Pb₆Ba₃Si₂S₈I₁₀: a new thiohalide with a quasi-two-dimensional structure and wide band gap

Abstract

Pb-based chalcogenides display abundant structural diversity and distinguished properties. Based on a mixed anion and dimensional reduction combined strategy, a wide band gap Pb-based thiohalide, Pb₆Ba₃Si₂S₈I₁₀, has been rationally designed and synthesized experimentally by the flux method. The compound crystallizes in the R3̄c space group with cell parameters a = 9.7925(2) Å, b = 9.7925(2) Å, and c = 70.628(3) Å and is composed of [SiS₄] tetrahedra and unprecedented [PbI₅S₂] polyhedral units, resulting in a unique quasi-two-dimensional structure, which enriches the chemical and structural diversity of Pb-based thiohalides. The experimental band gap of Pb₆Ba₃Si₂S₈I₁₀ was determined to be 2.80 eV. Based on statistical analyses and to the best of our knowledge, it is the largest experimental optical band gap among the known Pb-based thiohalides. The results demonstrate the feasibility of using highly electropositive Ba atoms to regulate the dimensions of the structural framework of thiohalides and give new insights into the structure and property modifications of thiohalides by the mixed anion and dimensional reduction combined strategy.

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Introduction

The development of functional materials and structural chemistry is highly dependent on the discovery of new compounds with distinctive crystal structures and physicochemical properties.^1–8 Introducing cations with stereochemically active lone-pair electrons, such as Pb²⁺, Sn²⁺, Bi³⁺, Sb³⁺, and As³⁺, has been demonstrated as an effective strategy to enhance the optical anisotropy of optoelectronic functional materials.^9–13 Among them, Pb²⁺-containing compounds with stereochemically active lone-pair electrons are conducive to inducing a significant second-harmonic generation (SHG) response, but they usually exhibit an adverse effect on the band gap, such as PbGa₂GeSe₆ (1.96 eV, 5 × AgGaS₂ (AGS)), Pb_0.65Mn_2.85Ga₃S₈ (1.68 eV, 1.5 × AGS), Pb_0.72Mn_2.84Ga_2.95Se₈ (1.65 eV, 4.4 × AGS), Pb₄Ga₄GeS₁₂ (2.18 eV, 2 × AGS), and PbGa₄Se₇ (2.1 eV, 3.3 × AGS).^14–20 As a critical parameter of optoelectronic functional materials, a wide band gap can effectively suppress two-photon/multi-photon absorption in IR nonlinear optical materials. For instance, a material with a band gap wider than 2.33 eV can only be effectively pumped by mature 1064 nm laser sources in the absence of two-photon absorption and free carrier absorption. Moreover, a larger band gap is advantageous for the compound to achieve a higher laser-induced damage threshold.^21–27 Thus, it is important to overcome the drawback of the narrow band gap in Pb-based chalcogenides.

Recently, introducing highly electronegative halogens has been proved to be a feasible strategy to increase the band gap of Pb-based chalcogenides,^28–37 and a series of Pb-based salt-inclusion chalcogenides have been developed, such as [K₂PbI][Ga₇S₁₂] (2.41 eV), [K₂PbBr][Ga₇S₁₂] (2.49 eV), [K₂PbCl][Ga₇S₁₂] (2.54 eV), [Na₂PbI][Ga₇S₁₂] (2.53 eV), Pb₃S₃Cl₂ (2.02 eV), Pb₃SBrI₃ (2.16 eV), Pb₂SbS₂I₃ (2.19 eV), Pb₄SeBr₆ (2.62 eV), Pb_3.5GeS₄Br₃ (2.6 eV), etc.^38–50 In addition, reducing the dimensions of the crystal structure using highly electro-positive/negative elements is also helpful in increasing the optical band gap of inorganic compounds, and this has been demonstrated in many systems, such as sulfides,^51,52 selenides,⁵³ tellurides and carbonates.⁵⁴

In this work, based on a mixed anion and dimensional reduction (introducing highly electropositive Ba atoms) combined strategy, a new Pb-based thiohalide Pb₆Ba₃Si₂S₈I₁₀ with unprecedented [PbI₅S₂] octahedra has been rationally designed and synthesized by the flux method. The compound crystallizes in the centrosymmetric R3̄c space group and shows a quite large cell parameter c = 70.628(3) Å (due to the presence of six distinct filling modes of Ba and I atoms in the layers), resulting in a quasi-two-dimensional (quasi-2D) crystal structure composed of tetrahedral [SiS₄] and polyhedral [PbI₅S₂] units. The formed quasi-2D structure is different from the three-dimensional (3D) frameworks in most of the Pb-based thiohalides. Moreover, due to the introduction of alkaline-earth metal (AEM) Ba and I atoms into the compound simultaneously, a relatively wide band gap (among the thiohalides) of 2.80 eV in the title compound is achieved.

Experimental section

Reagents

BaS (Aladdin, 99.7%), Pb (Aladdin, 99.9%), PbI₂ (Aladdin, 99.99%), Si (Aladdin, 99.99%), and S (Sinopharm, 99.9%) were purchased commercially and used as the starting materials without further purification.

Synthesis

The single crystal of the title compound for structural determination (Table 1) was grown by the flux method^55,56 with PbCl₂ as the flux in a sealed quartz tube at ∼950 °C. The details for the crystal growth are shown as follows: (1) the mixtures of BaS (0.094 g), Pb (0.038 g), Pbl₂ (0.427 g), Si (0.030 g) and S (0.010 g) were ground and loaded into a graphite crucible with an inner diameter of 7 mm; (2) the crucible was transferred into a 10 mm (inner diameter) quartz tube, and it was further sealed with a methane-oxygen flame under a high vacuum of 10⁻³ Pa; and (3) the sealed tube was placed into a computer-controlled furnace and then heated to 950 °C for 24 h and kept at this temperature for 48 h, cooled to 650 K at a rate of 1 °C h⁻¹, and then cooled to room temperature naturally. The pure phase powder samples were prepared in sealed quartz tubes with the starting materials of BaS : Pb : PbI₂ : Si : S = 3 : 1 : 5 : 2 : 5 (molar ratio). The temperature program for the chemical synthesis involved quickly heating up to 950 °C and maintaining that temperature for 48 h, and then cooling to room temperature rapidly. Finally, the polycrystalline powder samples of Pb₆Ba₃Si₂S₈I₁₀ were harvested at the bottom with some impurities on the top of the tubes.

Table 1 Crystallographic data and structure refinements for Pb₆Ba₃Si₂S₈I₁₀

a R 1 = ∑||Fo| − |Fc||/∑|Fo| and wR2 = [∑w(Fo^2 − Fc^2)^2/∑wFo^4]^1/2 for Fo^2 > 2σ(Fo^2).
Empirical formula	Pb6Ba3Si2S8I10
Formula weight	3236.76
Temperature	298.15 K
Crystal system, space group	Trigonal, R3̄c
Unit cell dimensions	a = 9.7925(2) Å
	b = 9.7925(2) Å
	c = 70.628(3) Å
Volume	5865.4(3) Å^3
Z, calculated density	6, 5.498 g cm^−3
Absorption coefficient	37.085 mm^−1
F(000)	8076
Data/restraints/parameters	1870/0/47
Index ranges	−13 ≤ h ≤ 12, −8 ≤ k ≤ 9, −89 ≤ l ≤ 98
Completeness	99.0%
Independent reflections	1870 [Rint = 0.0465, Rsigma = 0.0403]
GOF on F^2	1.074
2θ range for data collection	4.94° to 60.068°
Reflections collected	8103
Final R indices [Fo^2 > 2σ(Fo^2)]^a	R 1 = 0.0434, wR2 = 0.0869
R indices (all data)^a	R 1 = 0.0540, wR2 = 0.0911
Largest diff. peaks and holes	2.11/−2.19 e Å^−3

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Structural determination

A yellow and transparent Pb₆Ba₃Si₂S₈I₁₀ single crystal with a size of 0.15 × 0.05 × 0.04 mm³ was selected for the structural characterization. The data were collected on a Bruker Smart APEX III. The crystal structure was solved by direct methods and then refined by full-matrix least squares on F² with the SHELXTL-14 program.^57,58 The PLATON program was used to check the possible missing symmetry elements and no higher symmetry was suggested in the structure.⁵⁹ The crystallographic data are available at CCDC 2369466† in CIF format.

Powder X-ray diffraction (PXRD)

PXRD data of Pb₆Ba₃Si₂S₈I₁₀ were collected on a Bruker D2 PHASER X-ray diffractometer equipped with Cu Kα radiation (λ = 1.54056 Å) at room temperature and the diffraction patterns were taken from 10° to 70° (2θ).^60,61 Then, powder Rietveld refinement was performed using GSAS software.

Energy dispersive X-ray spectroscopy (EDS)

The EDS spectrum of the title compound was recorded on a field emission scanning electron microscope (FE-SEM, JEOL JSM-7610F Plus, Japan) with an energy dispersive X-ray spectrometer (Oxford, X-Max 50) at room temperature. It was operated at 5 kV.

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

The UV-vis-NIR diffuse reflectance spectrum of Pb₆Ba₃Si₂S₈I₁₀ was characterized on a Shimadzu SolidSpec-3700DUV spectrophotometer at room temperature within a wavelength range from 180 to 2600 nm. The reflectance spectrum was converted to pseudo-absorption data based on the Kubelka–Munk function: F(R) = (1 − R)²/2R = K/S (R = reflectance; K = absorption; and S = scattering).⁶² Before recording the IR spectrum, the polycrystalline Pb₆Ba₃Si₂S₈I₁₀ powder sample was made into a pellet with KBr powder. The IR spectrum of the compound was recorded on a Shimadzu IR Affinity-1 Fourier transform infrared spectrometer within a range of 4000–400 cm⁻¹.

Refractive index difference (RID)

The RID of the Pb₆Ba₃Si₂S₈I₁₀ single crystal was tested on a polarizing microscope equipped (ZEISS Axio Scope. 5 pol) with a Berek compensator. The wavelength of the used light source was 546 nm. The difference in the optical path (D) for one direction was determined according to the interference color with the maximum value of the crystal under polarized light.⁶³ The RID can be calculated from the following equation: D = |N₂ − N₁| × T = Δn × T, where Δn represents the difference in the refractive index and T is the thickness of the crystal.⁶⁴

Computational methods

The first-principles calculations were performed by the plane wave pseudopotential method implemented in the CASTEP package.^65,66 The generalized gradient approximation (GGA) and Perdew–Burke–Ernzerhof (PBE) functional were adopted for the calculations, and the norm-conserving pseudopotentials (NCPs) were used to calculate the electronic structure and optical properties.⁶⁷ The cutoff energy was set to 820 eV. In order to model the effective interactions between valence electrons and the atom cores in Pb₆Ba₃Si₂S₈I₁₀, the valence electrons were set as Ba-5s²5p⁶6s², Pb-5s²5p⁶5d¹⁰6s²6p², Si-3s²3p², S-3s²3p⁴, and I-5s²5p⁵. The Monkhorst–Pack k-point grid was set to 3 × 3 × 3 in the Brillouin zone (BZ). The residual calculation parameters and convergent criteria were set to the default values in the CASTEP package.

Results and discussion

The results of single crystal XRD show that Pb₆Ba₃Si₂S₈I₁₀ crystallizes in the trigonal R3̄c space group with cell parameters a = 9.7925(2) Å, b = 9.7925(2) Å, c = 70.628(3) Å, V = 5865.36(35) ų and Z = 6. The crystallographic data and structure refinement information for Pb₆Ba₃Si₂S₈I₁₀ are provided in Table 1, and the atomic coordinates, bond distances, angles, and isotropic displacement parameters are given in Tables S1 and S2.† The calculated bond valence sums (Pb²⁺: 1.83, Ba²⁺: 2.31, Si⁴⁺: 3.94, S²⁻: 2.12–2.15, and I⁻: 0.79–1.55 for Pb₆Ba₃Si₂S₈I₁₀) and small global instability indices GII = 0.178 (Table S1†) verify the reasonability of the crystal structure. The results of EDS spectroscopy and mapping (Fig. S1†) confirm the presence of Pb, Ba, Si, S, and I in the structure.

In the asymmetric unit of Pb₆Ba₃Si₂S₈I₁₀, there is one crystallographically independent Pb atom, one Ba atom, one Si atom, two S atoms and four I atoms. The Si atoms are coordinated with four S atoms to form [SiS₄] tetrahedral units with the bond lengths of d_(Si–S) = 2.112–2.152 Å (Fig. 1a). The Pb atoms are coordinated with two S and six I atoms to build [PbI₅S₂] polyhedral units (Fig. 1b) with d_(Pb–I) = 3.344–3.548 Å and d_(Pb–S) = 2.821–2.883 Å. The formed [PbI₅S₂] mixed anionic unit is totally different from the formed [PbS₅Cl₂] in Pb₅Sn₃S₁₀Cl₂,⁴⁷ [PbS₄Br₄], [PbS₅Br₃], [PbS₆Br₂] in Pb₄S₃Br₂,⁴¹ [PbSI₄] in Pb₃SBrI₃,⁴⁶ and [PbS₃I₅] and [PbSBr₂I₅] in Pb₃SBrI₃.⁴⁵ Based on statistical investigations using the Inorganic Crystal Structure Database (ICSD-5.2.0, the latest release of ICSD-2024/04/10) and to the best of our knowledge, [PbI₅S₂] is for the first time observed in Pb-based thiohalide compounds. Furthermore, the six [PbI₅S₂] octahedra are further interconnected with each other by edge-sharing to construct a [Pb₆I₁₃S₁₂] polymer (Fig. 1c). The formed [Pb₆I₁₃S₁₂] polymers are linked by tetrahedral [SiS₄] units in a vertex-sharing way to produce 2D Pb–Si–S–I layers (Fig. 1d). The Ba and partial I atoms are located in the gaps of 2D Pb–Si–S–I layers to balance the valence states, resulting in the final quasi-2D structure of the title compound.

Fig. 1 Crystal structure of Pb₆Ba₃Si₂S₈I₁₀. (a and b) The coordination mode of Si (a) and Pb atoms (b); (c) the formed [Pb₆I₁₃S₁₂] group; (d) the resultant 2D Pb–Si–S–I layer; and (e) the quasi-2D structure of Pb₆Ba₃Si₂S₈I₁₀.

Compared to the 3D Pb–S–Cl and Pb–S–Br frameworks in Pb₅Sn₃S₁₀Cl₂ ⁴⁷ and Pb₄S₃Br₂,⁴⁸ respectively, the Pb–S–I framework is reduced to a 2D structure in Pb₆Ba₃Si₂S₈I₁₀, due to the introduction of highly electropositive Ba atoms with a high atomic proportion. In addition, from the perspective of Ba–I layers, Ba atoms exhibit three distinct arrangements, referred to as A, B, and C in Fig. S2.† The arrangements give rise to A′, B′, and C′ through a combination of secondary screw axes and glide symmetry operations along the c-direction, preventing the Ba atom layer from repeating within the unit cell via translational operations. The interleaved arrangements of Ba and I atoms ultimately result in six unique Ba–I configurations along the c-axis. Hence, due to the presence of six distinct filling modes of Ba and I atoms between the inter-layers of 2D Pb–Si–S–I, a large cell parameter c = 70.628(3) Å is observed in Pb₆Ba₃Si₂S₈I₁₀. To confirm the chemical bonding and optical absorption regions, the IR spectrum of Pb₆Ba₃Si₂S₈I₁₀ was recorded on a Shimadzu IR Affinity-1 Fourier transform infrared spectrometer, and it shows three strong absorption peaks at 501 cm⁻¹, 534 cm⁻¹ and 1084 cm⁻¹, which can be attributed to the characteristic vibrations of Si–S bonding.⁶⁸ Besides, the absorption peak at 785 cm⁻¹ originates from the in-plane bending vibration of carbon dioxide (Fig. S3†).⁶⁹

To elucidate the influence of highly electropositive cations on the structural dimension in Pb-based thiohalides, we extended the investigation to the all-known alkali metal (AM)- and/or AEM-containing Pb-based thiohalides in the ICSD, and 16 Pb-based thiohalides without C, H, O, and N elements are summarized in Table S3.† It can be seen that, except Pb₆Ba₃Si₂S₈I₁₀ with a quasi-2D structure, all the known compounds including [Na₂PbI][Ga₇S₁₂], [K₂PbI][Ga₇S₁₂], [K₂PbBr][Ga₇S₁₂], and [K₂PbCl][Ga₇S₁₂] show 3D frameworks. Previous investigations indicate that the atomic ratio between electropositive AM/AEM cations and other central atoms capable of forming polyhedral units plays an important role in determining the dimensions of polyhedron-based frameworks.⁷⁰ Based on statistical investigations, when the A/(Pb + M) ratio (where A represents highly electropositive AM or AEM atoms, while M denotes other central atoms capable of forming polyhedral units) is smaller than about 0.3, the compounds show 3D frameworks (Fig. 2a). Taking [Na₂PbI][Ga₇S₁₂] as an example (Fig. 2c, where Pb₆Ba₃Si₂S₈I₁₀ is used as a reference in Fig. 2b), the Na/(Pb + Ga) atomic ratio is 0.25, smaller than the Ba/(Pb + Si) ratio of 0.375 in Pb₆Ba₃Si₂S₈I₁₀, resulting in a 3D pore-like structure in [Na₂PbI][Ga₇S₁₂], and the Na atoms are placed in the pores, different from the interlayered Ba atoms in Pb₆Ba₃Si₂S₈I₁₀ (Fig. 2b). The results highlight that the proportion of highly electropositive cations affects the crystal structures in thiohalides.

Fig. 2 (a) Relationship between the A/(Pb + M) ratio (A = Na, K, and Ba; M = Ga and Si) and the dimensions of Pb-based thiohalides; structural comparison between quasi-2D Pb₆Ba₃Si₂S₈I₁₀ (b) and 3D [Na₂PbI][Ga₇S₁₂] (c).

To detect the optical properties, the pure phase polycrystalline powder samples of the title compound were synthesized and characterized. As shown in Fig. 3a, the PXRD pattern of Pb₆Ba₃Si₂S₈I₁₀ matches well the theoretical result derived from the CIF file using Mercury. Moreover, to assess the purity of the powder samples, the results of powder Rietveld refinement (R_WP = 4.00% and R_P = 2.98%) have been analysed, which indicate high purity for the obtained polycrystalline samples and confirm the results of single crystal XRD. In addition, to check the thermal stability, XRD characterization before and after melting has been carried out, as shown in Fig. S5.† The results imply that Pb₆Ba₃Si₂S₈I₁₀ could be a congruently melting compound, which is favorable for the growth of large-sized single crystals. To detect the experimental optical band gap, the UV-vis-NIR diffuse reflectance spectrum of the title compound was recorded by using the pure phase polycrystalline powder samples. Moreover, based on the [(kνF(R))]^1/2 spectrum (Fig. S6†), the band gap of Pb₆Ba₃Si₂S₈I₁₀ was determined to be 2.13 eV, while it was 2.80 eV based on the [(kνF(R))]² spectrum (Fig. 3b). The latter shows good agreement with the color of Pb₆Ba₃Si₂S₈I₁₀ (yellow). It is worth noting that the value is larger than those of most of the Pb-based thiohalides, such as Pb₅Sn₃Se₁₀Cl₂ (1.44 eV), Pb₅Sn₃S₁₀Cl₂ (1.72 eV), Pb₅S₂I₆ (1.73 eV), Pb₄S₃I₂ (1.76 eV), Pb₄S₃Br₂ (1.91 eV), [K₂PbI][Ga₇S₁₂] (2.41 eV), [K₂PbBr][Ga₇S₁₂] (2.49 eV), [K₂PbCl][Ga₇S₁₂] (2.54 eV), [Na₂PbI][Ga₇S₁₂] (2.53 eV), Pb₃S₃Cl₂ (2.02 eV), Pb₃SBrI₃ (2.16 eV), Pb₂SbS₂I₃ (2.19 eV), Pb₄SeBr₆ (2.62 eV), and Pb_3.5GeS₄Br₃ (2.6 eV) (Fig. 3c).^38–50 The reason for the relatively wide band gap in Pb₆Ba₃Si₂S₈I₁₀ could be related to the low-dimensional quasi-2D structure, similar to the cases of A^IIHgM^IVS₄ (A^II = Sr and Ba; M^IV = Si and Ge) (2.94–3.06 eV),⁷¹ PbFIO₃ (3.87 eV),⁷² and AEGe₂O₄Se (AE = Sr and Ba) (3.57 and 3.81 eV).⁷³ Moreover, the RID of Pb₆Ba₃Si₂S₈I₁₀ was determined to be 0.006 at 546 nm (Fig. S7†).

Fig. 3 (a) Experimental and theoretical XRD patterns; (b) diffuse reflectance spectrum of Pb₆Ba₃Si₂S₈I₁₀ plotted as [(kνF(R))]² (direct band gap), where F(R) is the Kubelka–Munk function. Dotted lines show the fit used to extract the band gap; (c) the statistical investigations on the band gaps of typical Pb-based thiohalides (brown: 3D structured compounds and green: quasi-2D structured compounds).

To detect the origin of optical properties in Pb₆Ba₃Si₂S₈I₁₀, density functional theory (DFT) calculations were performed. The band structure indicates that Pb₆Ba₃Si₂S₈I₁₀ is an indirect band gap semiconductor with a calculated GGA band gap of 2.341 eV (Fig. 4a), smaller than the experimental value of 2.80 eV, because of the discontinuity of the exchange–correlation energy functional.⁷⁴ From the density of states (DOS) and partial DOS (PDOS) diagram in Fig. 4b, it can be seen that the top of the valence band (VB) is mainly derived from the S-3p and I-5p orbitals, while the bottom of the conduction band (CB) is occupied by the Pb-6p orbital (Fig. 4b). It indicates that the optical band gap in the title compound is mainly determined by the rare [PbI₅S₂] units, indicating the positive contribution of the mixed anions.³⁰

Fig. 4 (a) The band structure of Pb₆Ba₃Si₂S₈I₁₀ and (b) the total and partial densities of states of Pb₆Ba₃Si₂S₈I₁₀.

Conclusions

In summary, a new Pb-based thiohalide Pb₆Ba₃Si₂S₈I₁₀ with unprecedented [PbI₅S₂] mixed anionic units has been designed by a mixed anion and dimensional reduction combined strategy and synthesized experimentally. The crystal structure of the compound was determined by single crystal XRD, and it shows a quasi-2D structure that is composed of polyhedral [PbI₅S₂] and tetrahedral [SiS₄] units. More importantly, the introduction of highly electropositive Ba atoms with a high atomic proportion reduces the structural dimension, resulting in a relatively wide band gap of 2.80 eV in Pb₆Ba₃Si₂S₈I₁₀, which is higher than those of known Pb-based thiohalides. DFT calculations indicate that Pb₆Ba₃Si₂S₈I₁₀ is an indirect band gap semiconductor and that the band gap mainly originates from the interaction of the S-3p and I-5p orbitals in the unique [PbI₅S₂] mixed-anionic units. Experimental and theoretical results imply that introducing mixed anions and highly electropositive cations simultaneously is a feasible strategy to develop wide band gap Pb-based thiohalides.

Data availability

Supporting data for this article are presented in the ESI.† The raw data of this article can be obtained from the corresponding author upon reasonable request.

Conflicts of interest

The authors declare that they have no conflict of interest.

Acknowledgements

This work was supported by the Natural Science Foundation of the Xinjiang Uygur Autonomous Region (2024D01E30), the Open Fund of the Anhui Key Laboratory of Photonic Materials and Devices (AHKL2024KF02), and the National Natural Science Foundation of China (22475234, 52402017, 22335007, and 52002398).

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Footnote

† Electronic supplementary information (ESI) available: Related figures and tables. CCDC 2369466. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4dt02315c

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