Ca₂La(MS₄)(BS₃) (M = Ge/Si and Sn/Si): high-performance infrared nonlinear optical materials designed using an atomic site co-occupancy strategy

Abstract

Exploration of new material systems and optical performance enhancement are huge challenges for the study of infrared nonlinear optical (IR NLO) materials. In this work, the first thioborate-thiogermanate and thioborate-thiostannate compounds, Ca₂La(Ge_0.72Si_0.28S₄)(BS₃) and Ca₂La(Sn_0.75Si_0.25S₄)(BS₃), containing both co-occupied Ca²⁺/La³⁺ cation and [Ge/SiS₄]⁴⁻ or [Sn/SiS₄]⁴⁻ anion sites, respectively, were designed through an atomic site co-occupancy strategy. They inherited favourable 3D network structures in which the effectively aligned [MS₄]⁴⁻ and [BS₃]³⁻ functional anions were bridged by Ca²⁺/La³⁺ cations. Remarkably, the title compounds achieved excellent IR NLO properties, including good chemical and thermal stabilities, wide light transmission ranges (0.45–11 μm), strong second harmonic generation responses (1.5 and 2.0 times that of commercial AgGaS₂ at 2.05 μm) and high laser-induced damage thresholds (7 and 6 times that of AgGaS₂). Theoretical calculation and experimental results revealed that, on the basis of excellent structural framework, introducing more active functional groups through atomic site co-occupancy could simultaneously enhance the second harmonic generation effect and maintain a relatively high laser-induced damage threshold. This work not only offers an easier synthetic route for mixed anionic thioborates but also provides inspiration for the design of well-performed NLO materials.

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Introduction

There are urgent demands for infrared nonlinear optical crystals with various advanced optical applications, such as optical communication, optical imaging, laser guidance and so on.^1–4 However, the commercially available IR NLO crystals suffer from many drawbacks, such as the low laser-induced damage threshold (LIDT) of AgGaS₂, the poor phase matching ability of AgGaSe₂, and the detrimental two-photon absorption of ZnGeP₂.^5–7 Meanwhile, the conflict between large NLO coefficient and wide band gap hinders the achievement of overall excellent properties. Hence, the explorations of novel high-performance IR NLO materials with large nonlinear coefficients, phase matching ability (appropriate Δn), high laser-induced damage threshold (LIDT), wide optical transmission range, and high physical and chemical stability remain a hot topic.^8–10

To achieve unique non-centrosymmetric structures and improved IR NLO performance, increasing the diversity of components in chalcogenides has become a mainstream method.^11–16 Starting from the typical chalcopyrite configuration, numerous diamond-like IR NLO materials composed of different tetrahedral motifs have been explored, such as Hg₃P₂S₈ (4.2 × AGS) and Li₂ZnSiS₄ (1.2 × AGS).^17–20 In addition, cations with different coordination geometries have been combined with tetrahedral units, which led to the discovery of materials such as 3D close-packed Eu₂P₂S₆, 3D tunnel structural Li₄MgGe₂S₇ (0.7 × AGS), 2D layered SrZnGeS₄ (0.9 × AGS) or salt-inclusion [ABa₂Cl][Ga₄S₈] (A = Rb, Cs; 0.9 and 1 × AGS).^21–28 Besides the mainstream tetrahedral motifs, planar triangular groups, which exhibit large hyperpolarizabilities and polarizability anisotropies, have also caught researchers’ eye.^29–31 Among them, the π-conjugated BS₃ trigonal planar unit can not only produce strong second harmonic generation (SHG) response and large birefringence but simultaneously facilitate high LIDT, as exampled by BaB₂S₄ (0.8 × AGS) and LaBS₃ (1.2 × AGS).^32–35 However, due to the immature synthesis method and worrying stability of thioborates, there are still few IR NLO materials containing BS₃ groups up to now.³⁶ Committed to exploring new material systems, the first thioborate–thiosilicates, Ca₂Ln(BS₃)(SiS₄) (Ln = La, Ce, and Gd; ≈1 × AGS), containing both triangular BS₃ and tetrahedral SiS₄ units have been recently reported by our group.³⁷

Introducing defects or disorders has been demonstrated to be able to adjust the polarizations, alignments, and densities of functional groups.³⁸ For instance, in Pb_2.15Li_0.85Nb_4.85Ti_0.15O₁₅ and Pb_2.15Li_0.55Nb_4.85W_0.15O₁₅, doping transition metal Ti⁴⁺ or W⁶⁺ cations into the Nb⁵⁺ sites increased the distortion of the Nb/MO₆ octahedra, producing a significantly enhanced SHG effect (56 and 67 × KH₂PO₄).³⁹ Additionally, the vacancies in Pb_1.91K_3.22□_0.85Li_2.96Nb₁₀O₃₀ (□: vacancies) also contributed to its notable SHG effect (≈71.5 × KH₂PO₄).⁴⁰ Similar examples have also been found in IR NLO materials.^41,42 After doping the Na⁺ sites with Ag⁺ or doping Ag⁺ positions with Li⁺, Na₂Ba[(Ag_0.9Na_0.1)₂Sn₂S₇] and LiAgIn₂GeS₆ achieved a stronger SHG effect (1.6 and 0.8 × AGS, respectively).^43,44 However, these cases are based on the cation introduction, which led to weak performance changes.

Inspired by the above results, and considering the benefits of the structure frame on optical anisotropy (Δn = 0.149) and large laser damage threshold (10 × AGS) in Ca₂La(BS₃)(SiS₄), we aim to dope more flexible Ge⁴⁺ and Sn⁴⁺ into the Si⁴⁺ site in Ca₂Ln(BS₃)(SiS₄) to further enhance the IR NLO performances.³⁴ As a result, the first examples of thioborate-thiogermanates and thioborate-thiostannates, namely, Ca₂La(Ge_0.72Si_0.28S₄)(BS₃) and Ca₂La(Sn_0.75Si_0.25S₄)(BS₃), have been successfully designed and synthesized. They crystallize in the polar space group P6₃mc, featuring a 3D network structure in which the discrete [BS₃]³⁻ and [MS₄]⁴⁻ (M = Ge/Si and Sn/Si) anionic groups are interconnected by the co-occupied Ca²⁺ and La³⁺ cations. In addition, they exhibit stronger SHG effects (1.5 and 2.0 times that of commercial AGS), good chemical and thermal stabilities, wide light transmission range (0.45–11 μm) and high laser-induced damage thresholds (7 and 6 times that of AGS), which prove that Ca₂La(MS₄)(BS₃) (M = Ge/Si and Sn/Si) are promising infrared nonlinear optical materials.

Results and discussion

Syntheses

The single crystals of Ca₂La(MS₄)(BS₃) (M = Ge/Si and Sn/Si) were synthesized by solid-state reactions in sealed SiO₂ tubes at 900 and 950 °C with BaS, CaS, La₂S₃, GeO₂ (or SnO₂), B and S powders as reactants (ESI†). During our previous syntheses of Ca₂Ln(SiS₄)(BS₃) (Ln = La, Ce, and Gd), the replacement reaction between B (B₂S₃) and SiO₂ was utilized to form the Si–B–S system.⁴⁵ The successful syntheses of Ca₂La(MS₄)(BS₃) proved that this method can be extended to other group 14 element-B–S systems, which is of great significance in overcoming the synthetic difficulties of thioborates caused by the strong affinity of boron to oxygen. It is worth mentioning that similar reactions were also previously used by Guo et al. for the synthesis of thiosilates and thioganates.^46,47

Phase analysis

The purities of Ca₂La(MS₄)(BS₃) (M = Ge/Si and Sn/Si) were validated by the powder X-ray diffraction studies (Fig. 1a and b). Energy-dispersive X-ray spectroscopy (EDS) analyses revealed the presence of Ca, La, M and S atoms (Fig. 1c and d) with molar ratios 1.8 : 1: 0.7 : 0.3 : 5.5 and 1.8 : 1: 0.8 : 0.2 : 6.7 for Ca₂La(Ge_0.72Si_0.28S₄)(BS₃) and Ca₂La(Sn_0.75Si_0.25S₄)(BS₃), respectively, which are very close to the results of the X-ray structural analysis. The presence of the B element is confirmed by the presence of the infrared absorption bands associated with the [BS₃]³⁻ unit in the IR spectra.

Fig. 1 Powder X-ray diffraction patterns and EDS maps for Ca₂La(Ge_0.72Si_0.28S₄)(BS₃) (a and c) and Ca₂La(Sn_0.75Si_0.25S₄)(BS₃) (b and d).

Crystal structure

Ca₂La(MS₄)(BS₃) (M = Ge/Si and Sn/Si) are isostructural and crystallize in the hexagonal polar space group P6₃mc (No. 186). They are also isostructural to Ca₂Ln(BS₃)(SiS₄), but exhibit lattice distortion, which can be proved by the overall small-angle shift of their PXRD peaks (Fig. S1†).³⁴ The asymmetric unit of Ca₂La(MS₄)(BS₃) (M = Ge/Si and Sn/Si) is composed of one mixed site of Ca²⁺ and La³⁺, one mixed site of Si⁴⁺ and Ge⁴⁺ or Sn⁴⁺, one B, and three S atoms. In Ca₂La(Ge_0.72Si_0.28S₄)(BS₃), the refined occupancy factors of Ca²⁺ and La³⁺ are 0.652(3) and 0.348(3), respectively, and those of Ge⁴⁺ and Si⁴⁺ are 0.720(8) and 0.280(8), respectively. In Ca₂La(Sn_0.75Si_0.25S₄)(BS₃), the refined occupancy factors of Ca²⁺ and La³⁺ are 0.619(3) and 0.381(3), respectively, and those of Sn⁴⁺ and Si⁴⁺ are 0.755(9) and 0.245(9), respectively (Tables S1 and S2, ESI†). The Ca(1)/La(1), S(2) and S(3) atoms lie on the 6c site, whereas Ge(1)/Si(1), B(1) and S(1) occupy the 2a sites. As shown in Fig. 2a and Fig. S2a,† the structure of Ca₂La(MS₄)(BS₃) can be also viewed as a derivative of Sr₃[SnOSe₃][CO₃] where Sr, SnOSe₃, and CO₃ units are substituted by Ca/La, MS₄, BS₃ groups, respectively.⁴⁸ Both compounds feature a three-dimensional network composed of discrete tetrahedral and planar triangular anions interconnected by counter-metal cations (Fig. 2b and Fig. S2b†). Differently, in the Ca₂La(MS₄)(BS₃), the [BS₃]³⁻ triangles and MS₄ tetrahedra are arranged perfectly parallel to the ac plane, creating a sixfold rotation symmetry. While in Sr₃[SnOSe₃][CO₃] with a lower symmetric Pmn2₁ space group, the CO₃ and SnOSe₄ groups are not well aligned. The aligned active groups in Ca₂La(MS₄)(BS₃) are advantageous for nonlinear optical crystals, which facilitates the efficient superposition of the microscopic nonlinear coefficients, contributing to a stronger macroscopic SHG effect (1.5 and 2.0 × AGS) than that by Sr₃[SnOSe₃][CO₃] (1.0 × AGS).

Fig. 2 CaS₆, MS₄ and BS₃ functional motifs in Ca₂La(MS₄)(BS₃) (M = Ge/Si and Sn/Si) (a), and the overall crystal structure of Ca₂La(MS₄)(BS₃) viewed down the c-axis (b).

Thermal analysis

As shown in Fig. S3,† thermogravimetric analysis (TGA) and differential thermal analysis (DTA) curves measured under the N₂ atmosphere revealed that the Ca₂La(Ge_0.72Si_0.28S₄)(BS₃) and Ca₂La(Sn_0.75Si_0.25S₄)(BS₃) are stable up to about 700 °C, which are lower stabilities than that of Ca₂Ln(BS₃)(SiS₄) (880 °C).³⁴ This may be due to the weaker M–S covalent bonds compared to the Si–S bonds.

Optical properties

Raman spectra of both compounds exhibit no absorption peaks in the range of 500 to 4000 cm⁻¹ (Fig. 3a). The characteristic peaks at 366, 388, 445 cm⁻¹ for Ca₂La(Ge_0.72Si_0.28S₄)(BS₃) and 350, 422, 442 cm⁻¹ for Ca₂La(Sn_0.75Si_0.25S₄)(BS₃) are associated with vibrations of Ge–S and Sn–S bond, respectively, which are in good agreement with the reported data of Na₂Hg₃M₂S₈.⁴⁹ In addition, the characteristic peaks at low energy area (120, 146, 150 and 225 cm⁻¹) can be assigned to Ca/La–S bonds referring to La₃LiMS₇.⁵⁰

Fig. 3 Raman spectra (a), Fourier transform infrared spectra (b) and UV-vis-IR diffuse reflectance spectra (c) of the title compounds.

As shown in the IR spectra (Fig. 3b), Ca₂La(MS₄)(BS₃) (M = Ge/Si and Sn/Si) show good light transmission in the 4000–900 cm⁻¹ range. Since the IR absorption bands are dominated by the [BS₃]³⁻ and [SiS₄]⁴⁻ groups, their IR spectra are very similar. The absorption bands at ≈800–900 cm⁻¹ and 400 cm⁻¹ can be attributed to the asymmetrical B–S stretching E modes and symmetrical B–S stretching A′1 modes of the (BS₃)³⁻ units, respectively.⁵¹ In addition, the strong absorption peak at ≈500 cm⁻¹ can be attributed to Si–S bond vibrations.

UV-vis-NIR diffuse reflectance spectra revealed the band gaps of 3.15 and 2.62 eV for Ca₂La(Ge_0.72Si_0.28S₄)(BS₃) and Ca₂La(Sn_0.75Si_0.25S₄)(BS₃), respectively (Fig. 3c), which matches with their pale yellow and yellow colours. It is worth mentioning that their band gaps are close to those of LaLi₃GeS₇ (3.02 eV) and LaLi₃SnS₇ (2.40 eV), which indicate that their obviously different band gaps are mainly determined by the GeS₄ and SnS₄ groups.⁵⁰ Combining the IR spectra, Ca₂La(MS₄)(BS₃) (M = Ge/Si and Sn/Si) show good light transmittance in the visible to mid-IR spectrum range of 0.45–11 μm, suggesting their potential as IR NLO materials.

SHG and LIDT performances

Powder SHG measurements indicate that both Ca₂La(Ge_0.72Si_0.28S₄)(BS₃) and Ca₂La(Sn_0.75Si_0.25S₄)(BS₃) exhibit strong SHG effects, which are 1.5 and 2.0 times that of commercial AGS at 2.05 μm, respectively (Fig. 4a). Additionally, particle size-dependent SHG curves suggest that they are type-I phase-matchable (Fig. 4b). Compared to Ca₂La(SiS₄)(BS₃) (1.0 × AGS), Ca₂La(MS₄)(BS₃) achieved significant SHG enhancement, which may be attributed to the introduction of GeS₄ and SnS₄ functional motifs with larger polarization abilities and higher SHG activities. To directly prove this, the hyperpolarizabilities, which are generally considered to be positively correlated with SHG activity, of standard SiS₄, GeS₄ and SnS₄ units were calculated to be 67, 103 and 111, respectively, by using LANL2DZ base group in the Gaussian 09 program based on a semi-empirical method.⁵² The same trends of experimental and calculated results justify the atomic site co-occupancy strategy of introducing GeS₄ and SnS₄ groups to enhance the SHG effect.

Fig. 4 Particle size-dependent SHG intensity curves for the title compounds (a) and (b); SHG and LIDT comparison of reported thioborates (c).

Laser-induced damage threshold tests were performed by radiating the microcrystalline particles with 1 HZ 1064 nm laser. The LIDT values of Ca₂La(Ge_0.72Si_0.28S₄)(BS₃) and Ca₂La(Sn_0.75Si_0.25S₄)(BS₃) were 26.36 and 25.37 MW cm⁻², which are about 7 and 6 times that of AGS (4 MW cm⁻²) (Table S4†). To further explain their higher LIDTs, the temperature-dependent cell parameters of both compounds were measured from 100 to 400 K (Fig. S4†). Linear fitting analysis showed that their thermal expansion coefficients (TEC) in the a (b) axis and c axis are 1.09 × 10⁻⁵, 1.25 × 10⁻⁵ T⁻¹ and 1.03 × 10⁻⁵, 1.38 × 10⁻⁵ T⁻¹, respectively, which are much smaller than that of AGS (2.09 × 10⁻⁵, −1.07 × 10⁻⁵ T⁻¹). In addition, the much smaller TEC anisotropy (0.15 and 0.34 to 2.95) also gives the explanation of their higher LIDTs (Table 1). Compared with Ca₂La(SiS₄)(BS₃), the smaller LIDTs of title compounds are due to weaker covalent Ge/Sn–S bonds. However, the almost equally small TEC parameters indicate that the excellent crystal framework is the basis for them to achieve large polished damage thresholds. Furthermore, the SHG and LIDT performances comparison reveal that both Ca₂La(MS₄)(BS₃) exhibit obviously enhanced SHG effects among previously reported thioborates with considerable LIDTs.⁵³

Table 1 TECs and TEAs of Ca₂La(MS₄)(BS₃) and AGS

Compounds	TEC (*10^−5 T^−1)		TEA^a
	a	c
a TEA = (TECmax − TECmin)/TECmin.
Ca2La(Ge0.72Si0.28S4)(BS3)	1.09	1.25	0.15
Ca2La(Sn0.75Si0.25S4)(BS3)	1.03	1.38	0.34
AGS	2.09	−1.07	2.95

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Theoretical calculations

For further understanding of the relationship between structures and properties of title compounds, the first-principles calculations based on crystal structures and DFT methods were performed.^54,55 To facilitate the calculations, disorder-free structures of Ca₂La(GeS₄)(BS₃) and Ca₂La(SnS₄)(BS₃) were created by lowering the symmetry from the space group P6₃mc (No. 186) to Cmc2₁ (No. 36), in which the six Ln_1/3Ca_2/3 sites in the original cell were split into two Ln and four Ca atoms and the Ge/Si or Sn/Si sites were assigned as Ge or Sn, respectively. As shown in Fig. S5,† the indirect theoretical band gap values of Ca₂La(GeS₄)(BS₃) and Ca₂La(SnS₄)(BS₃) were calculated as 2.26 and 2.41 eV respectively, which are smaller compared to the experimental values due to the limitations of the GGA method. To calculate their optical properties more accurately, scissor operators of 0.89 and 0.11 eV were adopted during the subsequent optical property calculations.

The partial density of states (DOS) analysis (Fig. S6†) revealed that the band gap contributions of Ca₂La(GeS₄)(BS₃) and Ca₂La(SnS₄)(BS₃) are quite similar, which is due to their identical structures and similar chemical compositions. The topmost valence bands (VB) are dominated by the S-3p orbitals. The bottom of the conduction bands (CB), however, are contributed by La-5d, Ge-4s, Sn-5s and B-2p orbitals. Hence, the band gaps of the title compounds originate from the joint contribution of the LaS₆, MS₄, and BS₃ groups.

The theoretical birefringence values (Δn) of Ca₂La(GeS₄)(BS₃) and Ca₂La(SnS₄)(BS₃) were calculated to be 0.141 and 0.155 at 1064 nm, respectively (Fig. S7†). The equal refractive index values of the fundamental and double frequency light prove the phase matching ability of the title materials. Remarkably, the large Δn values of title compounds are close to those of Sr₃[SnOSe₃][CO₃] (0.16–0.13 at 400–700 nm) and Ca₂La(SiS₄)(BS₃) (0.149 at 1064 nm), indicating that combining anion groups with different geometries may be an effective way to achieve large birefringence.

Under the restriction of space group and Kleinman's symmetry, the largest independent SHG tensors of Ca₂La(GeS₄)(BS₃) and Ca₂La(SnS₄)(BS₃) were calculated to be d₃₂ = 7.48 and d₃₃ = −15.61 pm V⁻¹, respectively, which exhibit the same trend as the experimental results. Furthermore, the SHG-weighted electron density (SHG density) analyses for the largest independent SHG tensors were performed to reveal the distribution of the source of SHG effects. As shown in Fig. 5a and c, in the valence band, the SHG effects originate from S-3p (yellow) non-bonding states for both compounds; while in the conduction band (Fig. 5b and d), the SHG process is mainly dominated by the unoccupied La-5d (purple), Ge-4p or Sn-5p (pink) orbitals and B–S (green) π* anti-bonding states, with few contributions from Ca-3d (blue) and S-3p orbitals. Furthermore, the SHG density over VB and CB was integrated to evaluate the contribution percentages of each structure-building group. For Ca₂La(GeS₄)(BS₃), the SHG contribution percentages of Ca²⁺, LaS₆, GeS₄ and BS₃ units are 3.8%, 29.66%, 49.34%, 17.10% respectively; for Ca₂La(SnS₄)(BS₃), the SHG contribution percentages of Ca²⁺, LaS₆, SnS₄ and BS₃ units are 3.3%, 29.59%, 43.75% and 47.18%, respectively. Hence, the strong SHG responses of Ca₂La(MS₄)(BS₃) mainly originate from the synergistic effects of three SHG-active LaS₆, MS₄ and BS₃ groups.

Fig. 5 SHG density plots for Ca₂La(Ge_0.72Si_0.28S₄)(BS₃) [VB (a) and CB (b)] and Ca₂La(Sn_0.75Si_0.25S₄)(BS₃) [VB (c) and CB (d)].

Conclusions

In summary, the first examples of thioborate-thiogermanates and thioborate-thiostannates, namely, Ca₂La(Ge_0.72Si_0.28S₄)(BS₃) and Ca₂La(Sn_0.75Si_0.25S₄)(BS₃), were designed using an atomic site co-occupancy strategy. The syntheses of Ca₂La(MS₄)(BS₃) again support that the replacement reaction between boron and oxide is helpful for the syntheses of mixed anionic thioborates. In addition, Ca₂La(MS₄)(BS₃) inherit a favourable crystal structure, in which both the MS₄ tetrahedra and BS₃ triangles are arranged parallel to the ac plane, facilitating an efficient superposition of the SHG effect. Furthermore, both materials achieved good chemical and thermal stabilities, wide transmission range (0.45–11 μm), strong SHG responses (1.5 and 2.0 times that of commercial AGS) and high LIDTs (7 and 6 times that of AGS). Theoretical calculations revealed that the strong SHG effects of Ca₂La(MS₄)(BS₃) are contributed by the synergistic effects of LaS₆, MS₄ and BS₃ groups. These results demonstrate that introducing more active functional groups in an excellent structure frame could be an effective method to achieve overall high IR NLO performances.

Data availability

Supporting data for this article are presented in the ESI.† The raw data of this article can be obtained by contacting the corresponding author.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank Dr Bing-Xuan Li (Fujian Institute of Research on Structure of Matter, Chinese Academy of Sciences) for his help with the LIDT test. This work was supported by the National Natural Science Foundation of China (No. 22031009, 22375201, and 21921001) and the Self-deployment Project Research Program of Haixi Institutes, Chinese Academy of Sciences (CXZX-2022-GH06).

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Footnote

† Electronic supplementary information (ESI) available: Syntheses, methods, instrumentations, computational details, crystallographic data, LIDT test results, powder XRD, thermal analysis and computational results of title compounds. CCDC 2386136 and 2386137. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi03060e

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