Synthesis, crystal structure, linear and nonlinear optical properties of quaternary sulfides $Ba_6$$(Cu_2X)$$Ge_4$$S_{16}$ $(X=Mg, Mn, Cd)$

Abstract

Three new quaternary noncentrosymmetric sulfides, $Ba_6$$(Cu_{1.9}$$4Mg_{1.1})$$Ge_4$$S_{16,}$ $Ba_6$$(Cu_{2.2}$$Mn_{0.8})$$Ge_4$$S_{16,}$ and $Ba_6$($Cu_{1.6}$$Cd_{1.4}$)$Ge_4$$S_{16,}$ have been synthesized by a high-temperature solid-state method and flux growth method. The crystal structures of the compounds have been determined by single crystal X-ray diffraction. All compounds are isostructural and crystallize in the $Ba_6$$Ag_4$$Sn_4$$S_{16}$ structure type and the cubic space group I ͞43d (No. 220) with unit cell parameters of a ​= ​14.308 ​Å ($Ba_6$($Cu_{1.9}$$Mg_{1.1}$)$Ge_4$$S_{16}$), a ​= ​14.299 (2) Å ($Ba_6$($Cu_{2.2}$$Mn_{0.8}$)$Ge_4$$S_{16}$), and a ​= ​14.336 (2) Å ($Ba_6$($Cu_{1.6}$$Cd_{1.4}$)$Ge_4$$S_{16}$). The structure demonstrates good structural flexibility of mixing occupancy between Cu and X (X ​= ​Mg, Mn, Cd) at the Wyckoff site 12a. The structure consists of a distorted 3-dimensional framework constructed by (Cu/X)$S_4$ and $GeS_4$ tetrahedra. The divalent metals doped into the structure tune the band gaps of the compounds and contribute to good Second Harmonic Generation (SHG) response and high Laser Damage Threshold (LDT).