Li₂Mg₂Si₂S₆ and Li₂Mg₂Ge₂S₆: Two nonlinear optical sulfides featuring a unique, polar trigonal structure incorporating ethane‐like anions

Abstract

The new compounds Li 2 Mg 2 Si 2 S 6 and Li 2 Mg 2 Ge 2 S 6 have been prepared via traditional high-temperature, solid-state synthesis. The title compounds crystallize in the polar, noncentrosymmetric, trigonal space group P31m (No. 157) and adopt a new structure type featuring staggered, ethane-like (T 2 S 6 ) 6À units, where T = Si or Ge. These (T 2 S 6 ) 6À units are nestled within the holes of magnesium-sulfide "layers" that are created through the edge-sharing of MgS 6 octahedra. The holes found in the lithium-sulfide "layers", created by LiS 6 edge-sharing octahedra, remain vacant, containing no (T 2 S 6 ) 6À anionic group. Through the face sharing of the respective MgS 6 and LiS 6 octahedra, the magnesium-sulfide and lithium-sulfide "layers" are stitched together resulting in an overall three-dimensional structure. The optical bandgaps of Li 2 Mg 2 Si 2 S 6 and Li 2 Mg 2 Ge 2 S 6 are 3.24 and 3.18 eV, respectively, as estimated from optical diffuse reflectance UV-Vis-NIR spectroscopy. The compounds exhibit second harmonic generation responses of ~0.24 × KDP and ~2.92 × αquartz for Li 2 Mg 2 Si 2 S 6 and ~0.17 × KDP and ~2.08 × α-quartz for Li 2 Mg 2 Ge 2 S 6 , using a Nd:YAG laser at 1.064 μm. Electronic structure calculations were performed using density functional theory and the linearized augmented plane-wave approach within the WIEN2k software package. Examination of the electronic band structures shows that these compounds are indirect bandgap semiconductors.

[Sr4Cl2][Si3S9]: Ultrawide-bandgap salt-inclusion thiosilicate nonlinear optical material with unprecedented tri-polymerized [Si3S9] clusters

“…2). The other link modes of SiS 4 units can be divided into four different types: Si 2 S 6 (Li 2 Mg 2 Si 2 S 6 ), 46 Si 2 S 7 (Cu 4 NiSi 2 S 7 ), 47 Si 4 S 10 (Na 2 Si 2 S 5 ), 48 and 1D (SiS 3 ) n chain (Cs 2 ZnSi 3 S 8 ). 49 It is notable that there are two different link modes in the Si 2 S 6 units: Si 2 S 6 with the Si-Si bond or edge-sharing Si 2 S 6 without Si-Si bond (Fig.…”

“…3a) showed that its optical bandgap ( E g ) was 4.22 eV, which is larger than those of many famous NLO chalcohalides and thiosilicates, such as Li[LiCs 2 Cl][Ga 3 S 6 ] (4.18 eV), 41 [CsBa 3 Cl 2 ][Ga 5 S 10 ] (3.96 eV), 39 K 2 Ba 3 Ge 3 S 9 Cl 2 (3.69 eV), 50 NaBa 4 Ge 3 S 10 Cl (3.49 eV), 51 SrLi 2 SiS 4 (3.94 eV), 43 Li 2 ZnSiS 4 (3.9 eV), 52 BaGa 2 SiS 6 (3.75 eV), 42 Li 2 In 2 SiS 6 (3.61 eV), 53 and Li 2 Mg 2 Si 2 S 6 (3.24 eV). 46 We also summarized the bandgaps of known chalcohalides (>3.0 eV) in Table S3 (ESI†), and the detailed survey showed that [Sr 4 Cl 2 ][Si 3 S 9 ] exhibited the widest E g among the known IR NLO chalcohalides, which indicated that [Sr 4 Cl 2 ][Si 3 S 9 ] could break through the “4.0 eV wall” for the energy bandgap and thus has a large possibility to eliminate the harmful two-photon absorption. The electronic structure of [Sr 4 Cl 2 ][Si 3 S 9 ] was also studied by first-principles calculation and its theoretical E g was 3.633 eV (Fig.…”

An overview of Mg-based IR nonlinear optical materials

“…The first Li-containing members of the A 12−nx M x n+ (T 2 Q 6 ) 2 family, Li 2 Mg 2 Si 2 S 6 and Li 2 Mg 2 Ge 2 S 6 , were reported by Aitken and co-workers in 2022. 87 Transparent and colorless single crystals of Li 2 Mg 2 Si 2 S 6 and Li 2 Mg 2 Ge 2 S 6 were obtained by the traditional high-temperature solid-phase technology using stoichiometric amounts of the reagents at 1173 K. These two new compounds belong to the non-centrosymmetric polar space group P31m (No. 157) and represent a new structure type.…”

“…(iii) Due to the absence of d-d and ff electron transitions, Mg is beneficial for obtaining wide E g values and expanding the IR transparent region. [82][83][84][85][86][87][88][89][90][91][92][93][94][95][96] Moreover, highly polarizable Mg-based ABUs are helpful in maintaining strong d eff and suitable Δn. However, a specific summary of non-centrosymmetric Mg-based materials has not been provided, even though they have been discovered to display excellent IR-NLO performances.…”

Alkali halide flux synthesis, crystal structure, and photoelectric response of quaternary thiosilicates K3Ga3Si7S20 and K2ZnSi3S8

“…8,9 Over the past decades, more and more alkali metal containing multinary metal sulfides have been discovered. 10 Among them, quaternary metal sulfides A-M-M'-S (A = alkali metal; M = metal elements other than alkali metals; M' = group 14 elements Si, Ge, Sn) are of particular interest owing to their broad compositional and structural variability, [11][12][13] and interesting NLO, [14][15][16][17][18] semiconducting, [19][20][21] ion exchange, [22][23][24][25] and magnetic properties. 26,27 Unlike quaternary thiogermanates and thiostannates that have been well investigated, quaternary thiosilicates are still limited.…”