γ-P₄S₃I₂: a new metal-free infrared second-order nonlinear optical crystal designed by polymorphism strategy

Abstract

A non-centrosymmetric metal-free thiophosphate, γ-P₄S₃I₂, was successfully synthesized utilizing polymorphism strategy in this study. γ-P₄S₃I₂ crystallized in the space group of P4₃ and featured paralleled (P₄S₃I₂)_n molecular clusters. Importantly, it exhibited promising nonlinear optical (NLO) performances, such as a phase-matchable second harmonic generation (SHG) efficiency (0.5 × AgGaS₂), wide band gap (2.38 eV), large birefringence (0.118@2050 nm), and wide infrared transparency, suggesting that γ-P₄S₃I₂ may be a potential IR NLO candidate.

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Introduction

Exploring non-centrosymmetric (NCS) inorganic compounds is of great significance in the development of piezoelectric, pyroelectric, ferroelectric, and especially nonlinear optical (NLO) materials.^1–6 NLO materials are indispensable for applications in medical treatment, atmospheric detection and optical communication relying on frequency conversion.^7,8 To date, many well-known NLO crystals, such as β-BaB₂O₄, LiB₃O₅ and CsLiB₆O₁₀ as well as KTiOPO₄ and LiNbO₃, have basically met the needs in the UV and visible (vis) regions.^9–12 However, commercially available NLO crystals used in the infrared (IR) region are still inhibited by some intrinsic drawbacks, such as the poor laser damage threshold (LDT) of AgGaS₂ and AgGaSe₂ and the two-photon absorption (TPA) of ZnGeP₂,¹³ which have restricted their further practical applications. Therefore, exploring new IR NLO crystals is still of current research interest.

Generally, inorganic crystals possess a strict structurally non-centrosymmetric space group, which is the prerequisite for NLO materials. However, about 70% of inorganic compounds are crystallize in centrosymmetric space groups,¹⁴ which indicates that NCS compounds are not easily accessible. Thus, some strategies, including chemical substitution, mixing anions and salt-inclusion, have been proposed to obtain NCS compounds, which have resulted in plenty of IR NLO crystals being obtained, such as Li₂ZnSiS₄ (3.9 eV, 1.1 × AgGaS₂), Pb₄SeBr₆ (2.62 eV, 1.3 × AgGaS₂), Pb₁₈O₈Cl₁₅I₅ (2.82 eV, 1.05 × AgGaS₂) and Li[LiCs₂Cl][Ga₃S₆] (4.18 eV, 0.7 × AgGaS₂).^15–18 Recently, polymorphous modification has been regarded as an effective strategy for exploring new IR NLO crystals, which has been attributed to the following reasons: (i) polymorphism is a common phenomenon in crystalline materials; and (ii) the arrangement of the building units in the structure can be further optimized, which might enhance the SHG effect, for example, [Ga₄Se₁₁] C₂-type supertetrahedra in α-BaGa₄Se₇ can be reconstituted to T₂-type [Ga₄Se₁₀] supertetrahedra in β-BaGa₄Se₇ with a stronger SHG response.¹⁹ Until now, during the process of exploring new NLO polymorphism, most attention has been focused on borates and Pb-based compounds because the variable architectures of [B_xO_y] and the flexible coordination environment of the Pb²⁺ cation (2–10) were found to be favorable in forming polymorphs.^20,21 Thus, many polymorphous borates and Pb-based compounds were reported, such as α-BaB₄O₅F₄, α-BaBOF₃, α-LiPbB₉O₁₅ and β-PbGa₂S₄.^22–25 Moreover, using additives and adjusting temperatures were also helpful to obtain new polymorphs, for instance, β-Sc(IO₃)₃ ²⁶ can be obtained with Li₂CO₃ as an additive. Besides, β- and γ-BaGa₄S₇, β-BaGa₄Se₇ and β-BaGa₂Se₄ ^27,28 can be synthesized at differing appropriate temperatures. In this study, we proposed thiophosphates as a new system to explore NLO polymorphism.

Thiophosphates are a very promising system for mid-IR NLO applications because the existence of strong covalent P–S bonds not only improves the NLO response but can also drive the blue shift of the short-wave absorption edge;²⁹ for example, Hg₃P₂S₈ (2.72 eV, 4.2 × AgGaS₂), Eu₂P₂S₆ (2.54 eV, 0.9 × AgGaS₂), RbBiP₂S₆ (2.10 eV, 11.9 × AgGaS₂) and CuHgPS₄ (2.03 eV, 6.5 × AgGaS₂) exhibit good IR-NLO performances.^30–33 In addition, positive valence P atoms have a flexible coordination environment (2–4), which is favorable in forming polymorphs. For example, the polymorphism of P₂S₇ (α: P2₁/c, β: P1̄)³⁴ exhibits two different crystal structures. Moreover, until now, there have been no studies on the NLO properties of metal-free thiophosphates, and thus, we have a strong interest in studying the title compound. Therefore, in this work, we have focused on the P–S–I system and a new non-centrosymmetric polymorph of P₄S₃I₂ was discovered and named γ-P₄S₃I₂, with the previously reported phases named as α-P₄S₃I₂ (P1̄) and β-P₄S₃I₂ (Pnma).^35,36 γ-P₄S₃I₂ was successfully synthesized via solution evaporation at low temperature, and its linear and nonlinear optical properties were studied experimentally and theoretically. Results show that this compound exhibits a suitable SHG response (0.5 × AgGaS₂), wide transparency window, and large birefringence (0.118@2050 nm), indicating that γ-P₄S₃I₂ is a promising mid-IR NLO candidate.

Experimental section

Synthesis and crystal growth

All of the chemicals, carbon disulfide CS₂ (99.9%), S (99.99%), P (99.99%) and I₂ (99.99%), were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd and used without further purification. The synthesis of γ-P₄S₃I₂ was operated in two steps. First, P₄S₃ was prepared by heating a stoichiometric mixture of P and S in an evacuated silica tube at 300 °C for 24 h. Second, P₄S₃ and I₂ were evenly mixed with a ratio of 1 : 1 and loaded into an evacuated silica tube and then sealed under vacuum. After that, the mixture was heated to 180 °C in a drying oven, held for about 2 days, and then cooled to room temperature naturally, whereupon molten P₄S₃I₂ was obtained. γ-P₄S₃I₂ was prepared by recrystallizing amorphous P₄S₃I₂ in CS₂ at −10 °C. The millimeter-sized crystal of γ-P₄S₃I₂ was grown directly by an evaporation method. The recrystallized γ-P₄S₃I₂ (1.5 g) was dissolved in dried CS₂ (12 ml) in a round bottom centrifuge tube with a cover, and then the centrifuge tube was put into a refrigerator at −30 °C and held for about 5 days; the products were pure deep yellow block-shaped single crystals without byproducts.

Single crystal structure determination

Single crystal X-ray diffraction data for γ-P₄S₃I₂ were collected on a Rigaku Mercury CCD diffractometer with graphite-monochromatic Mo Kα radiation (λ = 0.71073 Å) at room temperature. Some γ-P₄S₃I₂ crystals were cut into appropriate sizes on a slide; for measurements, a high-quality single crystal was selected and mounted on top of a glass fiber using epoxy. The intensity data were corrected using a narrow-frame method in ω-scan mode. All data were integrated based on the CrystalClear program. The intensities were corrected for Lorentz polarization, air absorption, and absorption attributable to variation in the path length through the detector faceplate. Absorption corrections were also achieved by using a multi-scan technique. The crystal structures were determined by means of the direct methods and refined by full-matrix least-squares fitting on F² using SHELXL.^37,38 All the atoms were refined with anisotropic displacement parameters. The ADDSYM algorithm from the PLATON program³⁹ was used to monitor the correctness of the structures and no higher symmetry was found. Relevant crystallographic data and details of the experimental conditions are listed in Table S1.† Besides, the data of atomic coordinates and equivalent isotropic displacement parameters, selected bond lengths and angles, and anisotropic displacement parameters are summarized in Tables S2–S4.†

Powder X-ray diffraction

The powder X-ray diffraction analysis of compound γ-P₄S₃I₂ was implemented on a Miniflex-600 diffractometer with Cu Kα radiation (λ = 1.540598 Å) at room temperature. The angular range was 2θ = 10–70° with a scan step width of 0.02° and a fixed time of 0.2 s. The powder XRD pattern of polycrystalline materials matched with the calculated XRD pattern from the single crystal model (Fig. S1†), indicating that pure samples could be used for subsequent measurements.

Energy dispersive X-ray spectroscopy (EDS) analysis

The EDS analysis of γ-P₄S₃I₂ crystals was carried out on a field emission scanning electron microscope (FESEM, SU-8010) equipped with an energy dispersive X-ray spectrometer. The shaped crystals were rinsed using carbon disulfide and absolute ethyl alcohol, and then were affixed on the copper sample stage with a carbon conductive tape. Different regions on the crystals were tested with a focused beam, accelerating voltage of 20 kV and emission current of 12 μA (Fig. S2†).

Thermal analysis

The thermogravimetric (TG) analysis of γ-P₄S₃I₂ was conducted on a Netzsch STA449F3 simultaneous analyzer. The reference (Al₂O₃) and crystal samples (5–10 mg) were enclosed in an Al₂O₃ crucible, heated from 30 to 1000 °C at a rate of 10 °C min⁻¹ under a constant flow of nitrogen gas, and then cooled to room temperature naturally.

UV-vis-NIR diffuse reflectance spectroscopy

The UV-vis-NIR diffuse reflection data of γ-P₄S₃I₂ were collected on a PerkinElmer Lamda-950 ultraviolet/visible/near-infrared spectrophotometer at room temperature in the range of 400–800 nm with BaSO₄ as the standard of 100% reflectance. The reflectance values were converted to absorbance based on the Kubelka–Munk function F(R) = (1 − R)²/(2R) = K/S, where R is the reflectance, K is the absorption, and S is the scattering.^40,41

Fourier-transform infrared (FT-IR) spectroscopy

The Fourier-transform infrared (FT-IR) transmittance spectra were measured on a Bruker VERTEX 70 FT-IR spectrophotometer in the range of 4000–400 cm⁻¹. Dry KBr was ground into fine powder and then pressed into a transparent wafer as the reference. The powder sample and dry KBr were mixed and pressed into the same wafer for the measurements.

Powder SHG and LDT measurements

Polycrystalline SHG responses were measured using the Kurtz–Perry method⁴² with a Q-switched Nd:YAG solid-state laser at a laser radiation wavelength of 2050 nm. Polycrystalline γ-P₄S₃I₂ was ground and sieved into several different particle size ranges of 25–45, 45–63, 63–74, 74–106, 106–150, and 150–210 μm. Polycrystalline AgGaS₂ was also ground and sieved into the same particle size ranges as the reference. After that, samples were added into aluminous holders and pressed into 1 mm thick slides between two glass sheets bound with a 2 mm thick rubber ring, containing an 8 mm diameter hole in the center. Subsequently, the samples were then placed in a light-tight box and under the irradiation of a pulsed laser. The intensities of the SHG signals were measured with a photomultiplier tube attached to a RIGOL DS1052E 50 MHz oscilloscope. As a result, the ratio of the intensity of the SHG signals between the samples and the reference can eventually be calculated. The laser damage threshold (LDT) measurement was executed at room temperature using microcrystal samples of γ-P₄S₃I₂ and AgGaS₂ samples of similar sizes (150–212 μm) under a pulsed Nd:YAG laser (1064 nm, 1 Hz, 10 ns). The LDT was evaluated through increasing the energy of the laser gradually until the damage spot was observed. The value of the LDT can be calculated based on eqn (1):

I_(threshold) = E/(πr²t) (1)

where E, r, and t are the laser damage energy of a single pulse, the spot radius and the pulse width, respectively. Detailed data are listed in Table S5.†

Computational methods

The first-principles calculations for the physical properties of γ-P₄S₃I₂ were performed by using CASTEP,⁴³ a plane-wave pseudopotential total energy package based on density functional theory (DFT).⁴⁴ The exchange and correlative potential of electron–electron interactions were represented by generalized gradient approximation (GGA) in the scheme of Perdew–Burke–Ernzerhof (PBE).⁴⁵ Furthermore, the interaction of the electrons with ion cores was represented by the norm-conserving pseudopotentials, and the valence electrons were expressed as P: 3s² 3p³, S: 3s² 3p³ and I: 5s² 5p⁵. The k-point of the first Brillouin zone for γ-P₄S₃I₂ was sampled as the 2 × 2 × 1 Monkhorst–Pack scheme,⁴⁶ which was used to calculate the optical properties and density of states (DOS). The cut-off energy was set to be 650 eV and the self-consistent convergence of the total energy was 1.0 × 10⁻⁵ eV per atom. The scissor operation was adopted in the dielectric function calculation owing to the inherent underestimation of the band gap by the DFT method. In addition, the “velocity-gauge”^47,48 formula was employed to calculate the SHG coefficients of γ-P₄S₃I₂, and the SHG density of d₃₃ was calculated by the band-resolved method.⁴⁹

Results and discussion

Structural description and comparison

γ-P₄S₃I₂ crystallized in the tetragonal crystal system with an asymmetric space group of P4₃ (no. 78), with a = 7.3415(2) Å, b = 7.3415(2) Å, and c = 39.1942(19) Å (crystallographic details are shown in Tables S1–S4†). As shown in Fig. 1a, the basic building unit (BBU) of γ-P₄S₃I₂ was a unique molecular cluster (P₄S₃I₂)_n (n = 4) constructed from P₄S₃I₂ molecules with two-dimensional (2D) layers (layer A and layer B). The BBUs in γ-P₄S₃I₂ were arranged along the c direction with C₄ symmetry and the 4₃ screw axis is paralleled to the c-axis. In addition, layers A and B in the (P₄S₃I₂)_n molecular cluster were interconnected through weak van der Waals interactions between the two layers, and both layers were constructed by using the paralleled P₄S₃I₂ molecules in the a–b plane, where adjacent P₄S₃I₂ molecules are connected with each other also via van der Waals interactions (Fig. 1b and c). The boat-like P₄S₃I₂ molecule in γ-P₄S₃I₂ contained four P atoms, three S atoms and two I atoms with the I–P bonds ranging from 2.471(3) to 2.480(3) Å, the P–S bonds ranging from 2.082(5) to 2.136(5) Å and the P–P bonds ranging from 2.202(5) to 2.212(5) Å, which are comparable to I–P bonds, P–S bonds and P–P bonds in α-P₄S₃I₂ and β-P₄S₃I₂.^35,36 Moreover, the structural feature of the boat-like P₄S₃I₂ molecules in γ-P₄S₃I₂ was similar to that in α-P₄S₃I₂, yet quite different from that in β-P₄S₃I₂ (Fig. 1d, c and e). The P₄S₃I₂ molecules in γ-P₄S₃I₂ and α-P₄S₃I₂ both showed C₂ local symmetry, while the molecules in β-P₄S₃I₂ exhibited C_s local symmetry.

Fig. 1 Crystal structure of γ-P₄S₃I₂ viewed along the a-axis (a). Layer A of the (P₄S₃I₂)_n basic building unit in γ-P₄S₃I₂ (b). Layer B of the (P₄S₃I₂)_n basic building unit in γ-P₄S₃I₂ (c). P₄S₃I₂ molecules in γ-P₄S₃I₂ (d), α-P₄S₃I₂ (e) and β-P₄S₃I₂ (f).

To better understand the structure of γ-P₄S₃I₂, α-P₄S₃I₂ was chosen to compare with γ-P₄S₃I₂ (Fig. 2) because their P₄S₃I₂ molecules have the same local symmetry. The structural evolution of γ-P₄S₃I₂ could be regarded as that where P₄S₃I₂ molecules in α-P₄S₃I₂ (Fig. 2a) were partially picked out and regularly rearranged according to a new symmetry, leading to γ-P₄S₃I₂, which crystallizes in another space group (P4₃). However, as shown in Fig. 2b and c, the distance between the two layers in the (P₄S₃I₂)_n (n = 4) molecular cluster of γ-P₄S₃I₂ (4.661 Å) is greater than that of α-P₄S₃I₂ (4.648 Å). In addition, the literature values of the van der Waals radii for these atoms are 2.15 Å for I and 1.90 Å for P, which give a sum of 4.05 Å, while the experimental I⋯P distance in the (P₄S₃I₂)_n (n = 4) molecular cluster of γ-P₄S₃I₂ (3.82 Å) and α-P₄S₃I₂ (3.87 Å) was considerably less than the sum of their van der Waals radii (Fig. S3†), indicating the existence of intermolecular van der Waals interactions between their P₄S₃I₂ molecules.

Fig. 2 Crystal structure of α-P₄S₃I₂ viewed along the a-axis (a). A (P₄S₃I₂)_n (n = 4) molecular cluster in α-P₄S₃I₂ (b). A (P₄S₃I₂)_n (n = 4) molecular cluster in γ-P₄S₃I₂ (c). Crystal structure of γ-P₄S₃I₂ viewed along the a-axis (d).

Thermal analysis

Thermogravimetric (TG) and differential thermal analysis (DTA) curves revealed that γ-P₄S₃I₂ was decomposed at 120–300 °C with an exothermic peak at 120 °C (Fig. S5†), indicating that this compound was incongruent. Solid γ-P₄S₃I₂ was transformed into liquid amorphous P₄S₃I₂ at 120 °C and began to decompose, and γ-P₄S₃I₂ can be obtained by recrystallizing the liquid amorphous P₄S₃I₂ in CS₂ (Fig. S4†). γ-P₄S₃I₂ exhibited considerable thermal stability compared to some metal halide IR NLO crystals, such as HgBr₂ (100 °C), Cs₂Hg₃I₈–H₂O (110 °C) and RbHgI₃ (120 °C).^50–53

Optical properties

The optical band gap of γ-P₄S₃I₂ was determined to be 2.38 eV (Fig. S6†), which is consistent with the deep yellow colour of its crystals. The band gap of γ-P₄S₃I₂ is larger than those of some IR NLO thiophosphates, such as SnPS₃ (2.35 eV), RbBiP₂S₆ (2.10 eV), CuHgPS₄ (2.03 eV) and AgHg₃PS₆ (1.85 eV).^32,33,54,55 Powder laser damage threshold (LDT) measurements suggested that the LDT of γ-P₄S₃I₂ was 11.97 MW cm⁻², which was about 2.8 times that of AgGaS₂ (4.25 MW cm⁻²) under the same conditions (Table S5†). In addition, no obvious absorption from 3 to 20 μm was observed in the IR transmittance spectra of γ-P₄S₃I₂ (Fig. S7†), indicating that this compound could potentially be applied in the IR region.

NLO properties

According to the anionic group theory, the orientated arrangement of NLO-active structural units is beneficial for achieving a large SHG reponse.⁵⁶ Thus, γ-P₄S₃I₂ may be expected to exhibit suitable SHG responses because the (P₄S₃I₂)_n (n = 4) molecular clusters were arranged in parallel along the same direction (Fig. 1a). The size-dependent SHG measurements of the polycrystalline samples of γ-P₄S₃I₂ and standard AgGaS₂ were performed using a 2.05 μm laser. The intensities of the SHG signals were gradually increased along with an increase of particle sizes (Fig. 3b), indicating that the required type-I phase-matching behavior could be realized in γ-P₄S₃I₂. Furthermore, the SHG response of γ-P₄S₃I₂ was about 0.5 times that of AGS within the same particle size range of 150–210 μm, which was considerable among some reported IR NLO chalcogenides, such as SnI₄·S₈ (0.5 × AgGaS₂), CH₃I·S₈ (0.7 × AgGaS₂), LiGa₂PS₆ (0.5 × AgGaS₂), Rb₂GaP₂S₉ (0.1 × AgGaS₂) and AgHg₃PS₆ (0.5 × AgGaS₂).^55,57–59 These results demonstrated that γ-P₄S₃I₂ had great potential as an IR NLO candidate.

Fig. 3 Measured SHG signals of γ-P₄S₃I₂ and AgGaS₂ with a 150–210 μm particle size (a). Measured SHG intensities of γ-P₄S₃I₂ and AgGaS₂ under 2.05 μm laser irradiation at room temperature (b).

Theoretical calculations

To further understand the electronic structure and optical properties, systematic theory calculations for γ-P₄S₃I₂ were performed based on density functional theory (DFT). The calculated band structure suggests that the direct band gap of γ-P₄S₃I₂ was 2.123 eV as both the top of the valence band (VB) and the bottom of the conduction band (CB) were located at the X point (Fig. S8†). As can be seen in the partial density of state (PDOS) curves of γ-P₄S₃I₂ (Fig. S9†), the top of the valence band was dominated by I 5p, P 3p and S 3p orbitals, and the bottom of the conduction band was also occupied by I 5p, P 3p and S 3p orbitals, indicating that the linear optical properties of γ-P₄S₃I₂ were determined by the P₄S₃I₂ molecules.

The space group P4₃ of γ-P₄S₃I₂ belonged to point group 4. According to the Kleinman symmetry, two independent nonzero SHG tensors, d₃₁ and d₃₃ remained (Fig. S10†). The largest tensor d₃₃ value at 2.05 μm in γ-P₄S₃I₂ was 9.60 pm V⁻¹, which was about 0.63 times that of AgGaS₂ (15.3 pm V⁻¹), matching with the corresponding measured results. In addition, to clearly illustrate the origin of the contribution of the SHG effect in γ-P₄S₃I₂, the SHG-weighted densities of the largest tensor d₃₃ were calculated. As shown in Fig. 4, the I atoms, S atoms, P atoms and P–P bonds in the P₄S₃I₂ molecule contributed to both occupied and unoccupied electronic states, which confirmed that the (P₄S₃I₂)_n molecular clusters determined the SHG response.

Fig. 4 The SHG-weighted densities for occupied (a, c) and unoccupied (b, d) electronic states in γ-P₄S₃I₂.

Additionally, the calculated dispersion of the refractive indices curves of γ-P₄S₃I₂ is displayed in Fig. S11.† The results were n_c < n_a and the birefringence (n_c − n_a) of γ-P₄S₃I₂ was calculated to be 0.118 at 2050 nm, which contributed to the type-I phase-matching behavior over a wide IR region and matched the experimental results. These results indicated that the P₄S₃I₂ molecules may be potential fundamental building units for birefringence materials.

Conclusions

In summary, a non-centrosymmetric metal-free thiophosphate, γ-P₄S₃I₂, was successfully synthesized and investigated for NLO materials based on polymorphism strategy. The crystal structure of γ-P₄S₃I₂ featured paralleled (P₄S₃I₂)_n molecular clusters connecting with each other through van der Waals interactions. γ-P₄S₃I₂ also showed excellent nonlinear optical performances including a suitable SHG response (0.5 × AgGaS₂), wide band gap (2.38 eV), large birefringence (0.118@2050 nm), and wide infrared transparency. More importantly, the polymorphism strategy in this work may provide new thoughts for exploring new NLO materials with enhanced properties.

Author contributions

Xin Zhao performed the experiments, data analysis, and paper writing; Shunda Yang and Chao Wang offered help in synthesizing the compounds; Tao Yan and Jian Zhang offered help in analyzing the experimental data; Chensheng Lin and Haotian Tian performed the theoretical calculations; Bingxuan Li performed the powder laser damage threshold (LDT) measurements; Min Luo revised the manuscript. Ning Ye and Min Luo guided and supervised the experiments. All authors contributed to the general discussion.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 22222510, 21975255 and 21921001), Natural Science Foundation of Fujian Province (2021J01514), the Foundation of Fujian Science & Technology Innovation Laboratory (2021ZR202), Youth Innovation Promotion Association CAS (2019303) and Open Project of State Key Laboratory of Crystal Materials, Shandong University (KF1907).

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Footnote

† Electronic supplementary information (ESI) available: Crystallographic data, measurements of physical properties, and theoretical calculations. CCDC 2216006. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2qi02313j

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