Na₆Sn₃P₄S₁₆: Sn(II)-chelated PS₄ groups inspired an ultra-strong SHG response

Abstract

The discovery of new infrared nonlinear optical (IR NLO) materials with a large second harmonic generation (SHG) effect and broad optical bandgap is still a challenging task. Based on this, we propose a rational strategy for the design of new NLO materials: (i) the coupling of stereochemically active lone-pair (SCALP) Sn(II) cations and typical PS₄ units into a structure enhances the SHG response; (ii) the incorporation of an alkali metal (Na⁺) cation benefits the preservation of a relatively wide bandgap, which affords the discovery of a new promising IR NLO thiophosphate, Na₆Sn₃P₄S₁₆. It exhibits an ultra-strong SHG response (6.6 × AgGaS₂) and relatively wide bandgap (2.52 eV) that is better than those of known NLO SnPS₃ (1.1 × AgGaS₂ and 2.35 eV). Remarkably, Na₆Sn₃P₄S₁₆ exhibits the largest SHG response among all known IR NLO thiophosphates with bandgaps greater than 2.50 eV. Such an ultra-strong SHG response in Na₆Sn₃P₄S₁₆ originates from the collaborative polarization of edge-sharing SnS₄ and PS₄ units. This work inspires a new way to design new IR NLO thiophosphates with enhanced SHG responses by introducing Sn(II)-chelated PS₄ groups into structures.

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Introduction

Nonlinear optical (NLO) crystals have come to play critical roles in the fabrication of tunable lasers for the development of modern civil–military technology.^1–7 In past decades, NLO crystals with excellent performance in the ultraviolet (UV) and visible regions (e.g., β-BaB₂O₄ (β-BBO), LiB₃O₅ (LBO), KTiOPO₄ (KTP) and KH₂PO₄ (KDP)) have been rapidly developed and industrialized.^8,9 Mid-infrared (IR) lasers have also shown significant applications in remote communication and IR detection.^10–13 Unfortunately, a few conventional commercial IR NLO crystals, such as AgGaQ₂ (Q = S, Se)¹⁴ and ZnGeP₂,¹⁵ with several inherent defects such as a low laser damage threshold (LDT), harmful two-photon absorption and small birefringence, have hindered their range of application. Limited by the property requirements (wide IR transmission range, strong second harmonic generation (SHG) response, broad optical bandgap and large optical anisotropy) for an excellent IR NLO crystal, there is still a lack of outstanding NLO crystals for IR applications.^16–18 So far, extensive work has been undertaken to explore IR NLO materials and several effective design strategies have been proposed: for example, coupling highly distorted anionic units into a structure, such as various anionic groups with centered d⁰ or d¹⁰ transition metals or stereochemically active lone-pair (SCALP) cations, has been proven to have a positive impact on the SHG response.^19–23 In addition, introducing alkali or alkaline-earth metals as cations into crystal structures can effectively broaden the optical bandgap of targeted crystals.²⁴ Recently, chalcogenides containing SCALP metal cations (Sn²⁺, Pb²⁺, As³⁺, Sb³⁺ and Bi³⁺) were selected as optimal systems to search for IR NLO materials with strong SHG effects. As for cations with SCALP, their outermost electrons are not shared with other ions that generate repulsive interactions for the formation of highly distorted polyhedral groups, which can induce positive symmetry-breaking for the potential development of NCS or even chiral structures. This SCALP strategy can definitely be beneficial for improving the probability of obtaining NCS structures, but cannot completely guarantee an NCS structure. However, SCALP cation-centered polyhedra usually exhibit large distortion and a uniform polarized arrangement, which indicates that they have huge potential for an enhancement of inherent NLO effects.^25–30 Notably, introducing two or more types of NLO-active functional units into one structure is an effective way to strengthen the SHG effect.^31–33 Moreover, strong covalent P–S bonds in the tetrahedral PS₄ also contribute to the improved NLO response, and a series of thiophosphates have become more established as IR NLO candidates.^34–39 On the basis of combining the above strategies, we have successfully synthesized a new NCS compound, Na₆Sn₃P₄S₁₆, in the thiophosphate system by coupling the SCALP cation Sn(II) and a PS₄ unit. Na₆Sn₃P₄S₁₆ exhibits a peculiar low-dimensional cluster structure consisting of edge-sharing SnS₄ and PS₄ tetrahedra. Remarkably, Na₆Sn₃P₄S₁₆ exhibits an ultra-strong SHG response (6.6 × AgGaS₂) and a relatively wide bandgap (2.52 eV), showing the largest SHG effect among all reported IR NLO thiophosphates with a bandgap greater than 2.50 eV (Fig. 1a).^{34–37,40–51} Compared with known SnPS₃,⁵² Na₆Sn₃P₄S₁₆ also shows an obvious improvement in bandgap and SHG effect (Fig. 1b), illustrating the importance of the Sn(II)S₄ unit for material performance. Theoretical calculation further confirms that the collaborative polarization of edge-sharing Sn(II)S₄ and PS₄ units provides a huge contribution to the strong SHG response, which also offers us a new way to design new IR NLO candidates through coupling Sn(II)-chelated PS₄ groups into structures.

Fig. 1 (a) Summary of the SHG response and bandgap among Na₆Sn₃P₄S₁₆ and reported IR NLO thiophosphates with bandgaps greater than 2.50 eV. Detailed data are listed in Table S1.† (b) Histogram comparison for the NLO data of AgGaS₂, Na₆Sn₃P₄S₁₆ and SnPS₃.

Experimental methods

Materials

All raw reagents (Na, SnS, P and S powder) were purchased from the Beijing Hawk Science & Technology Co., Ltd with a high-purity level (99.9%). Herein, the storage and weighing processes were carried out in an Ar-filled glovebox (the oxygen and water vapour contents were lower than 0.1 ppm) to prevent oxidation of the Na metal.

Synthesis

Microcrystals of Na₆Sn₃P₄S₁₆ were synthesized in a stoichiometric proportion by the high-temperature solid-state method. The detailed steps are as follows: (1) A mixture of Na (6 mmol), SnS (3 mmol), P (4 mmol), and S (13 mmol) was first weighed and loaded into a graphite crucible, then put into a silica tube; (2) The silica tube was pumped to a 10^–3 Pa vacuum and sealed with a flame; (3) The silica tubes were placed in a muffle furnace; (4) A rising temperature program was set to heat the tube to 750 °C at 15 °C h⁻¹ and it was kept at this temperature for about 100 h, then slowly cooled down to room temperature over 3 days. Finally, many millimeter-level yellow crystals for Na₆Sn₃P₄S₁₆ (the yield >90%) were obtained and found to be stable in air for several months.

Single crystal X-ray diffraction

Selected high-quality single crystals of Na₆Sn₃P₄S₁₆ were fixed on glass fibers with epoxy. Diffraction data were collected on a Bruker D8 VENTURE diffractometer using Mo Kα radiation (λ = 0.71073 Å) at 296 K. The crystal structure was determined by the direct method and refined by full-matrix least-squares fitting on F² using the SHELXTL program package.⁵³ The PLATON program was used to verify the final structures, and no higher symmetries were found.⁵⁴ A multi-scan method was used for absorption correction. Anisotropic refinement and extinction corrections achieved rational anisotropic thermal parameters for all atoms.

Powder X-ray diffraction

In order to determine the purity of the samples, the powder X-ray diffraction (XRD) patterns of Na₆Sn₃P₄S₁₆ were collected using a Bruker D2 X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å) at room temperature. The 2θ range was 10–70° with a step size of 0.02° and a fixed counting time of 1 s per step. Using Mercury software, the calculated XRD patterns were derived from the respective single-crystal data. The experimental XRD patterns are consistent with the corresponding calculated patterns (Fig. 4a), indicating the purity of the samples and the accuracy of the structural model.

UV-vis-near-IR (NIR) diffuse-reflectance spectra

Diffuse-reflectance spectra were measured with a Shimadzu SolidSpec-3700DUV spectrophotometer in the wavelength range of 200–1100 nm at room temperature, with BaSO₄ as the standard reference. The absorption data were calculated using the Kubelka–Munk formula: α/S = (1 − R)²/2R, where α, S and R represent the absorption, scattering and reflection coefficients.

IR and Raman spectra

The infrared spectrum was carried out on a Shimadzu IR Affinity Fourier transform infrared spectrometer in the range of 400–4000 cm⁻¹ with a resolution of 4 cm⁻¹. The crystals were mixed well with spectrally pure KBr in a ratio of approximately 1 : 100 as the sample for measurement. Then, the sample was dried and ground into a fine powder under an IR lamp. Powder samples were pressed into a transparent sheet. Finally, the sheet was loaded into the sample chamber, and the IR spectral data were recorded.

High-quality crystals were placed on object slides for measurement. Then, the Raman spectrum was recorded with a 532 nm laser using a LabRAM HR Evolution spectrometer equipped with a CCD detector. The integration time was set to 5 seconds.

Second-harmonic generation measurement

The powder SHG response was measured using the Kurtz and Perry method with a Q-switch laser (2.09 μm, 3 Hz, 50 ns). The polycrystalline sample was ground and sieved into different particle sizes: 38–55, 55–88, 88–105, 105–150, 150–200, and 200–250 μm. AgGaS₂ crystals with the same size range were used as the reference.

Theoretical calculations

The electronic structure and partial density of states (PDOS) of Na₆Sn₃P₄S₁₆ were studied using density functional theory (DFT) calculations to investigate the structure–property relationship further.⁵⁵ The exchange–correlation potential of Na₆Sn₃P₄S₁₆ was calculated using the Perdew–Burke–Ernzerhof (PBE) functional within the generalized gradient approximation (GGA) method.⁵⁶ The following orbital electrons were treated as valence electrons: Na: 2p⁶ 3s¹; Sn: 5s² 5p²; P: 3s² 3p³; S: 3s² 3p⁴. The plane-wave basis set energy cutoff was 720 eV within the normal-conserving pseudo-potential (NCP) to achieve energy convergence.⁵⁷ As important parameters for IR NLO material, the theoretical SHG coefficients and birefringence (Δn) of Na₆Sn₃P₄S₁₆ were also calculated using a suitable scissor operator. The dipole moment and SHG-density calculations were also performed to further analyze the SHG contributions of different functional units.

Results and discussion

Na₆Sn₃P₄S₁₆ crystallizes in the non-centrosymmetric (NCS) trigonal space group R3m (no. 160) with lattice parameters a = b = 19.304(4) Å, c = 6.181(2) Å and Z = 3. Its asymmetric unit consists of one crystallographically independent Na atom, one Sn atom, two P atoms and five S atoms, with Na1 and S2 in the Wyckoff 18c position, P1 and S1 at 3a and the other atoms (Sn1, P2, S3, S4 and S5) at 9b. Table S2 in the ESI† provides more detailed information on crystal data and structural refinement. As shown in Fig. 2b, Na atoms are coordinated with six S atoms, forming slightly distorted NaS₆ octahedra with bond lengths of 2.809(6)–3.138(7) Å. NaS₆ are linked together through shared corners and edges to create closed 6-membered rings (MRs), which allows the unique isolated pinwheel-shaped [Sn₃(PS₄)₄] cluster to fill the channels and finally form the final three-dimensional (3D) structure of Na₆Sn₃P₄S₁₆ (Fig. 2a). In the process of experimental synthesis, we also attempted to synthesize other alkali-metal-based analogues. For example, we attempted to synthesize the hypothetical K₆Sn₃P₄S₁₆, but only the reported KSnPS₄ was obtained. As for (Rb,Cs)-based analogues, neither of them was synthesized in this work after we made several attempts. Seen from the structure of Na₆Sn₃P₄S₁₆, NaS₆ units link together to compose 6-MR tunnel structures, and isolated [Sn₃(PS₄)₄] clusters are located within the tunnels. While Na cations are substituted by other alkali metals (A = K, Rb, Cs) with larger radii, even if their longer A–S (A = K, Rb, Cs) bond lengths can still form 6-MRs with larger channels, these tunnel structures may not be suitable for the existence of previous [Sn₃(PS₄)₄] clusters. Therefore, the introduction of other alkali metals into the structure of Na₆Sn₃P₄S₁₆ may destabilize the original structures and make it more difficult for us to achieve the experimental synthesis. Note that these [Sn₃(PS₄)₄] clusters are stacked in the same direction along the c-axis, which may enhance the polarization rate and NLO effect. Sn and P atoms are quadruply coordinated with S atoms to form SnS₄ and PS₄ tetrahedra. The three SnS₄ are connected through the longest Sn–S bond to share corners, forming an [Sn₃S₁₀] trimer. The coordination environment of Sn atoms can be specifically described as an irregular trigonal pyramid, which demonstrates the high distortion of SnS₄ (Fig. 2c and d). Herein, we also investigated the structures of 12 known Sn(II)-based chalcogenides in the Inorganic Crystal Structure Database (ICSD) (Table S3†) and the results show that they exhibit the following functional groups: SnS₃,^58,59 SnS₄,^28,60 SnS₅,^61,62 SnS₆⁶³ and SnS₈⁵² units in their structures; only Sn₂Ga₂S₅⁵⁵and SnGa₄S₇²⁸ have a similar SnS₄ unit to that of titular Na₆Sn₃P₄S₁₆. The Sn–S bond lengths were in the range from 2.759(3) to 3.018 Å, which is also in accordance with those of 2.636(4) to 3.157(6) Å in Sn₂Ga₂S₅,⁶⁰ 2.6394(17) to 3.3552(21) Å in Sn₂SiS₄⁶³ and 2.7041(12) to 3.0180(2) Å in LaSnGa₃S₇.⁶² In addition, P1–S bonds are 2.071(5) Å and 2.004(12) Å, and P2–S bonds are 2.064 (4) Å, 2.032(7) Å and 2.028(6) Å, which can be also found in other thiophosphates, such as Ag₇Sn(PS₄)₃:⁵⁹ 2.018–2.083 Å; AgZnPS₄:⁴³ 2.037(2) Å to 2.058(3) Å; or AgCd₃(PS₄)S₂:⁵⁰ 2.038(3) Å to 2.064(3) Å (Fig. 2d and e). Herein, we also compared the structural differences between Na₆Sn₃P₄S₁₆ and SnPS₃ (Fig. 3). In SnPS₃, Sn(II) atoms are in eight-coordinated mode with S atoms to form an Sn(II)S₈ dodecahedron, which is different from the Sn(II)S₄ unit in Na₆Sn₃P₄S₁₆. Besides, SnPS₃ exhibits a P₂S₆ unit with a P–P bond and P₂S₆ units connected with Sn(II)S₈ to form a 3D network, which is also different from the 0D [Sn₃(PS₄)₄] clusters in Na₆Sn₃P₄S₁₆.

Fig. 2 (a) Whole structure of Na₆Sn₃P₄S₁₆ along the c-axis. (b) NaS₆ octahedra connected together to form channel-shaped closed rings. A 6-MR is indicated with black lines. (c) The pinwheel-like [Sn₃(PS₄)₄] cluster consists of one P(1)S₄, three P(2)S₄, and three SnS₄ units. (d) The 4-membered ring formed by P(2)S₄ and SnS₄. (e) The 4-membered ring formed by P(1)S₄ and SnS₄.

Fig. 3 (a) The combinatorial arrangement of structural units SnS₄ and PS₄ in Na₆Sn₃P₄S₁₆. (b) The combinatorial arrangement of structural units SnS₈ and P₂S₆ in SnPS₃. (c) The SnS₄ and P(1)S₄, P(2)S₄ units are connected through edge sharing in the structure of Na₆Sn₃P₄S₁₆. (d) The Sn(1)S₈ and Sn(2)S₈ are connected to P₂S₆ units by edge sharing in the structure of SnPS₃.

The measured diffuse-reflectance spectrum (Fig. 4b) shows that the experimental optical bandgap of Na₆Sn₃P₄S₁₆ is 2.52 eV, which is consistent with its crystal color (yellow). Also, in combination with the IR transmission spectrum analysis, Na₆Sn₃P₄S₁₆ has a broad IR transparent region (2.5–15.0 μm) covering the two most critical atmospheric windows of 3–5 and 8–14 μm. Moreover, the presence of IR absorption peaks at 526 and 568 cm⁻¹ can be attributed to P–S bond vibrations (Fig. 4c), and similar cases can be found in previously reported thiophosphates, such as AgGa₂PS₆⁴⁶ with IR absorption peaks at 531, 560, 580, and 590 cm⁻¹ and CuCd₃PS₆⁶⁴ at 524, 542, and 573 cm⁻¹. The experimental Raman spectrum measured under 532 nm laser radiation indicates that the Raman peaks (410–584 cm⁻¹) can also be attributed to the same characteristic absorption of the P–S bonding mode (Fig. 4d) as for Hg₃P₂S₈³⁴ (360–600 cm⁻¹) and K₃ReP₂S₈⁴⁵ (400–600 cm⁻¹). While the peaks at 222, 217, and 313 cm⁻¹ originate from the vibration of the Sn(II)–S bond. In addition, the thermal behavior of Na₆Sn₃P₄S₁₆ was measured using a tiny custom-made vacuum-sealed silica tube, which has good thermal stability below 400 °C. As shown from its DSC curve (Fig. S2†), Na₆Sn₃P₄S₁₆ has an endothermic peak at 412 °C, which can be attributed as its melting point. This is much lower than the melting point of commercial IR NLO crystals (AgGaS₂: 996 °C; ZnGeP₂: 1298 °C). To gain insights into the intrinsic relationship between structure and properties, we performed theoretical calculations on the electronic structure and density of states (DOS) of Na₆Sn₃P₄S₁₆. The theoretical band structure shows a theoretical E_g of 2.05 eV for Na₆Sn₃P₄S₁₆ (Fig. S3a†), which is smaller than the experimental value due to the inherent defects of GGA calculations. As can be seen from the DOS diagram (Fig. S3b†), the valence band top (VB) and conduction band bottom (CB) regions are mainly occupied by S-p, Sn-p, and P-p orbitals. Therefore, optical absorption of Na₆Sn₃P₄S₁₆ is determined by the combined influence of Sn(II)S₄ and PS₄ units.

Fig. 4 Experimental XRD patterns (a), optical bandgap (b), IR spectrum (c) and Raman spectrum (d) of Na₆Sn₃P₄S₁₆.

Based on the typical Kurtz–Perry method, SHG response measurement was performed under a 2.09 μm Q-switched laser with six different particle sizes. AgGaS₂ microcrystals with similar particle sizes were selected as a reference. Na₆Sn₃P₄S₁₆ exhibits a strong powder SHG response of about 6.6 times that of benchmark AgGaS₂ and its SHG intensities show an enhanced trend with increasing particle size, showing its essential phase-matching (PM) behaviour (Fig. 5a). It should also be noted that the SHG effect of Na₆Sn₃P₄S₁₆ is greater than those of other known PM IR NLO thiophosphates, such as HgCuPS₄ (6.5 × AgGaS₂),⁶⁵ AgHgPS₄ (5 × AgGaS₂),³⁶ KHgPS₄ (4.15 × AgGaS₂),³⁵ or Hg₃P₂S₈ (3.6 × AgGaS₂),³⁴ achieving a further breakthrough in this system. Moreover, Na₆Sn₃P₄S₁₆ exhibits the largest SHG response among all known IR NLO thiophosphates with bandgaps greater than 2.50 eV. We also calculated the theoretical NLO coefficient and the maximum value is about 69.3 pm/V, which is consistent with the experimental result. Furthermore, the theoretical birefringence (Δn) of Na₆Sn₃P₄S₁₆ was estimated to be 0.12 at a wavelength of 2 μm (Fig. 5b), which is in good agreement with the experimental PM behavior. SHG-density calculation is also used to analyze the origin of the SHG response and the result shows that its SHG origin can be attributed to the synergistic contribution between Sn(II)S₄ and PS₄ units (Fig. 6). Dipole-moment calculation (Table S4†) also shows that the dipole-moment of Sn(II)S₄ is about twice that of the PS₄ unit, showing the huge influence of the SCALP Sn(II)S₄ group on the origin of the NLO response. Moreover, an electron localization function (ELF) map of Na₆Sn₃P₄S₁₆ was calculated and the result further confirms the existence of lone-pair orbitals for the Sn(II) atoms. Note that the stereochemically active lone-pair of an Sn(II) atom induces strong distortion in the Sn(II)S₄ unit and further regulates the structural anisotropy, which leads to the final large birefringence of Na₆Sn₃P₄S₁₆ (Fig. S4†). This work inspires us that coupling of Sn(II)S₄ and PS₄ units into a structure provides a feasible way to explore IR NLO candidates with strong SHG effects.

Fig. 5 (a) Powder SHG response versus particle size in Na₆Sn₃P₄S₁₆ with AgGaS₂ as a reference. (b) Calculated birefringence in Na₆Sn₃P₄S₁₆.

Fig. 6 Calculated SHG-density diagrams in occupied and unoccupied states of Na₆Sn₃P₄S₁₆.

Conclusions

In summary, a promising IR NLO crystal, Na₆Sn₃P₄S₁₆, was successfully designed and synthesized by introducing a SCALP Sn(II) cation into the structure of a thiophosphate. Na₆Sn₃P₄S₁₆ exhibits the largest powder SHG response of about 6.6 times that of AgGaS₂ with PM behavior among all known PM IR NLO thiophosphates (>2.50 eV) and still maintains a relatively broad optical bandgap (2.52 eV). Theoretical calculation indicates that the synergistic contribution of edge-sharing SnS₄ and PS₄ units affords the origin of SHG, which also verifies that incorporation of an Sn(II)S₄ unit into thiophosphate could be regarded as an effective method to design and synthesize IR NLO crystals with large SHG effects.

Author contributions

The manuscript was written through contributions of all authors. 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 Nos. 52372009, 51872324), the Natural Science Foundation of Hebei Province (Grant No. E2020201005), the Science Foundation for Distinguished Young Scholars of Hebei Province (B2021201045), the Central Guidance on Local Science and Technology Development Fund of Hebei Province (226Z2402G).

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Footnote

† Electronic supplementary information (ESI) available: Performance comparison, crystal data, structural summary, BS and DOS. CCDC 2278142. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3qi01257c

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