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 A2MIIMIV3Q8 family (2-1-3-8 system; A = monovalent alkali metal; MII = divalent transition metal; MIV = group 14 metal; Q = chalcogen). The A2MIIMIV3Q8 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 A2MIIBMIV3Q8 (MIIB = 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 (MIIBQ4 and MIVQ4) 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.