Crystal structure, electronic structure, and optical properties of the novel Li₄CdGe₂S₇, a wide-bandgap quaternary sulfide with a polar structure derived from lonsdaleite

Abstract

The novel quaternary thiogermanate Li4CdGe2S7 (tetralithium cadmium digermanium heptasulfide) was discovered from a solid-state reaction at 750 °C. Single-crystal X-ray diffraction data were collected and used to solve and refine the structure. Li4CdGe2S7 is a member of the small, but growing, class of I4–II–IV2–VI7 diamond-like materials. The compound adopts the Cu5Si2S7 structure type, which is a derivative of lonsdaleite. Crystallizing in the polar space group Cc, Li4CdGe2S7 contains 14 crystallographically unique ions, all residing on general positions. Like all diamond-like structures, the compound is built of corner-sharing tetrahedral units that create a relatively dense three-dimensional assembly. The title compound is the major phase of the reaction product, as evidenced by powder X-ray diffraction and optical diffuse reflectance spectroscopy. While the compound exhibits a second-harmonic generation (SHG) response comparable to that of the AgGaS2 (AGS) reference material in the IR region, its laser-induced damage threshold (LIDT) is over an order of magnitude greater than AGS for λ = 1.064 µm and τ = 30 ps. Bond valence sums, global instability index, minimum bounding ellipsoid (MBE) analysis, and electronic structure calculations using density functional theory (DFT) were used to further evaluate the crystal structure and electronic structure of the compound and provide a comparison with the analogous I2–II–IV–VI4 diamond-like compound Li2CdGeS4. Li4CdGe2S7 appears to be a better IR nonlinear optical (NLO) candidate than Li2CdGeS4 and one of the most promising contenders to date. The exceptional LIDT is likely due, at least in part, to the wider optical bandgap of ∼3.6 eV.

Accurate X-ray diffraction data required for proper evaluation of bond valence sums and global instability indexes: redetermination of the crystal structures of diamond-like Cu2CdSiS4 and Cu2HgSnS4 as a case study

“…A similar range is found for the isostructural Li 2 CdGeS 4 , where the �(R) values are in the range 0.020-0.072 A ˚ (Craig et al, 2022). Low absolute values of S, the ellipsoidal shape parameter, are also observed for Cu 2 CdSiS 4 and Li 2 CdGeS 4 (Craig et al, 2022). In Cu 2 CdSiS 4 , the S values are negative for the CuS 4 and CdS 4 tetrahedra, indicating axial compression, while the slight positive value for SiS 4 implies axial stretching.…”

“…As shown in Table 3, all polyhedra are four-coordinate with limited distortion indicated by the relatively low �(R) values, which are in the range 0.030-0.087 A ˚, with the largest values found for Cu and S2. A similar range is found for the isostructural Li 2 CdGeS 4 , where the �(R) values are in the range 0.020-0.072 A ˚ (Craig et al, 2022). Low absolute values of S, the ellipsoidal shape parameter, are also observed for Cu 2 CdSiS 4 and Li 2 CdGeS 4 (Craig et al, 2022).…”

“…The centre displacements (D) of all the ions in Cu 2 CdSiS 4 are in the range 0.011-0.334 A ˚, where the values obtained for the anions are �2-30� those observed for the cations. For Li 2 CdGeS 4 , a similar pattern is found, with the anion D values being �4-7� those of the cations (Craig et al, 2022).…”

Phase Competition and Strong SHG Responses of the Li2MIIMIVSe4 Family: Atom Response Theory Predictions versus Experimental Results

“…The crystallization of the DL structure in the noncentrosymmetric space groups makes this system naturally suitable for exploring NLO materials. In addition, DL structures can be classified into normal DL structures and defect DL structures, which results in the structural diversity and provides more possibilities for materials with excellent NLO properties. − Recently, many promising NLO materials of DL structures were synthesized, e.g., Li 4 M II M IV 2 Q 7 ( M II = Cd, Hg; M IV = Ge, Sn; Q = S, Se), − Li 2 Mg M IV Se 4 ( M IV = Ge, Sn), Hg 3 P 2 S 8, , M I M II PS 4 ( M I = Li, Ag, Cu; M II = Zn, Hg), − and M II 3 PnI 3 (M II = Zn, Cd; Pn = P, As) …”