The role of the alkali and chalcogen atoms on the stability of the layered chalcogenide [Formula: see text] (A = alkali-metal; M = metal-cations; Q = chalcogen) compounds: a density functional theory investigation within van der Waals corrections

Triclinic Layered A2ZnSi3S8 (A = Rb and Cs) with Large Optical Anisotropy and Systematic Research on the Inherent Structure-Performance Relationship in the A2MIIBMIV3Q8 Family

Two triclinic A₂ZnSi₃S₈ (A = Rb and Cs) with layered structures were successfully synthesized, and their physicochemical performances including optical bandgap, thermal behavior, and optical anisotropy were investigated. A₂ZnSi₃S₈ could be viewed as the first discovered Si-based examples in the known A₂M^IIM^IV₃Q₈ family (2-1-3-8 system; A = monovalent alkali metal; M^II = divalent transition metal; M^IV = group 14 metal; Q = chalcogen). The A₂M^IIM^IV₃Q₈ family members crystallize in five different space groups (P1̅, P2₁, P2₁/n, P2₁2₁2₁, and Pa3̅), and their structural transformation and optical performances (bandgap, NLO coefficient, and birefringence) were systematically studied based on the first-principles calculation among 13 A₂M^IIBM^IV₃Q₈ (M^IIB = Zn, Cd, and Hg) compounds without cubic β-K₂ZnSn₃S₈. Research result shows that the above 13 compounds exhibit the layered structures, but diverse wavelike layers and their optical anisotropy (Δn) undergo an increasing trend range from the triclinic to orthorhombic systems. Moreover, P2₁2₁2₁ compounds have very weak NLO effects compared with those of the P2₁ compounds since the polarization directions of anionic groups (M^IIBQ₄ and M^IVQ₄) in P2₁2₁2₁ compounds are directing oppositely and almost completely canceled out by the dipole moment calculation, which further indicates that P2₁ compounds exhibiting the relatively strong NLO effect and large optical anisotropy could be expected as potential IR NLO candidates.

New compounds of the Li2 MSn3S8 type

The substitution of Cu/Ag by lithium in complex thiospinels with the general formula A I 2 B II C IV 3 X VI 8 was achieved by ball milling and a subsequent annealing step in an atmosphere of H2S. Four hitherto unknown compounds Li2 MSn3S8 with M = Mg, Mn, Fe, Ni were obtained without side phases and have been structurally investigated. From X-ray powder diffraction experiments, space group Fd 3 ¯ $\overline{3}$ m and a spinel-type structure are suggested. In these so-called normal spinels, lithium occupies one eighth of the tetrahedral voids (Wyckoff position 8a) of the cubic closest packing of the sulfide ions whereas M and Sn can be found on one half of the octahedral voids (Wyckoff position 16d).