From oxides to oxysulfides: the mixed-anion GeS3O unit induces huge improvement in the nonlinear optical effect and optical anisotropy for potential nonlinear optical materials.

Oxysulfides combining intrinsic performance advantages between sulfides (strong NLO response) and oxides (wide optical bandgap) are proposed as potential infrared (IR) NLO materials. Theoretical calculation shows that the mixed-anion GeS3O tetrahedron has a stronger polarizability anisotropy and hyperpolarizability than that of the typical GeO4 unit. Based on this, two Sr2MGe2S6O (M = Zn, Cd) oxysulfides with the GeS3O unit show dozens of times improvement in critical birefringence and the NLO effect compared with those of isostructural Sr2ZnGe2O7. Moreover, structure-property study further verifies that the mixed-anion GeS3O ligand is a useful NLO-active unit and can offer great influence over the NLO origin. This research result also gives us a feasible design strategy and research system to explore new IR NLO candidates. This journal is © The Royal Society of Chemistry.

Nonlinear optical (NLO) crystals have shown extensive applications in extending the conventional laser wavelengths to unusual short deep-ultraviolet (DUV) or mid- and far infrared (MFIR) regions through frequency-conversion technology.[1-4] As for the IR region, inherent performance drawbacks (low laser damage threshold (LDT) and harmful two-photon absorption (TPA)) in commercial NLO crystals have seriously limited their applications.[5] As we know, the optical bandgap is proportional to the LDT but shows an inverse relationship with the NLO response; thus, it is extremely challenging to design new promising IR NLO crystals with balanced performances, such as wide bandgap (≥3.0 eV) and large second harmonic generation (SHG) effect (≥0.5 × AgGaS2).[6] Recently, researchers have proposed several feasible design strategies to regulate the crystal structures and performances. Incorporation of mixed-anion functional groups into crystal structure was regarded as one good way to solve the above problem.[7] Based on this, several of chalcohalides have been synthesized and shown the excellent performances compared with halogen-free analogues.[8] Nowadays, oxysulfides have been also attracted increasing attentions because this system can be viewed as the modification of sulfides and oxides, and oxysulfides exhibit the performance advantages in both of them (good NLO response and wide bandgap).[9] Besides, chalcohalides often appear the structural changes by anion-substitution of S2− with halogen (X−) owing to different valence states, but the atom substitution with same valence (such as O2− to S2−) can maintain the similar structural features, thus, oxysulfide system provides one good way to investigate the influence of mixed-anion ligand on property while compared with their isostructural oxides. With this in mind, two oxysulfides Sr2MGe2S6O (M = Zn, Cd) were successfully synthesized and their properties were systematically compared with Sr2MGe2O7 in this work. Among them, crystal structure of Sr2ZnGe2S6O was reported in 1985 but its performances have not been studied so far.[10] Besides, optical performances of Sr2CdGe2S6O were reported in the Lin's master's thesis in 2019.[11] However, in view of the similar structures between Sr2MGe2S6O oxysulfides and Sr2ZnGe2O7 oxides, this is good case to compare the performance change rule and study the inherent ligand-property relationship, such as from typical GeO4 to mixed-anion GeS3O unit. Herein, we have done the detailed performance comparison between title oxysulfides and isostructural oxides based on experimental and theoretical methods. Critical performances in Sr2MGe2S6O (such as optical anisotropy and SHG response) show the obvious enhancement compared with those of Sr2ZnGe2O7, which indicates that oxysulfides have good potential to be used as IR NLO candidates.

Sr2MGe2S6O (M = Zn, Cd) crystallize in the tetragonal P4̄21m space group (Table S1†). Among them, we have chosen Sr2CdGe2S6O as the representative to discuss their structural features (Fig. 1). In its asymmetric unit, there is composed of one Sr, one Cd, one Ge, one O and two S atoms (Table S2†). Ge atom exhibits the four-coordination mode with one O and three S atoms to form mixed-anion GeS3O tetrahedron with d(Ge–S) = 2.147–2.206 Å and d(Ge–O) = 1.834 Å (Fig. 1d). Two GeS3O units link together by sharing one O atom to compose the isolated Ge2S6O dimer. Typical CdS4 units link with Ge2S6O dimers to form the two-dimensional (2D) [CdGe2S10O] layers (Fig. 1e). Sr atoms connect with one O and seven S atoms to form the SrS7O dodecahedron with d(Sr–S) = 3.014–3.278 Å and d(Sr–O) = 2.829 Å (Fig. 1c). SrS7O units are located at the interlayers and further link with these 2D layers to form a 3D network (Fig. 1b). From another point of view, Sr atoms are located within the tunnels seen from the c-axis (Fig. 1a). Sr2MGe2S6O are isostructural and their whole structures are composed of 2D [MGe2S10O] layers and Sr atoms are located within the interlayers. Note that MS4 (M = Zn, Cd) are regular tetrahedral units and one MS4 unit is connected with four (Ge2S6O) dimers to form the windmill shapes. We have added the structural diagrams of Sr2MGe2S6O from the same direction (along the c-axis) (Fig. S1†). Compared with the distortion degree of GeS3O unit, they have the tiny differences, such as Δd = 5.42 × 10−3 in Sr2CdGe2S6O and Δd = 5.05 × 10−3 in Sr2ZnGe2S6O. Besides, ZnS4 and CdS4 can be viewed as the regular tetrahedral units with the same d(Zn–S) = 2.327 Å and d(Cd–S) = 2.494 Å. We have surveyed the compounds with O–Ge–S tetrahedron based on the Inorganic Crystal Structure Database and the result shows that MGeOS2 (M = Sr, Ba)[9] have another special GeO2S2 unit and these GeO2S2 units further link together to compose the (GeO2S2) chains, which is different with that (isolated Ge2S6O dimer) in Sr2MGe2S6O. Note that GeO2S2 unit shows close relationship with SHG effect but the specific contribution of GeS3O unit on the SHG origin has not been studied. In view of the disparity between Ge–S and Ge–O bond length, GeS3O exhibits the higher distortion degree than that of GeO4, which is conducive to the generation of large SHG response.

Fig. 1

(a) Crystal structure of Sr2CdGe2S6O seen from c-axis (Sr–S/Sr–O bonds were omitted for clearly); (b) crystal structure of Sr2CdGe2S6O seen from b-axis (Sr–S/Sr–O bonds were omitted for clearly); coordination modes of SrS7O (c) and GeS3O (d) units with bond-length (Å); (e) 2D layer composed of CdS4 and GeS3O units.

Microcrystals of Sr2MGe2S6O were successfully synthesized in vacuum-sealed silica tubes and their experimental powder XRD patterns are in consistence with corresponding calculated ones (Fig. 2a and b). Their polycrystalline samples show the good chemical stability and can be stably stored in air more than half a year. We have also investigated their thermal behaviour and measured the differential thermal curves in the customized tiny silica tubes. Sr2MGe2S6O have the explicit endothermic and exothermic peaks in the heating and cooling process, for example, Sr2ZnGe2S6O has the single melting temperature (1030 °C) and crystallization point (832 °C), whereas the melting and crystallization points of Sr2CdGe2S6O are 941 and 776 °C, respectively (Fig. 2c and d). We have also studied the XRD patterns before and after recrystallization process and they still have the similar XRD patterns, which verifies that Sr2MGe2S6O are congruent-melting compounds (Fig. 2a and b). Thus, Sr2MGe2S6O oxysulfides could be grown as large-size single-crystals by the conventional Bridgman–Stockbarger (BS) method. Their diffuse-reflection spectra were measured and their experimental bandgaps are 3.30 eV for Sr2ZnGe2S6O (colorless) and 3.13 eV for Sr2CdGe2S6O (pale-yellow) (Fig. 2e), respectively, which are smaller than that of Sr2ZnGe2O7 (4.31 eV) (Fig. S2 and S3†). First principles calculation was used to analyze the inherent structure–property relationship.[12] Seen from their electronic structures, Sr2MGe2S6O are indirect bandgap compounds and their theoretical bandgaps are calculated to be 2.77 and 2.67 eV, respectively (Fig. 3a and b), such theoretical values are often estimated due to the GGA calculation problem. Besides, they have the similar density of states (DOS) and Sr2ZnGe2S6O was chosen as representative to be discussed (Fig. 3c and d). Near the top of valence band (VB: −5 to 0 eV), this region is mainly composed of S-p orbital with minor contribution from Ge-p and Zn-p orbitals. On the bottom of conduction band (CB), Zn-s, Ge-s, Ge-p and S-p orbitals produce the major contribution on this region, thus, optical absorption of Sr2ZnGe2S6O can be attributed as the synergistic effect between ZnS4 and GeS3O units. Compared with the DOS diagram of Sr2ZnGe2O7 (Fig. S5†), S-p orbital makes the great influence on the Fermi-level and further induces the obvious red shift of short absorption edge in Sr2ZnGe2S6O.

Fig. 2

Powder XRD patterns of Sr2ZnGe2S6O (a) and Sr2CdGe2S6O (b); DSC curves of Sr2ZnGe2S6O (c) and Sr2CdGe2S6O (d); (e) optical bandgaps of Sr2MGe2S6O; (f) SHG response versus particle size in Sr2MGe2S6O with AgGaS2 as reference.

Fig. 3

(a) Band structure of Sr2ZnGe2S6O; (b) band structure of Sr2CdGe2S6O; (c) PDOS diagram of Sr2ZnGe2S6O; (d) PDOS diagram of Sr2CdGe2S6O.

In view of the isostructural NCS structures between Sr2MGe2S6O and Sr2ZnGe2O7, their powder SHG responses have been tested with a 2.09 μm Q-switch pulse laser and commercial AgGaS2 was chosen as the reference (Fig. 2f). SHG signal intensities of Sr2MGe2S6O show the continuously increasing trend with the enhanced particle sizes range from 38 to 250 μm, which shows that they satisfy the phase-matching (PM) condition. And at the maximum particle size (200–250 μm), they exhibit the good SHG responses about 0.6 for Sr2ZnGe2S6O and 0.7 times that of AgGaS2 for Sr2CdGe2S6O, respectively, which are comparable to those of other reported NLO oxysulfides, such as Sr5Ga8O3S14 (0.8 × AgGaS2),[9] SrGeOS2 (0.4 × AgGaS2)[9] and BaGeOS2 (0.5 × AgGaS2)[9] signal under the 2.09 μm about 1/20 times than of AgGaS2 at the maximum particle size and they cannot achieve the PM behavior (Fig. S4†). Theoretical NLO coefficients are calculated to be −3.71 for Sr2ZnGe2S6O and −3.53 pm V−1 for Sr2CdGe2S6O, respectively, which are much larger than that (−0.144 pm V−1) of Sr2ZnGe2O7. Therefore, SHG responses of Sr2MGe2S6O undergo the great promotion about 25 times that of Sr2ZnGe2O7, which agree well with the As for the previously Lin’ reported Sr2CdGe2S6O,[11] its SHG effect about 0.3 × AgGaS2 was measured at one particle size at 125–150 μm. Unfortunately, Sr2ZnGe2O7 only shows the very weak SHG data. In order to verify the specific PM condition, we have also calculated their optical anisotropy (Δn) between Sr2MGe2S6O and Sr2ZnGe2O7 (Fig. S6†). The result shows that Sr2MGe2S6O exhibit the significant increase (0.126–0.141 @ 2 μm) about up to dozens of times that of Sr2ZnGe2O7 (0.011 @ 2 μm), thus, mixed-anion units have a good chance to enhance the birefringence for critical phase-matching demand, which is also corresponding to the change rules of polarizability anisotropy from GeO4 to GeS3O ligand (Fig. 5a). SHG-density calculation (Fig. 4) was also used to analyse the origin of NLO effect in oxysulfides and their SHG responses can be attributed as the collaborative contribution from MS4 and GeS3O units, which indicates that GeS3O unit is one useful NLO-active unit and offer the positive effect on the SHG origin. Moreover, gaussian calculation shows that GeS3O exhibits the stronger hyperpolarizability (βmax) and polarizability anisotropy (δ) than those of GeO4, which are consistence with the variation of experimental results from Sr2ZnGe2O7 to Sr2MGe2S6O (Fig. 5). Moreover, increase optical bandgaps and decreased SHG responses were appeared from Sr2ZnGe2S6O to Sr2CdGe2S6O with the Cd was replaced with Zn atoms. This study also verifies the oxysulfides as the optimal research system for the exploring the new IR NLO materials.

Fig. 5

Calculated hyperpolarizability (βmax) and polarizability anisotropy (δ) of GeO4 and GeS3O units (upper); property comparison between birefringence and SHG response among Sr2ZnGe2O7 and Sr2MGe2S6O (lower).

Fig. 4

SHG-density maps of Sr2ZnGe2S6O. (a) Occupied and (b) unoccupied states in the virtual-electron (VE) process; (c) occupied and (d) unoccupied states in the virtual-hole (VH) process.

Conclusions

In summary, through the partial substitution of O with S atoms in the structure of Sr2ZnGe2O7, Sr2MGe2S6O IR NLO oxysulfides with specific GeS3O unit were successfully synthesized. Research result shows that Sr2MGe2S6O achieve the good balanced performances between wide bandgap (3.13–3.30 eV) and good SHG response (0.6–0.7 × AgGaS2), showing the potential application as IR NLO materials. Besides, Sr2MGe2S6O satisfy the PM condition and their birefringence (Δn = 0.126) are much larger than that (Δn = 0.011) of Sr2ZnGe2O7 due to the great contribution of mixed-anion units. Theoretical analysis indicates that mixed-anion GeS3O unit offers the positive influence on the SHG origin. Furthermore, oxysulfides coexisting the property advantages between oxides and sulfides could be regarded as a good system choice for the future exploration of new IR NLO crystals.

Conflicts of interest

There are no conflicts to declare.