A high-performance IR nonlinear optical material Sr₃MnGe₂S₈ achieved via partial cation substitution-induced structural transformation

Abstract

The development of Mn-based infrared nonlinear optical (IR-NLO) materials that concurrently exhibit a large second-harmonic generation response (SHG > 1.0 × AgGaS₂) and a wide band gap (E_g > 3.0 eV) remains a formidable challenge. Herein, we demonstrate a partial cation substitution strategy to address this issue, successfully transforming centrosymmetric Sr₂GeS₄ into a novel non-centrosymmetric quaternary sulfide, Sr₃MnGe₂S₈. The compound features a three-dimensional framework constructed from corner-sharing [MnS₄] and [GeS₄] tetrahedra, with Sr²⁺ cations residing in the interstitial spaces. Sr₃MnGe₂S₈ achieves an outstanding balance of key IR-NLO properties: it shows a strong SHG response (2.4 × AgGaS₂ at 2050 nm), a high laser-induced damage threshold (13.4 × AgGaS₂), and a wide E_g of 3.06 eV—ranking it among the best-performing Mn-based chalcogenides. Theoretical calculations and structural analysis reveal that the excellent NLO performance originates from the synergistic effects between the two distinct types of functional units. This work provides a valuable paradigm for designing high-performance IR-NLO materials through rational structural transformation, highlighting the power of chemical substitution in overcoming property trade-offs.

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Introduction

Nonlinear optical (NLO) crystals serve as the cornerstone materials for laser frequency conversion and play a pivotal role in a variety of applications, including environmental monitoring, medical imaging, infrared (IR) remote sensing, and national defense and security.¹ Currently, commercially available NLO crystals operating in the mid- to far-IR region—such as AgGaS₂, AgGaSe₂, and ZnGeP₂—suffer from intrinsic limitations, notably a narrow bandgap (E_g), which leads to a low laser-induced damage threshold (LIDT) and undesirable two-photon absorption (TPA).² These drawbacks significantly restrict their application in high-power infrared laser systems. Consequently, there is an urgent need to explore and synthesize new IR-NLO crystals that exhibit superior overall performance, particularly those possessing both a wide E_g and a large second-harmonic generation (SHG) response.³

Transition metal-based chalcogenides have garnered significant attention as a compelling class of materials for IR-NLO applications, distinguished by their structurally tunable frameworks, adaptable electronic band structures, strong nonlinear susceptibilities, high LIDTs, and excellent physicochemical stability.⁴ While over 500 compounds in this family have been explored to date, research efforts remain heavily concentrated on systems containing d¹⁰-configured metal centers, leaving those with alternative electron configurations—such as manganese-based compounds with a d⁵ configuration—relatively underexplored. Despite their limited investigation, the known Mn-based IR-NLO materials already exhibit remarkable structural diversity.^5–22 These range from simple chalcogenides (e.g., RbMn₄In₅Se₁₂,⁵ Pb_0.65Mn_2.85Ga₃S₈,⁷ and Li₂MnSnS₄ ¹⁰) to complex heteroanionic systems including oxychalcogenides (e.g., Ba₁₀In₂Mn₁₁Si₃O₁₂S₁₈¹² and Eu₂MnGe₂OS₆ ¹³) and salt-inclusion chalcogenides (e.g., [Ba₄Cl₂][MnGa₄Se₁₀]¹⁶). However, a critical challenge persists across these systems: their relatively narrow E_g fundamentally limits practical utility by inducing TPA and constraining the LIDT. Consequently, achieving a balanced combination of wide E_g (>3.0 eV) and strong SHG response (>1.0 × AgGaS₂) represents a pivotal yet unresolved pursuit in the development of high-performance Mn-based IR-NLO materials.

Beyond the performance limitations previously discussed, a more fundamental challenge in the development of Mn-based IR-NLO materials lies in the intentional design of noncentrosymmetric (NCS) structures—a strict prerequisite for any second-order NLO material.²³ Over decades, multiple synthetic strategies have been developed to promote NCS formation, among which chemical substitution has gained considerable attention due to its operational simplicity and structural predictability.²⁴ This approach typically employs a known centrosymmetric (CS) parent phase as a template, where targeted replacement of cationic or anionic motifs induces symmetry breaking, thereby activating the SHG response. Notable successes of this strategy include the transformation from CS-CsGaS₂ to NCS-[CsBa₂Cl][Ga₄S₈],²⁵ CS-Rb₄P₂S₆ to NCS-RbBiP₂S₆,²⁶ CS-SnBr₂ to NCS-Sn₇Br₁₀S₂,²⁷ and CS-PbBr₂ to NCS-Pb₄SeBr₆.²⁸ Building on this methodology, our group recently demonstrated that chemical substitution can be strategically applied to wide-E_g CS templates to simultaneously achieve a large SHG response and retain a wide E_g. Representative examples include: CS-SrGeO₃ to NCS-SrGeOSe₂ (E_g = 3.16 eV, SHG = 1.3 × AgGaS₂),²⁹ CS-Sr₂GaS₄ to NCS-SrCdSiS₄ (E_g = 3.61 eV, SHG = 1.1 × AgGaS₂),³⁰ and CS-Cs₃[Sb₃O₆][Ge₂O₇] to NCS-Nd₃[Ga₃O₃S₃][Ge₂O₇] (E_g = 4.35 eV, SHG = 0.8 × AgGaS₂).³¹ These results highlight the potential of rational substitution strategies to overcome the traditional trade-off between wide E_g and large SHG response—paving a viable pathway toward high-performance Mn-based NLO systems.

Based on the aforementioned design strategy, we herein report the rational synthesis of a novel Mn-based IR-NLO candidate, Sr₃MnGe₂S₈, through a partial cation substitution approach, starting from the wide-E_g CS phase Sr₂GeS₄ as a structural template.³² The resulting Sr₃MnGe₂S₈ achieves an outstanding balance of key IR-NLO properties, exhibiting a strong SHG response (2.4 × AgGaS₂ at 2050 nm), a high LIDT (13.4 × AgGaS₂), and a wide E_g of 3.06 eV. These merits place it among the top-performing Mn-based chalcogenides reported to date. This work comprehensively describes its synthesis, crystal structure, optical properties, and the underlying structure–property relationships.

Results and discussion

The successful implementation of a chemical substitution strategy for structural transformation and IR-NLO material design critically depends on the judicious selection of a parent structure. In this work, the CS Sr₂GeS₄ was chosen as the template based on two key considerations. First, it contains conventional NLO-active motifs—specifically, isolated [GeS₄] tetrahedra—that are essential for generating a SHG response.³³ Second, its zero-dimensional (0D) structural architecture (Fig. 1a), characterized by well-separated structural units, contributes to a wide experimental E_g (3.46 eV), which is highly desirable for achieving a high LIDT.³⁴ By partially substituting Sr²⁺ with transition-metal Mn²⁺, we aimed to leverage the inherent size mismatch to break inversion symmetry—inducing a CS-to-NCS transition—while largely retaining the wide E_g characteristic imparted by the parent structure. Single-crystal X-ray diffraction (XRD) analysis confirms that Sr₃MnGe₂S₈ crystallizes in the cubic crystal system with the NCS space group I4̄3d (no. 220). The unit cell parameters are a = b = c = 13.9794(2) Å (Table 1). The asymmetric unit comprises one independent Sr atom (Wyckoff site 24d), one Ge atom (16c), one Mn atom (12a), and two S atoms (sites 48e and 16c) (Table S1). As depicted in Fig. 1b, the crystal structure of Sr₃MnGe₂S₈ is characterized by Mn²⁺ and Ge⁴⁺ ions in tetrahedral coordination with S atoms, forming [MnS₄] and [GeS₄] units. These tetrahedra interconnect through vertex-sharing, assembling into a three-dimensional (3D) anionic [MnGe₂S₈]⁶⁻ framework, charge-balanced by Sr²⁺ cations residing in the interstitial sites. Fig. 1c and d illustrate the local coordination geometry of the [MnS₄] and [GeS₄] tetrahedra, respectively. It is noteworthy that each [MnS₄] tetrahedron shares its four S1 atoms with four adjacent [GeS₄] units. In contrast, the [GeS₄] tetrahedron on the three-fold axis is surrounded by three distorted [MnS₄] tetrahedra via shared S1 atoms, whereas its S2 atom remains as the terminal atom.

Fig. 1 Structural evolution from (a) CS Sr₂GeS₄ to (b) NCS Sr₃MnGe₂S₈via partial cation substitution. The local coordination environments of the (c) [MnS₄] and (d) [GeS₄] tetrahedral units in the resulting Sr₃MnGe₂S₈ structure are also displayed.

Table 1 Crystal data and structure refinement for Sr₃MnGe₂S₈

a R 1 = ∑||Fo| − |Fc||/∑|Fo|, wR2 = {∑w[(Fo)^2 − (Fc)^2]^2/∑w[(Fo)^2]^2}^1/2.
Molecular formula	Sr3MnGe2S8
CCDC number	2488264
Formula weight	719.46
Crystal system	Cubic
Space group	I4̄3d
Temperature (K)	293(2)
Radiation	Mo Kα (λ = 0.71073)
F(000)	2648.0
a (Å)	13.97941(15)
α (°)	90
V (Å^3)	2731.91(9)
Z	8
ρ cal. (g cm^−3)	3.50
μ (mm^−1)	25.6
GOOF on F^2	1.08
Flack factor	0.04(3)
R 1, wR2 [I > 2σ(I)]^a	0.0413, 0.1056
R 1, wR2 (all data)^a	0.0428, 0.1079
Largest diff. peak and hole (e Å^−3)	0.82/−1.66

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A key structural consequence of the partial substitution of Sr²⁺ by Mn²⁺ is the pronounced distortion observed in both the [GeS₄] and [MnS₄] tetrahedra, attributable to the ionic size mismatch between the substituents. In Sr₃MnGe₂S₈, the degree of distortion of the [GeS₄] unit is larger than that in the parent Sr₂GeS₄ structure (15.44° vs.14.88°), as quantified by the formula .³⁵ Similarly, the newly formed [MnS₄] tetrahedron displays substantial angular distortion, with a maximum S–Mn–S bond angle of 139.67° and a corresponding gaint degree of distortion of 110.94° (Tables S2 and S3). The local dipole moment also serves as a metric for the tetrahedral distortion, yielding values of 0.84 D and 28.55 D for the [GeS₄] and [MnS₄] units, respectively.³⁶ Moreover, the charge-balancing Sr atoms exhibit notable differences in their coordination configuration and structure assembly. Specifically, Sr is six-coordinate within the CS Sr₂GeS₄, constituting a 2D layered structure. Conversely, in the NCS Sr₃MnGe₂S₈, Sr adopts an eight-coordinate geometry, resulting in a 3D network structure, as shown in Fig. S1. These geometric perturbations collectively underscore the efficacy of chemical substitution in modulating local symmetry and bonding geometry, which is critical for activating and enhancing second-order SHG responses.

The pale yellow crystalline compound Sr₃MnGe₂S₈ was synthesized via a high-temperature solid-state reaction under vacuum, using stoichiometric mixtures of Sr, Mn, Ge, and S. The resulting product (with a yield of approximately 70%) demonstrated excellent stability, remaining unaltered after more than one year of storage in ambient air (see the SI for details). Energy dispersive X-ray spectroscopy (EDX) confirmed the homogeneous distribution of Sr, Mn, Ge, and S throughout the crystal, with an average atomic ratio of approximately 3 : 1.1 : 2 : 7.9 (Fig. 2a and Fig. S2), in good agreement with the stoichiometry derived from single-crystal XRD structure analysis. Phase purity was further verified by powder XRD, as shown in Fig. 2b. UV-vis-NIR absorption spectroscopy revealed that Sr₃MnGe₂S₈ possesses a wide optical E_g of 3.06 eV, as estimated using the Kubelka–Munk function³⁷ (Fig. 2c). This value represents one of the largest E_g reported to date among Mn-based IR-NLO materials, exceeding the threshold of 3.0 eV (see Table 2 for details). Notably, the optical E_g of Sr₃MnGe₂S₈ is substantially broader than those of well-established IR-NLO crystals such as AgGaS₂ (2.56 eV),³⁸ AgGaSe₂ (1.83 eV),³⁹ and ZnGeP₂ (2.0 eV).² A large E_g is beneficial for minimizing TPA and achieving a high LIDT. In light of this, we evaluated Sr₃MnGe₂S₈'s performance in high-power applications by measuring its powder LIDT. The obtained value of 37.52 MW cm⁻² at 1064 nm is 13.4 times that of AgGaS₂ (2.8 MW cm⁻²).⁴⁰ Additionally, transmission measurements based on the single-crystal sample indicate that Sr₃MnGe₂S₈ exhibits a broad transparency window spanning from 0.37 μm (UV-Vis) to 14.2 μm (far-IR) (Fig. 2d). This value is comparable to recently reported IR-NLO materials, such as Cs₃In(In₄Se₇)(P₂Se₆) (0.41–19.5 μm), [Ba₂₂(SO₄)₅][Zn₁₄Ga₁₈S₅₈] (0.35–10.2 μm), [K₄BaCl₂][In₆Se₁₁] (0.7–17.1 μm), and [Ba₄Cl₂][CdGa₄S₁₀] (0.28–18.6 μm).⁴¹ The compound also displays remarkable thermal stability, maintaining its structural integrity up to 1100 K under a nitrogen atmosphere, as evidenced by thermogravimetric analysis (Fig. S3). The exceptional thermal stability of this compound—outperforming recent chalcogenides such as LiPb₃GeS₄Cl₃ (693 K), β-EuZnGeS₄ (1073 K), and (Na₃Rb)Hg₂Ge₂S₈ (717 K)⁴²—makes it highly promising for applications demanding harsh-environment operation. Owing to its NCS crystal structure, the NLO properties of Sr₃MnGe₂S₈ were systematically evaluated using the powder SHG method. Polycrystalline samples of Sr₃MnGe₂S₈, along with AgGaS₂ as a reference, were carefully ground and sieved into five distinct particle size ranges. SHG measurements were conducted using a 2050 nm laser. As shown in Fig. 2e, the SHG intensity of Sr₃MnGe₂S₈ does not increase monotonically with the increase of particle size, indicating non-phase-matching behavior—consistent with its cubic crystal system, which lacks sufficient birefringence for phase matching. Notably, Sr₃MnGe₂S₈ exhibits a strong SHG response, reaching approximately 2.4 times that of AgGaS₂ in the particle size range of 75–100 μm (Fig. 2e).

Fig. 2 Characterization of Sr₃MnGe₂S₈: (a) SEM image with the corresponding elemental maps, (b) experimental (blue) and simulated (black) powder XRD patterns, (c) solid-state diffuse reflectance spectrum, (d) optical transmission spectra across UV-Vis-NIR and mid-IR regions, and (e) particle size-dependent SHG responses relative to the reference AgGaS₂.

Table 2 Properties comparison of the reported Mn-based IR-NLO chalcogenides

Number	Compounds	Unit cell	Space group	E g (eV)	SHG (×AgGaS2)	Ref.
1	CsMn4In5Te12	R3 (no. 146)	Trigonal	1.48	0.3	5
2	K3Mn2Sb3S8	I4̄2m (no. 121)	Tetragonal	1.58	0.4	6
3	Rb3Mn2Sb3S8	I4̄2m (no. 121)	Tetragonal	1.61	0.3	6
4	Pb0.72Mn2.84Ga2.95Se8	P6̄ (no. 174)	Hexagonal	1.65	4.4	7
5	RbMn4In5Se12	R3 (no. 146)	Trigonal	1.76	4.4	5
6	CsMn4In5Se12	R3 (no. 146)	Trigonal	1.79	4.0	5
7	BaMnSnS4	Fdd2 (no. 43)	Orthorhombic	1.90	1.2	8
8	CsMnAs3S6	I4̄ (no. 82)	Trigonal	2.02	0.9	9
9	Li2MnSnS4	Pmn21 (no. 33)	Orthorhombic	2.03	0.5	10
10	RbMnAs3S6	R3 (no. 146)	Trigonal	2.16	1.1	9
11	Pb0.65Mn2.85Ga3S8	P6̄ (no. 174)	Hexagonal	2.25	1.5	7
12	Sr3MnSn2S8	I4̄3d (no. 220)	Cubic	2.33	1.0	11
13	Ba10In2Mn11Si3O12S18	I4̄3m (no. 217)	Cubic	2.38	3 × α-SiO2	12
14	Eu2MnGe2OS6	P4̄21m (no. 113)	Tetragonal	2.40	0.3	13
15	Ba6Cu2.2Mn0.8(SnS4)4	I4̄3d (no. 220)	Cubic	2.5	2.4	14
16	Ba6(Cu2Mn)Ge4S16	I4̄3d (no. 220)	Cubic	2.70	2.2	15
17	[Ba4Cl2][MnGa4Se10]	I4̄ (no. 82)	Tetragonal	2.78	1.0	16
18	[K4Cl][MnGa9S16]	P1 (no. 1)	Triclinic	3.02	0.6	17
19	Li2MnGeS4	Pmn21 (no. 33)	Orthorhombic	3.07	0.5	18
20	[K3Cl][Mn2Ga6S12]	P31c (no. 159)	Trigonal	3.17	3.10	19
21	CsMn3Ga5S11	Pc (no. 7)	Monoclinic	3.23	0.8	20
22	RbMn3Ga5S11	Pc (no. 7)	Monoclinic	3.27	0.7	20
23	K3Rb3[K3Cl][Li2Mn4Ga12S27]	P63mc (no. 186)	Hexagonal	3.31	1.1	21
24	Sr2MnGe2OS6	P4̄21m (no.113)	Tetragonal	3.51	0.3	22
★	Sr 3 MnGe 2 S 8	I4̄3d (no. 220)	Cubic	3.06	2.4	This work

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To contextualize the performance of the newly discovered Sr₃MnGe₂S₈, we have benchmarked it against known Mn-based IR-NLO materials. As shown in Fig. 3 and summarized in Table 2, sulfides represent the majority of materials in this category, comprising 80% of reported compounds, whereas selenides and tellurides account for 16% and 4%, respectively (Fig. 3a). When classified by anionic composition, these chalcogenides can be categorized into two groups: single-anion chalcogenides, which constitute the predominant fraction (72%), and mixed-anion chalcogenides, representing a smaller proportion (28%). The latter includes oxychalcogenides (12%) and salt-inclusion chalcogenides (16%) (Fig. 3b). Statistical analysis indicates that Mn-based IR-NLO chalcogenides crystallize in all six crystal systems, with the following distribution: cubic (24%), tetragonal (20%), orthorhombic (12%), trigonal (24%), hexagonal (12%), and monoclinic (8%) (Fig. 3c). This distribution reflects a pronounced preference for high-symmetry systems (approximately 80%), whereas low-symmetry structures, such as the triclinic and monoclinic system, are relatively uncommon. Fig. 3d summarizes two critical performance indicators for these compounds—the SHG response and the E_g—with detailed data provided in Table 2. Notably, Sr₃MnGe₂S₈ achieves a remarkable balance between a wide E_g (>3.0 eV) and a strong SHG response (>1.0 × AgGaS₂), a combination that effectively overcomes the typical performance trade-off in Mn-based NLO systems.

Fig. 3 Statistical distribution of Mn-based IR-NLO chalcogenides categorized by (a and b) chemical composition and (c) crystallographic system. (d) Mapping the balance between SHG and E_g for Mn-based IR-NLO chalcogenides. The outlined area identifies the most promising candidates with a wide E_g (>3.0 eV) and large SHG efficiency (>1.0 × AgGaS₂). Detailed data are provided in Table S2.

To elucidate the optical properties and structure–performance correlations of the title compound, first-principles calculations of DFT were conducted using the VASP package. The electronic band structure along high-symmetry paths (Fig. S4) confirms that Sr₃MnGe₂S₈ is an indirect bandgap semiconductor with a theoretical E_g of 2.23 eV. The computed value is moderately lower than the experimental result, a discrepancy commonly associated with the discontinuity in the exchange–correlation energy inherent to the GGA.⁴³ The partial density of states (PDOS) in Fig. 4 reveals pronounced orbital hybridization between Mn and S, as well as between Ge and S, across both the valence bands (VBs) and conduction bands (CBs), indicative of strong covalent character. The upper VB is mainly constituted by Ge 4s, Ge 4p, Mn 3d, and S 3p orbitals, while the lower CB is primarily contributed by Ge 4s, Ge 4p, Mn 3d, and S 3p orbitals. Since electronic transitions near the band edges are critical in governing optical behavior, the Ge–S and Mn–S groups are considered key structural units responsible for the optical E_g and NLO properties of Sr₃MnGe₂S₈. Furthermore, the SHG coefficients of Sr₃MnGe₂S₈ were evaluated based on the electronic structure using the length-gauge formalism.⁴⁴ In accordance with Kleinman's symmetry⁴⁵ and the I4̄3d space group, only one independent SHG tensor element, d₁₄, is present, and its value is calculated as 15.72 pm V⁻¹, showing good consistency with experimental findings.

Fig. 4 The partial density of states (PDOS) of Sr₃MnGe₂S₈.

Conclusions

In summary, a novel NCS Mn-based quaternary sulfide, Sr₃MnGe₂S₈, has been successfully synthesized via an equivalent partial cation substitution strategy, using the CS compound Sr₂GeS₄ as the parent template. This material demonstrates outstanding IR-NLO performance, characterized by a strong SHG response (2.4 × AgGaS₂ at 2050 nm), a wide E_g (3.06 eV), a high LIDT (13.4 × AgGaS₂), and a broad transparency range (0.37–14.2 μm). These properties are attributed to the synergistic effect arising from the distortion and favorable alignment of the constituent [MnS₄] and [GeS₄] tetrahedral units. Notably, Sr₃MnGe₂S₈ stands out as one of the very few Mn-based chalcogenides that simultaneously surpass the benchmark thresholds of large SHG efficiency (>1.0 × AgGaS₂) and wide E_g (>3.0 eV). Through detailed structural and theoretical analysis, we establish that the partial substitution of Sr by Mn not only triggers a CS-to-NCS structural transformation but also significantly enhances the NLO activity. This work provides an effective chemical design strategy for achieving balanced NLO performance and opens a viable pathway for the discovery of high-performance IR-NLO materials through targeted structural engineering.

Author contributions

A-Lan Xu: investigation, formal analysis, writing – original draft. Mao-Yin Ran: investigation, formal analysis, writing – review & editing. Zuju Ma: methodology, validation. Xin-Tao Wu: conceptualization, writing – review & editing. Hua Lin: supervision, conceptualization, writing – review & editing. Qi-Long Zhu: supervision, writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: additional experimental and theoretical results, together with additional tables and figures. See DOI: https://doi.org/10.1039/d5qi02182k.

CCDC 2488264 contains the supplementary crystallographic data for this paper.⁴⁶

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22175175), Natural Science Foundation of Fujian Province (2022L3092 and 2023H0041). The authors thank Prof. Bing-Xuan Li at FJIRSM for helping with the NLO measurements, Prof. Long-Hua Li at Jiangsu University and Prof. Yong-Fan Zhang at Fuzhou University for helping with the DFT calculations.

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