, this bandgap is significantly higher than those observed in previously reported lead-based compounds such as Pb₃SeO₅ (3.3 eV),⁵³ CdPb₈(SeO₃)₄Br₁₀ (3.32 eV),⁴⁵ Pb₂Bi(SeO₃)₂Cl₃ (3.45 eV),²⁰ Pb₂NbO₂(SeO₃)₂Br (3.17 eV),⁵⁴ and Pb₃(SeO₃)Br₄ (3.35 eV).⁵⁵ This significant bandgap can be primarily attributed to the contribution of the perfluorinated group SiF₆. Combined with the IR spectrum, the transparency range for Pb₂(SeO₃)(SiF₆) powder samples is from 0.25 to 12.6 μm (Fig. 4). However, considering the potential deviation in infrared cutoff edge due to multi-phonon absorption in powder and crystal measurements, a 50% correction was applied.⁵⁶ Therefore, the estimated transparency range for Pb₂(SeO₃)(SiF₆) should be from 0.25 μm to 6.3 μm, covering the ultraviolet, visible, and mid-infrared regions. This suggests that Pb₂(SeO₃)(SiF₆) serves as a promising optical functional material.

Fig. 4 UV-vis-NIR diffuse reflectance and IR transmittance spectra of Pb₂(SeO₃)(SiF₆).

A large bandgap is advantageous for enhancing the laser-induced damage threshold (LIDT) of compounds. Therefore, we measured the LIDT of Pb₂(SeO₃)(SiF₆) within the particle size range of 150 to 210 nm. The experiments revealed that the LIDT value for Pb₂(SeO₃)(SiF₆) is 50.8 MW cm⁻², approximately 19.5 times higher than that of AgGaS₂ (2.6 MW cm⁻²). This value is slightly higher than those previously reported for the compounds Hg₃Se(SeO₃)(SO₄) (23.35 MW cm⁻²)⁵⁷ and Pb₃(TeO₃)Br₄ (21.5 MW cm⁻²).⁴⁵

The birefringence of Pb₂(SeO₃)(SiF₆) at 546 nm was measured using a Zeiss Axio Scope A1 polarizing microscope. Under positive polarization, Pb₂(SeO₃)(SiF₆) exhibited complete extinction. The optical path difference was determined to be 1.365 μm, with a certain crystal plane at a thickness of 9.31 μm (Fig. S5†). Utilizing the formula R = Δn × T,⁵⁸ the experimental birefringence of Pb₂(SeO₃)(SiF₆) at 546 nm was determined to be 0.147 (Fig. 5). This result aligns with the theoretical birefringence value calculated in the subsequent text.

Fig. 5 Experimental birefringence at 546 nm for Pb₂(SeO₃)(SiF₆).

We have conducted an investigation into the electronic structure and optical properties of Pb₂(SeO₃)(SiF₆) utilizing density functional theory.⁶⁷ The results show that Pb₂(SeO₃)(SiF₆) is a direct bandgap compound, exhibiting a bandgap of 4.17 eV, which aligns closely with the experimental value of 4.4 eV (Fig. S6†). To ensure the precision of subsequent optical property calculations, we used a scissor operator of 0.23 eV. The partial and total density of states of Pb₂(SeO₃)(SiF₆) are shown in Fig. 6a. Notably, the valence band top predominantly consists of O-2p and Se-4s4p orbitals, while the conduction band bottom is primarily composed of Se-4p and Pb-6p orbitals. This indicates that the bandgap of Pb₂(SeO₃)(SiF₆) is mainly determined by Se, Pb, and O atoms.

Fig. 6 The total and partial density of states (a) and the calculated refractive indices and birefringence values (b) of Pb₂(SeO₃)(SiF₆).

To elucidate the theoretical birefringence of Pb₂(SeO₃)(SiF₆), we calculated its birefringence using the formula n²(ω) = ε(ω).⁶⁸ As a biaxial crystal, Pb₂(SeO₃)(SiF₆) exhibits refractive index order: n₀₀₁ > n₀₁₀ > n₁₀₀. The theoretical birefringence values for Pb₂(SeO₃)(SiF₆) are 0.161 at 546 nm, 0.143 at 1064 nm, and 0.139 at 2000 nm, showing a close agreement with the measured value of 0.147 at 546 nm (Fig. 6b). Fig. 7 illustrates the pronounced asymmetry of lobes on Se(IV), indicating of the high stereo–chemical activity of lone pairs, which is much larger than those of Pd(II). From the electron density difference (EDD) map, we can also find that the Se(IV) and Pb(II) groups are almost arranged in straight lines, which leads to the large birefringence of Pb₂(SeO₃)(SiF₆). So, the large birefringence of Pb₂(SeO₃)(SiF₆) was contributed by SeO₃, PbO₄ and PbO₃F₄ polyhedrons primarily. Although the SiF₆ groups contribute little to the birefringence of Pb₂(SeO₃)(SiF₆) directly, the scissor effect of SiF₆ octahedrons make an important role in regulation of the arrangement of lone pair containing polyhedrons.

Fig. 7 The electron density difference (EDD) map of Pb₂(SeO₃)(SiF₆).

Table 1 provides a comprehensive summary of the birefringence of selenite compounds with bandgaps exceeding 4.2 eV reported to date.¹⁸ It can be observed that Pb₂(SeO₃)(SiF₆) exhibits a large birefringence in these selenites, second only to KCl·(H₂SeO₃)₂ (0.17 @ 532 nm).⁶⁴ However, Pb₂(SeO₃)(SiF₆) demonstrates the largest birefringence among the selenites without hydrogen, and it is the sole example of lead selenite with a large birefringence (Δn ≥ 0.1) and wide bandgap (E_g ≥ 4.2 eV). This result highlights the effectiveness of perfluorinated group modification in achieving dual enhancement of the bandgap and birefringence in selenite systems.

Table 1 The birefringence of reported selenite compounds with bandgap ≥4.2 eV

Compound	Space group	E g (eV)	Birefringence	Ref.
K4(H2SeO3)(HSO4)2(SO4)	P-1	—	Cal.: 0.017 @ 1064 nm	59
Pb2Ga3F6(SeO3)2Cl3·2H2O	R32	4.59	Cal.: 0.02 @ 1064 nm	60
NaGa3(HSeO3)6(SeO3)2	P21/n	5.2	Cal.: 0.03 @ 1064 nm	29
Y3F(SeO3)4	P63	5.03	Cal.: 0.036 @ 1064 nm	37
Ga2(Se2O5)2(HSeO3)(H2SeO3)F	Pna21	4.71	Cal.: 0.037 @ 1064 nm	61
Na8(SeO3)(SO4)3	Pna21	5.69	Cal.: 0.038 @ 1064 nm	59
Y(HSeO3)(SeO3)(H2O)·(H2O)	P212121	5.5	Cal.: 0.041 @ 1064 nm	30
Cd5Ga4(HSeO3)2(SeO3)10	P-1	4.7	Cal.: 0.044 @ 1064 nm	29
Al(B(SeO3)3)H2O	Pbca	4.86	Cal.: 0.063 @ 1064 nm	62
Ga(B(SeO3)3)H2O	Pbca	4.79	Cal.: 0.064 @ 1064 nm	62
Ga2(SeO3)3(H2O)3	R3c	5.3	Cal.: 0.079 @ 1064 nm	29
Ga(Se2O5)(HSeO3)	P21/c	4.34	Cal.: 0.079 @ 1064 nm	61
Na2(H2SeO3)(SO4)	P21/c	5.04	Cal.: 0.082 @ 1064 nm	59
Al2(SeO3)3(H2O)3	R3c	5.5	Cal.: 0.084 @ 1064 nm	29
Hf(SeO3)2	P21/c	5.2	Cal.: 0.096 @ 1064 nm	63
CsCl·(H2SeO3)2	P-1	5.46	Cal.: 0.1 @ 532 nm	64
Zr(SeO3)2	P21/c	5.1	Cal.: 0.1025 @ 1064 nm	63
Hg3(SeO3)2(SO4)	P21	4.7	Cal.: 0.103 @ 1064 nm	34
Sc(HSeO3)3	Cc	5.28	Cal.: 0.105 @ 1064 nm	19
Gd(NO3)(Se2O5)·3H2O	P212121	5.53	Cal.: 0.109 @ 1064 nm	65
CaYF(SeO3)2	Pbcn	5.06	Cal.: 0.127 @ 1064 nm	37
NaLu(SeO3)2	Pna21	5.3	Cal.: 0.128 @ 1064 nm	28
CsCl H2SeO3	P21/c	5.23	Cal.: 0.13 @ 532 nm	64
RbCl·(H2SeO3)2	P-1	5.39	Cal.: 0.14 @ 589 nm	66
CsBr·(H2SeO3)2	P21/m	5.19	Cal.: 0.14 @ 532 nm	64
Pb 2 (SeO 3 )(SiF 6 )	Pnma	4.4	Cal.: 0.161 @ 546 nm	This work
KCl·(H2SeO3)2	P-1	5.44	Cal.: 0.17 @ 532 nm	64

---

Conclusions

In summary, the first selenite fluorosilicate, Pb₂(SeO₃)(SiF₆), has been successfully obtained through modification using perfluorinated groups. The compound exhibits a unique 3D structure composed of 2D lead selenite layers interconnected by SiF₆ groups. Excitingly, Pb₂(SeO₃)(SiF₆) achieves a balance between a wide bandgap and large birefringence. It represents a new birefringent crystal with large birefringence (0.161 @ 546 nm), a short UV cut-off edge (247 nm), a broad transmittance window (0.25–6.3 μm), a high laser-induced damage threshold (50.8 MW cm⁻²) and excellent environmental stability. Notably, its birefringence is the largest among non-hydrogen selenite materials with bandgap ≥ 4.2 eV and stands as the only case of a lead selenite material with bandgap ≥4.2 eV and birefringence ≥ 0.1. Theoretical calculations proved that the large birefringence of Pb₂(SeO₃)(SiF₆) primarily originates from SeO₃ groups, PbO₄ and PbO₃F₄ polyhedrons. Our work not only pioneers a new selenite system, but also more importantly demonstrates the significance of perfluorinated groups modification in the development of novel birefringent materials with a large bandgap.

Data availability

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

Author contributions

Peng-Fei Li: investigation, formal analysis, and writing – original draft. Chun-Li Hu: theoretical calculations. Jiang-Gao Mao: supervision, resources, and funding acquisition. Fang Kong: conceptualization, project administration, and writing – review & editing. All authors have given approval to the final version of the manuscript.

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 NSF of Fujian Province (Grant No. 2023J01216).

Notes and references

1. M. Mutailipu, J. Han, Z. Li, F. Li, J. Li, F. Zhang, X. Long, Z. Yang and S. Pan, Nat. Photonics, 2023, 17, 694–701.
2. H. Y. Wu, C. L. Hu, M. B. Xu, Q. Q. Chen, N. Ma, X. Y. Huang, K. Du and J. Chen, Chem. Sci., 2023, 14, 9533–9542.
3. S. J. Han, A. Tudi, W. B. Zhang, X. L. Hou, Z. H. Yang and S. L. Pan, Angew. Chem., Int. Ed., 2023, 62, e202302025.
4. Z. Fang, W. H. Ma, Q. Y. Chen, X. T. Zhu, X. M. Zeng, P. B. Li, Q. F. Zhou, T. T. Song and M. H. Duan, Inorg. Chem. Front., 2024, 11, 1775–1780.
5. G. Zhou, J. Xu, X. Chen, H. Zhong, S. Wang, X. Ke, P. Deng and F. Gan, J. Cryst. Growth, 1998, 191, 517–519.
6. G. Ghosh, Opt. Commun., 1999, 163, 95–102.
7. M. J. Cohen, H. Luo, E. L. Dereniak, T. Tkaczyk, R. Sampson and E. L. Dereniak, presented in part at the Semiconductor Photodetectors III, 2006.
8. M. J. Dodge, Appl. Opt., 1984, 23, 1980–1985.
9. J. R. DeVore, J. Opt. Soc. Am., 1951, 41, 416–419.
10. M. Cheng, W. Q. Jin, Z. H. Yang and S. L. Pan, Chem. Sci., 2022, 13, 13482–13488.
11. J. Guo, A. Tudi, S. Han, Z. Yang and S. Pan, Angew. Chem., Int. Ed., 2021, 60, 24901–24904.
12. Y. Lan, J. Ren, P. Zhang, X. Dong, L. Huang, L. Cao, D. Gao and G. Zou, Chin. Chem. Lett., 2024, 35, 108652.
13. M. Mutailipu, M. Zhang, Z. H. Yang and S. L. Pan, Acc. Chem. Res., 2019, 52, 791–801.
14. W. Zhou and S. P. Guo, Acc. Chem. Res., 2024, 57, 648–660.
15. H. Sha, D. Yang, Y. Shang, Z. Wang, R. Su, C. He, X. Yang and X. Long, Chin. Chem. Lett., 2024, 109730.
16. M. Y. Ran, A. Y. Wang, W. B. Wei, X. T. Wu, H. Lin and Q. L. Zhu, Coord. Chem. Rev., 2023, 481, 215059.
17. J. Chen, C. Lin, X. Jiang, G. Yang, M. Luo, X. Zhao, B. X. Li, G. Peng, N. Ye, Z. Hu, J. Wang and Y. Wu, Mater. Horiz., 2023, 10, 2876–2882.
18. P. F. Li, J. G. Mao and F. Kong, Mater. Today Phys., 2023, 37, 101197.
19. Z. Bai, J. Lee, H. Kim, Y. Kuk, M. H. Choi, C. L. Hu and K. M. Ok, Small, 2023, 19, 2207709.
20. Y. J. Jia, X. Y. Zhang, Y. G. Chen, X. X. Jiang, J. N. Song, Z. S. Lin and X. M. Zhang, Inorg. Chem., 2022, 61, 15368–15376.
21. S. S. Shi, C. S. Lin, G. S. Yang, L. L. Cao, B. X. Li, T. Yan, M. Luo and N. Ye, Chem. Mater., 2020, 32, 7958–7964.
22. Y. P. Gong, C. L. Hu, F. Kong and J. G. Mao, Chem.–Eur. J., 2019, 25, 3685–3694.
23. L. Lin, X. X. Jiang, C. Wu, Z. S. Lin, Z. P. Huang, M. G. Humphrey and C. Zhang, Dalton Trans., 2021, 50, 7238–7245.
24. Y. X. Ma, P. F. Li, C. L. Hu, J. G. Mao and F. Kong, Adv. Sci., 2023, 10, 2304463.
25. J. Ren, H. Cui, L. Cheng, Y. Zhou, X. Dong, D. Gao, L. Huang, L. Cao and N. Ye, Inorg. Chem., 2023, 62, 21173–21180.
26. Y. P. Gong, C. L. Hu, Y. X. Ma, J. G. Mao and F. Kong, Inorg. Chem. Front., 2019, 6, 3133–3139.
27. P. F. Li, C. L. Hu, F. Kong and J. G. Mao, Inorg. Chem., 2023, 62, 8494–8499.
28. P. F. Li, C. L. Hu, J. G. Mao and F. Kong, Mater. Horiz., 2024, 11, 1704–1709.
29. P. F. Li, Y. P. Gong, C. L. Hu, B. Zhang, J. G. Mao and F. Kong, Adv. Opt. Mater., 2024, 12, 2301426.
30. P. F. Li, C. L. Hu, B. X. Li, F. Kong and J. G. Mao, J. Alloys Compd., 2023, 959, 170570.
31. P. Zhang, X. Mao, X. Dong, L. Huang, L. Cao, D. Gao and G. Zou, Chin. Chem. Lett., 2023, 109235,  DOI:10.1016/j.cclet.2023.
32. P. F. Li, C. L. Hu, Y. F. Li, J. G. Mao and F. Kong, J. Am. Chem. Soc., 2024, 146, 7868–7874.
33. P. F. Li, C. L. Hu, F. Kong and J. G. Mao, Mater. Chem. Front., 2022, 6, 3567–3576.
34. J. Ren, Y. Chen, L. Ren, Y. Zhou, X. Dong, D. Gao, L. Huang, L. Cao and N. Ye, Inorg. Chem., 2023, 62, 9130–9138.
35. H. Qiu, S. Pan and M. Mutailipu, Fundam. Res., 2023 DOI:10.1016/j.fmre.2023.08.017.
36. C. Shen, F. Zhang, T. Sasaki, C. Eerdun, M. Hayashi, H.-w. Wang, K. Tominaga, M. Mutailipu and S. Pan, Angew. Chem., Int. Ed., 2024, 63, e202319121.
37. P. F. Li, C. L. Hu, F. Kong and J. G. Mao, Angew. Chem., Int. Ed., 2023, 62, e202301420.
38. Y. Yang, Y. Xiao, B. Li, Y. G. Chen, P. Guo, B. Zhang and X. M. Zhang, J. Am. Chem. Soc., 2023, 145, 22577–22583.
39. G. Xu, H. Li, J. Han, X. Hou, Z. Yang and S. Pan, Inorg. Chem., 2023, 63, 852–859.
40. H. Qiu, F. Li, C. Jin, Z. Yang, J. Li, S. Pan and M. Mutailipu, Angew. Chem., Int. Ed., 2023, 63, e202316194.
41. Y. Hu, X. Jiang, T. Wu, Y. Xue, C. Wu, Z. Huang, Z. Lin, J. Xu, M. G. Humphrey and C. Zhang, Chem. Sci., 2022, 13, 10260–10266.
42. M. Yan, R. L. Tang, W. D. Yao, W. Liu and S. P. Guo, Chem. Sci., 2024, 15, 2883–2888.
43. R. L. Tang, X. Lian, W. D. Yao, W. Liu and S. P. Guo, Dalton Trans., 2021, 50, 16562–16567.
44. Y. Shen, Y. Zhou, X. Xue, H. Yu, S. Zhao and J. Luo, Inorg. Chem. Front., 2022, 9, 5226–5230.
45. P. F. Li, C. L. Hu, B. X. Li, J. G. Mao and F. Kong, Inorg. Chem. Front., 2023, 10, 7343–7350.
46. C. Bai, Y. Chu, J. Zhou, L. Wang, L. Luo, S. Pan and J. Li, Inorg. Chem. Front., 2022, 9, 1023–1030.
47. Y. Y. Yang, Y. Guo, B. B. Zhang, T. Wang, Y. G. Chen, X. H. Hao, X. X. Yu and X. M. Zhang, Inorg. Chem., 2022, 61, 1538–1545.
48. C. Stoll, M. Seibald, D. Baumann, J. Bandemehr, F. Kraus and H. Huppertz, Eur. J. Inorg. Chem., 2021, 2021, 62–70.
49. J. Rienmüller, J. Bandemehr and F. Kraus, Z. Naturforsch., B: J. Chem. Sci., 2021, 76, 559–565.
50. W. Hinteregger, H. Niederwieser and Z. Huppertz, Kristallogr, 2014, 229, 77–82.
51. B. N. E. Brese and M. O'Keeffe, Acta Crystallogr., 1991, B47, 192–197.
52. P. F. Li, F. Kong and J. G. Mao, J. Solid State Chem., 2020, 286, 121288.
53. S. H. Kim, J. Yeon and P. S. Halasyamani, Chem. Mater., 2009, 21, 5335–5342.
54. H. Zhao, P. Gong, X. Zhang, Z. Lin, Z. Hu and Y. Wu, Dalton Trans., 2020, 49, 14046–14051.
55. X. X. Wang, X. X. Jiang, H. M. Liu, L. Yang, Z. S. Lin, Z. G. Hu, X. G. Meng, X. G. Chen and J. G. Qin, Dalton Trans., 2018, 47, 1911–1917.
56. Z. Yang, C. Hu, M. Mutailipu, Y. Sun, K. Wu, M. Zhang and S. Pan, J. Mater. Chem. C, 2018, 6, 2435–2442.
57. P. F. Li, C. L. Hu, B. X. Li, J. G. Mao and F. Kong, Inorg. Chem., 2024, 63, 4011–4016.
58. H. Y. Sha, Y. R. Shang, Z. J. Wang, R. B. Su, C. He, X. M. Yang and X. F. Long, Small, 2023, e2309776,  DOI:10.1002/smll.202309776.
59. X. W. Zhang, Z. X. Wang, C. L. Hu, Y. F. Li, J. G. Mao and F. Kong, Inorg. Chem., 2024, 63, 6067–6074.
60. L. Liu, J. Wang, B. Xiao, X. Li, Y. Chu, T. Sun and P. S. Halasyamani, J. Mater. Chem. C, 2024, 12, 4986–4994.
61. J. Zhang, H. Yu, Z. Hu, J. Wang, Y. Wu and H. Wu, Mater. Today Phys., 2023, 35, 101145.
62. M. Y. Cao, C. L. Hu, F. Kong, Z. Y. Xiong and J. G. Mao, Dalton Trans., 2021, 50, 15057–15061.
63. J. Lv, Q. Li, X. Guan, N. Lin, J. Zhang, Z. Jia and X. Tao, CrystEngComm, 2023, 25, 1675–1682.
64. M. Zhu, J. Wang, L. Hou, Y. Yuan, L. Liu, Y. Chu and C. Huang, Inorg. Chem., 2024, 63, 2289–2297.
65. C. Wu, L. H. Li, L. Lin, Z. P. Huang, M. G. Humphrey and C. Zhang, Dalton Trans., 2020, 49, 3253–3259.
66. H. Wang, L. Liu, Z. Hu, J. Wang, M. Zhu, Y. Meng and J. Xu, Inorg. Chem., 2022, 62, 557–564.
67. C. L. Hu, Y. X. Han, Z. Fang and J. G. Mao, Chem. Mater., 2023, 35, 2647–2654.
68. H. Chen, M. Y. Ran, S. H. Zhou, X. T. Wu, H. Lin and Q. L. Zhu, Chin. Chem. Lett., 2023, 34, 107838.

---

Footnote

† Electronic supplementary information (ESI) available: Experimental section, computational method, crystal data, important bond distances and bond valences, element analysis, PXRD patterns, TGA curves, IR spectrum, UV-visible-NIR diffuse reflectance spectrum and band structures. CCDC 2327037. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4sc01376j

---