Li 2 HgMS 4 ( M = Si , Ge , Sn ) : New Quaternary Diamond-Like Semiconductors for Infrared Laser Frequency Conversion

A new family of quaternary diamond-like semiconductors (DLSs), Li2HgMS4 (M = Si, Ge, Sn), were successfully discovered for the first time. All of them are isostructural and crystallize in the polar space group (Pmn21). Seen from their structures, they exhibit a three-dimensional (3D) framework structure that is composed of countless 2D honeycomb layers stacked along the c axis. An interesting feature, specifically, that the LiS4 tetrahedra connect with each other to build a 2D layer in the ac plane, is also observed. Experimental investigations show that their nonlinear optical responses are about 0.8 for Li2HgSiS4, 3.0 for Li2HgGeS4, and 4.0 for Li2HgSnS4 times that of benchmark AgGaS2 at the 55–88 μm particle size, respectively. In addition, Li2HgSiS4 and Li2HgGeS4 also have great laser-damage thresholds that are about 3.0 and 2.3 times that of powdered AgGaS2, respectively. The above results indicate that title compounds can be expected as promising IR NLO candidates.


Introduction
Solid-state lasers have shown a wide range of applications in the fields of military, industry, medical treatment and information communications [1,2].However, traditional laser sources, such as Ti:Al 2 O 3 and Nd:YAG lasers, mainly cover the wavelengths range from visible to near infrared, not including the important ultraviolet (UV < 400 nm) and middle-far infrared (MFIR, 3-20 µm) region [3,4].To extend the laser wavelength ranges, frequency-conversion technology on nonlinear optical (NLO) materials was invented and has been further developed for decades [5].Recently, many promising NLO materials have been discovered and have basically solved the demand of UV region .However, for the IR region, outstanding IR NLO materials were rarely discovered and only several ternary diamond-like semiconductors (DLSs), such as AgGaS 2 , AgGaSe 2 and ZnGeP 2 , have been commercially used [35][36][37].Although they have high second harmonic generation (SHG) coefficients and wide IR transmission regions, some of the self-defects including the low laser-damage thresholds (LDTs) or strong two-photon absorption (TPA) still seriously hinder their practical application.Researchers have done a lot of work to explore new NLO materials for the IR application, and the combination of two or more different building units into crystal structures can be viewed as a feasible way to obtain new NLO compounds.Up to now, many reports indicate that cations with second order Jahn-Teller distortions, lone electron pairs or d 10 configuration can contribute to good SHG response with the cooperative effects of typical tetrahedral Crystals 2017, 7, 107 2 of 12 units MQ 4 (M = Ga, In, Si, Ge, Sn; Q = S, Se) .Note that diamond-like semiconductors (DLSs) with inherently noncentrosymmetrical (NCS) structures, with all the tetrahedral units in crystal structures oriented in the same direction, have been further investigated on the ternary and quaternary systems [74][75][76][77][78][79][80].Among them, the d 10 cations (Zn 2+ , Cd 2+ ) containing quaternary DLSs with outstanding performances, such as Li 2 CdGeS 4 , Li 2 ZnGeSe 4 , and Li 2 ZnSnSe 4 [74,78], were proven as promising IR NLO candidates and belong to the general formula I 2 -II-IV-VI 4 , where I are the monovalent elements, II are the divalent elements, IV are the group 14 elements, and VI are the chalcogen elements.Previous reports indicate that other types of Hg-containing metal chalcogenides have shown good NLO performances in the IR field, such as HgGaS 4 [81].BaHgQ 2 (Q = S, Se), [82,83] A 2 Hg 3 M 2 S 8 (A = Na, K; M = Si, Ge, Sn) [61,68], and Li 4 HgGe 2 S 7 [84].However, up to now, the Hg-containing DLSs have been rarely investigated in the IR frequency conversion region.Thus, it is meaningful to explore new Hg-containing DLSs and study their important NLO properties.From this background, we have chosen the Li-Hg-M-S (M = Si, Ge, Sn) as the research system and successfully prepared three new IR NLO materials, Li 2 HgMS 4 (M = Si, Ge, Sn).They are isostructural and crystallize in the Pmn2 1 polar space group.Overall properties investigation shows that they can be expected to be promising IR NLO candidates owing to their large NLO coefficients, high LDTs, wide IR transparent regions, and good chemical stability.

Crystal Structure
The title compounds are isostructural and crystallize in the NCS polar space group Pmn2 1 .In order to ensure the reasonability of crystal structures of title compounds, the bond valence [85,86] and the global instability index (GII) [87][88][89] were also systemically studied (Table 1).Calculated results (Li, 1.085-1.125;Hg, 2.090-2.133;Si/Ge/Sn, 4.030-4.152;S, 2.055-2.229)indicate that all atoms are in reasonable oxidation states.In addition, GII can be derived from the bond valence concepts, which represent the tension of lattice parameters and are always used to evaluate the rationality of structure.When the value of GII is less than 0.05 vu (valence unit), the tension of structure is not proper, whereas when the value of GII is larger than 0.2 vu, its structure is not stable.Thus, the value of GII in a reliable structure should be limited at 0.05-0.2 in general.As for the title compounds, calculated GII values are in the range of 0.10-0.14vu, which illustrates that the crystal structures of all compounds are reasonable.Herein, Li 2 HgGeS 4 is chosen as the representative for the structural discussion.In its structure, each cation is linked to four S atoms, forming the typical LiS 4 , HgS 4 , and GeS 4 tetrahedra.These units connect with each other to make up a two-dimensional (2D) honeycomb layer structure, which is located at the ab plane (Figure 1b).Then, the layers are further stacked along the c axis to form a three-dimensional (3D) framework structure (Figure 1a).In addition, an interesting feature is that the LiS 4 tetrahedra connect with each other to build a 2D layer in the ac plane (Figure 1c).The whole structure is composed of tetrahedral ligands that align along the c axis.Note that the discovered quaternary DLSs in the I 2 -II-IV-VI 4 systems normally crystallize in the one of following space groups: I-42m (Cu 2 CdSnS 4 ) [76], I-4 (Cu 2 ZnSnS 4 ) [90], Pmn2 1 (Li 2 CdGeS 4 ) [74], Pna2 1 (Li 2 MnGeS 4 ) [75], and Pn (Li 2 CoSnS 4 ) [75], to our best knowledge, which represent the stannite, kesterite, wurtz-stannite, and wurtz-kesterite structural features.

Optical Properties
As for an IR NLO crystal, its optical parameters, such as optical bandgap, IR absorption edge, NLO response, and LDT, are necessary to be determined for the assessment of application prospect.The detailed mechanism for laser damage in a given material is not fully clear yet, however it has been normally accepted that strong optical absorption of the materials will cause thermal and electronic effects and finally lead to laser damage.Note that the optical breakdown can be attributed to the effect of electron avalanche that has a close relationship with optical bandgap in a given material [75].Figure 2 shows that the optical bandgaps are 2.68 for Li2HgSiS4, 2.46 for Li2HgGeS4, and 2.32 eV for Li2HgSnS4, respectively.All of them are much larger than those of commercial AgGaSe2 (1.80 eV) [86] and ZnGeP2 (1.75 eV) [37], which may be conducive to improve the laser damage resistance of the title compounds compared with the commercially available IR NLO materials.Recently, the assessment of the LDTs on powder samples has been developed as a feasible and semi-quantitative method [52,55].Thus, in this work, based on a pulse laser (1.06 μm, 10 Hz and 10 ns), the LDTs of the title compounds were measured with AgGaS2 as the reference and corresponding results are shown in Table 2. From this table, it can be found that the title compounds have great LDTs, such as 91.6 for Li2HgSiS4, 70.6 for Li2HgGeS4, and 30.5 MW/cm 2 for Li2HgSnS4, and are about 3.0, 2.3, and ~1 times that of powdered AgGaS2 (29.6 MW/cm 2 ), respectively.Moreover, Li2HgSiS4 and Li2HgGeS4 are comparable to those of PbGa2GeSe6 (3.7 × AgGaS2) [51], Na2In2GeS6 (4.0 × AgGaS2) [56], Na2Hg3Ge2S8 (3 × AgGaS2) [61], and SnGa4Se7 (4.6 × AgGaS2) [52].Therefore, Li2HgSiS4 and Li2HgGeS4 can be expected to have application with high-power lasers, compared with the commercial IR NLO materials.

Optical Properties
As for an IR NLO crystal, its optical parameters, such as optical bandgap, IR absorption edge, NLO response, and LDT, are necessary to be determined for the assessment of application prospect.The detailed mechanism for laser damage in a given material is not fully clear yet, however it has been normally accepted that strong optical absorption of the materials will cause thermal and electronic effects and finally lead to laser damage.Note that the optical breakdown can be attributed to the effect of electron avalanche that has a close relationship with optical bandgap in a given material [75].Figure 2 shows that the optical bandgaps are 2.68 for Li 2 HgSiS 4 , 2.46 for Li 2 HgGeS 4 , and 2.32 eV for Li 2 HgSnS 4 , respectively.All of them are much larger than those of commercial AgGaSe 2 (1.80 eV) [86] and ZnGeP 2 (1.75 eV) [37], which may be conducive to improve the laser damage resistance of the title compounds compared with the commercially available IR NLO materials.Recently, the assessment of the LDTs on powder samples has been developed as a feasible and semi-quantitative method [52,55].Thus, in this work, based on a pulse laser (1.06 µm, 10 Hz and 10 ns), the LDTs of the title compounds were measured with AgGaS 2 as the reference and corresponding results are shown in Table 2. From this table, it can be found that the title compounds have great LDTs, such as 91.

Optical Properties
As for an IR NLO crystal, its optical parameters, such as optical bandgap, IR absorption edge, NLO response, and LDT, are necessary to be determined for the assessment of application prospect.The detailed mechanism for laser damage in a given material is not fully clear yet, however it has been normally accepted that strong optical absorption of the materials will cause thermal and electronic effects and finally lead to laser damage.Note that the optical breakdown can be attributed to the effect of electron avalanche that has a close relationship with optical bandgap in a given material [75].Figure 2 shows that the optical bandgaps are 2.68 for Li2HgSiS4, 2.46 for Li2HgGeS4, and 2.32 eV for Li2HgSnS4, respectively.All of them are much larger than those of commercial AgGaSe2 (1.80 eV) [86] and ZnGeP2 (1.75 eV) [37], which may be conducive to improve the laser damage resistance of the title compounds compared with the commercially available IR NLO materials.Recently, the assessment of the LDTs on powder samples has been developed as a feasible and semi-quantitative method [52,55].Thus, in this work, based on a pulse laser (1.06 μm, 10 Hz and 10 ns), the LDTs of the title compounds were measured with AgGaS2 as the reference and corresponding results are shown in Table 2. From this table, it can be found that the title compounds have great LDTs, such as 91.6 for Li2HgSiS4, 70.6 for Li2HgGeS4, and 30.5 MW/cm 2 for Li2HgSnS4, and are about 3.0, 2.3, and ~1 times that of powdered AgGaS2 (29.6 MW/cm 2 ), respectively.Moreover, Li2HgSiS4 and Li2HgGeS4 are comparable to those of PbGa2GeSe6 (3.7 × AgGaS2) [51], Na2In2GeS6 (4.0 × AgGaS2) [56], Na2Hg3Ge2S8 (3 × AgGaS2) [61], and SnGa4Se7 (4.6 × AgGaS2) [52].Therefore, Li2HgSiS4 and Li2HgGeS4 can be expected to have application with high-power lasers, compared with the commercial IR NLO materials.In addition, Raman spectra (Figure 3) are also measured to determine the IR absorption edges for the title compounds.The results show that all of them exhibit the wide IR transmission regions, such as 2.5-19 µm (530 cm −1 ) for Li 2 HgSiS 4 , 2.5-22 µm (450 cm −1 ) for Li 2 HgGeS 4 , and 2.5-23.5 µm (425 cm −1 ) for Li 2 HgSnS 4 , which cover the two important atmospheric windows (3-5 and 8-12 µm) that can be used in telecommunications, laser guidance, and explosives detection.Note that the IR absorption edges gradually get longer from the Si to Sn compounds, which are consistent with the IR data for other related mental chalcogenides [61].Although the measured IR absorption data on powder samples have some deviations with the results on single-crystals, they can give the preliminary assessment for the transmission region of IR materials.Overall view on Raman spectra shows similar patterns for the title compounds, and a shift to lower absorption energies from the Si to Sn compounds that are severely affected by the tetravalent (M IV ) metals.The absorption peaks above approximately 300 cm −1 , including Li 2 HgSiS 4 (520, 396 cm −1 ), Li 2 HgGeS 4 (430, 387, 360, 327 cm −1 ), and Li 2 HgSnS 4 (402, 346 cm −1 ), can be assigned to the characteristic absorptions of the Si-S, Ge-S, and Sn-S modes, respectively.Moreover, several peaks located between 200 and 300 cm −1 , such as Li 2 HgSiS 4 (285, 258 cm −1 ), Li 2 HgGeS 4 (257 cm −1 ), and Li 2 HgSnS 4 (258 cm −1 ), are attributed to the Hg-S bonding interactions.In addition, the absorptions below 200 cm −1 are primarily corresponding to the Li-S vibrations for the title compounds.In addition, Raman spectra (Figure 3) are also measured to determine the IR absorption edges for the title compounds.The results show that all of them exhibit the wide IR transmission regions, such as 2.5-19 μm (530 cm −1 ) for Li2HgSiS4, 2.5-22 μm (450 cm −1 ) for Li2HgGeS4, and 2.5-23.5 μm (425 cm −1 ) for Li2HgSnS4, which cover the two important atmospheric windows (3-5 and 8-12 μm) that can be used in telecommunications, laser guidance, and explosives detection.Note that the IR absorption edges gradually get longer from the Si to Sn compounds, which are consistent with the IR data for other related mental chalcogenides [61].Although the measured IR absorption data on powder samples have some deviations with the results on single-crystals, they can give the preliminary assessment for the transmission region of IR materials.Overall view on Raman spectra shows similar patterns for the title compounds, and a shift to lower absorption energies from the Si to Sn compounds that are severely affected by the tetravalent (M IV ) metals.The absorption peaks above approximately 300 cm −1 , including Li2HgSiS4 (520, 396 cm −1 ), Li2HgGeS4 (430, 387, 360, 327 cm −1 ), and Li2HgSnS4 (402, 346 cm −1 ), can be assigned to the characteristic absorptions of the Si-S, Ge-S, and Sn-S modes, respectively.Moreover, several peaks located between 200 and 300 cm −1 , such as Li2HgSiS4 (285, 258 cm −1 ), Li2HgGeS4 (257 cm −1 ), and Li2HgSnS4 (258 cm −1 ), are attributed to the Hg-S bonding interactions.In addition, the absorptions below 200 cm −1 are primarily corresponding to the Li-S vibrations for the title compounds.
Second harmonic generation (SHG) responses for the title compounds were investigated on powder samples and the results are shown in Figure 4. From this figure, it can be found that the SHG intensities of the title compounds are not enhanced gradually with the increase of particle sizes, which indicates the nonphase matching behaviour for these compounds.In addition, their SHG responses are about 0.8 for Li2HgSiS4, 3.0 for Li2HgGeS4, and 4.0 for Li2HgSnS4 times that of benchmark AgGaS2 at the 55-88 μm particle size, respectively, which shows that the title compounds may have great NLO potential in the IR region as promising frequency-conversion candidates.Second harmonic generation (SHG) responses for the title compounds were investigated on powder samples and the results are shown in Figure 4. From this figure, it can be found that the SHG intensities of the title compounds are not enhanced gradually with the increase of particle sizes, which indicates the nonphase matching behaviour for these compounds.In addition, their SHG responses are about 0.8 for Li 2 HgSiS 4 , 3.0 for Li 2 HgGeS 4 , and 4.0 for Li 2 HgSnS 4 times that of benchmark AgGaS 2 at the 55-88 µm particle size, respectively, which shows that the title compounds may have great NLO potential in the IR region as promising frequency-conversion candidates.

Synthesis
All the starting materials were used as purchased without further refinement.In the preparation process, a graphite crucible was added into the vacuum sealed silica tube to avoid the reaction between metal Li and the silica tube at a high temperature.

Li2HgSiS4 and Li2HgSnS4
Target compounds were prepared with a mixture with the ratio of Li:HgS:(Si or Sn):S = 2:1:1:3, respectively.The temperature process was set as follows: first, it was heated to 700 °C in two days, and kept at this temperature about four days, then slowly down to 300 °C within four days, and finally quickly cooled to room temperature by turning off the furnace.Obtained products were washed by the N,N-dimethylformamide (DMF) solvent to remove the other byproducts.Yellow crystals for Li2HgSiS4 and orange-red crystals for Li2HgSnS4 appeared, and both of them remained stable in air over half a year.In addition, the yield of Li2HgSiS4 was about 80%.

Li2HgGeS4
Initially, we attempted to prepare Li2HgGeS4 with the ratio of Li:HgS:Ge:S = 2:1:1:3 at the reaction temperature of 700 °C.After the single crystal X-ray diffraction measurement, Li4HgGe2S7 (main product, yellow) [79] and Li2HgGeS4 (a small amount, reddish) were interestingly obtained.In addition, we had further adjusted the ratio of reactants and interestingly found that the pure-phase of Li2HgGeS4 could be obtained while the ratio of Li:HgS is greater than 2:1.Moreover, the Li2HgGeS4 crystals were repeatedly washed with DMF solvent and they also remained stable in air.

Structure Determination
Selected single-crystals were used for data collections with a Bruker SMART APEX II 4K CCD diffractometer (Bruker Corporation, Madison, WI, USA) using Mo Kα radiation (λ = 0.71073 Å) at room temperature.Multi-scan method was used for absorption correction [91].All the crystal structures were solved by the direct method and refined using the SHELXTL program package [92].As for the structural refinement of Li2HgSiS4, the initial refinement result gave the formula Li2HgSiS4, but the site of the Li atom showed abnormal anisotropy parameter (almost zero).Thus, we attempted to set the Li1 and Hg2 atoms to occupy the same site with the ratio of 0.97:0.03(Li1:Hg2) by random refinement.In view of the low occupancy (0.025) of Hg2 atom, we consider using the

Synthesis
All the starting materials were used as purchased without further refinement.In the preparation process, a graphite crucible was added into the vacuum sealed silica tube to avoid the reaction between metal Li and the silica tube at a high temperature.

Li 2 HgSiS 4 and Li 2 HgSnS 4
Target compounds were prepared with a mixture with the ratio of Li:HgS:(Si or Sn):S = 2:1:1:3, respectively.The temperature process was set as follows: first, it was heated to 700 • C in two days, and kept at this temperature about four days, then slowly down to 300 • C within four days, and finally quickly cooled to room temperature by turning off the furnace.Obtained products were washed by the N,N-dimethylformamide (DMF) solvent to remove the other byproducts.Yellow crystals for Li 2 HgSiS 4 and orange-red crystals for Li 2 HgSnS 4 appeared, and both of them remained stable in air over half a year.In addition, the yield of Li 2 HgSiS 4 was about 80%.

Li 2 HgGeS 4
Initially, we attempted to prepare Li 2 HgGeS 4 with the ratio of Li:HgS:Ge:S = 2:1:1:3 at the reaction temperature of 700 • C.After the single crystal X-ray diffraction measurement, Li 4 HgGe 2 S 7 (main product, yellow) [79] and Li 2 HgGeS 4 (a small amount, reddish) were interestingly obtained.In addition, we had further adjusted the ratio of reactants and interestingly found that the pure-phase of Li 2 HgGeS 4 could be obtained while the ratio of Li:HgS is greater than 2:1.Moreover, the Li 2 HgGeS 4 crystals were repeatedly washed with DMF solvent and they also remained stable in air.

Structure Determination
Selected single-crystals were used for data collections with a Bruker SMART APEX II 4K CCD diffractometer (Bruker Corporation, Madison, WI, USA) using Mo Kα radiation (λ = 0.71073 Å) at room temperature.Multi-scan method was used for absorption correction [91].All the crystal structures were solved by the direct method and refined using the SHELXTL program package [92].As for the structural refinement of Li 2 HgSiS 4 , the initial refinement result gave the formula Li 2 HgSiS 4 , but the site of the Li atom showed abnormal anisotropy parameter (almost zero).Thus, we attempted to set the Li1 and Hg2 atoms to occupy the same site with the ratio of 0.97:0.03(Li1:Hg2) by random refinement.In view of the low occupancy (0.025) of Hg2 atom, we consider using the "ISOR" order to treat the Li1 atom as isotropic instead of a positional disorder (Li1:Hg2).Moreover, the subsequent analysis of the element contents in the title compounds with energy dispersive X-ray (EDX) equipped Hitachi S-4800 SEM (Tokyo, Japan) showed the approximate molar ratio of 1:1:4 for Hg, Si/Ge/Sn, and S (Li is undetectable in EDX).The final structures were carefully checked with PLATON software (Glasgow, UK) and no other symmetries were found [93].Table 3 shows the crystal data and structure refinement of the title compounds.

Powder XRD Measurement
A Bruker D2 X-ray diffractometer (Madison, USA) with Cu Kα radiation (λ = 1.5418Å) was used to measure the powder X-ray diffraction (XRD) patterns of title compounds at room temperature.The measured range is 10-70 • with a step size of 0.02 • .Compared with the calculated and experiment results, it can be concluded that they are basically consistent with each other, except for Li 2 HgSiS 4 with a small number of the Hg 4 SiS 4 impurities (Figure 5)."ISOR" order to treat the Li1 atom as isotropic instead of a positional disorder (Li1:Hg2).Moreover, the subsequent analysis of the element contents in the title compounds with energy dispersive X-ray (EDX) equipped Hitachi S-4800 SEM (Tokyo, Japan) showed the approximate molar ratio of 1:1:4 for Hg, Si/Ge/Sn, and S (Li is undetectable in EDX).The final structures were carefully checked with PLATON software (Glasgow, UK) and no other symmetries were found [93].Table 3 shows the crystal data and structure refinement of the title compounds.

Powder XRD Measurement
A Bruker D2 X-ray diffractometer (Madison, USA) with Cu Kα radiation (λ = 1.5418Å) was used to measure the powder X-ray diffraction (XRD) patterns of title compounds at room temperature.The measured range is 10-70° with a step size of 0.02°.Compared with the calculated and experiment results, it can be concluded that they are basically consistent with each other, except for Li2HgSiS4 with a small number of the Hg4SiS4 impurities (Figure 5).

UV-Vis-NIR Diffuse-Reflectance Spectroscopy
Diffuse-reflectance spectra were measured by a Shimadzu SolidSpec-3700DUV spectrophotometer (Shimadzu Corporation, Beijing, China) in the wavelength range of 190-2600 nm at room temperature.The absorption spectra were converted from the reflection spectra via the Kubelka-Munk function.

Raman Spectroscopy
Hand-picked crystals were first put on an object slide, and then a LABRAM HR Evolution spectrometer equipped with a CCD detector (HORIBA Scientific, Beijing, China) by a 532-nm laser was used to record the Raman spectra.The integration time was set to be 10 s.

Second-Harmonic Generation Measurement
By the Kurtz and Perry method, powder SHG responses of the title compounds were investigated by a Q-switch laser (2.09 µm, 3 Hz, 50 ns) with ground micro-crystals on different particle sizes.AgGaS 2 single-crystal was also ground and sieved into the same size range as the reference.SHG signals were detected by a digital oscilloscope.

LDT Measurement
Ground micro-crystals samples (55-88 µm) were used to evaluate the LDTs of the title compounds under a pulsed YAG laser (1.06 µm, 10 ns, 10 Hz).Similar sizes of the AgGaS 2 crystal were chosen as the reference.By adjusting the laser output energy, colour change of the test sample was carefully observed by an optical microscope to determine the LDTs.

Conclusions
A new family of new DLSs, Li 2 HgMS 4 (M = Si, Ge, Sn), were successfully synthesized by the solid-state method in vacuum-sealed silica tubes.They are isostructural and crystallize in the orthorhombic Pmn2 1 space group.Seen from their structures, they have the similar 3D framework and 2D honeycomb-like layer structures with the interconnection of three types of tetrahedral units (LiS 4 , HgS 4 , and MS 4 ).Corresponding optical properties for the title compounds are systemically studied and the results show that they have great potential as promising IR NLO candidates.

Figure 1 .
Figure 1.(a) View of the crystal structure of Li2HgGeS4 along the b axis; (b) A honeycomb-like layer composed of the LiS4, HgS4, and GeS4 units located at the ab plane; (c) A layer composed of the LiS4 units located at the ac plane.

Figure 2 .
Figure 2. Experimental bandgaps of the title compounds.

Figure 1 .
Figure 1.(a) View of the crystal structure of Li 2 HgGeS 4 along the b axis; (b) A honeycomb-like layer composed of the LiS 4 , HgS 4 , and GeS 4 units located at the ab plane; (c) A layer composed of the LiS 4 units located at the ac plane.

11 Figure 1 .
Figure 1.(a) View of the crystal structure of Li2HgGeS4 along the b axis; (b) A honeycomb-like layer composed of the LiS4, HgS4, and GeS4 units located at the ab plane; (c) A layer composed of the LiS4 units located at the ac plane.

Figure 2 .
Figure 2. Experimental bandgaps of the title compounds.

Figure 2 .
Figure 2. Experimental bandgaps of the title compounds.

Figure 3 .
Figure 3. Raman spectra of the title compounds.

Figure 3 .
Figure 3. Raman spectra of the title compounds.

Figure 4 .
Figure 4. Second-harmonic generation (SHG) intensity versus particle size for the title compounds and AgGaS2.

Figure 4 .
Figure 4. Second-harmonic generation (SHG) intensity versus particle size for the title compounds and AgGaS 2 .

Table 1 .
Bond Valence Sum (vu) and Global Instability Index (GII) of title compounds.

Table 2 .
LDTs of the title compounds and AgGaS 2 (as the reference).

Table 2 .
LDTs of the title compounds and AgGaS2 (as the reference).

Table 3 .
Crystal data and structure refinement for the title compounds.

Table 3 .
Crystal data and structure refinement for the title compounds.