La3Ga3Ge2S3O10: An Ultraviolet Nonlinear Optical Oxysulfide Designed by Anion-Directed Band Gap Engineering.

Chalcogenide-containing compounds have been widely studied as infrared nonlinear optical (NLO) materials. However, they have never been applied in the ultraviolet (UV) region owing to the high energy levels of chalcogen anions, leading to band gap narrowing. In this paper, we report the synthesis of a new UV NLO oxysulfide La 3 Ga 3 Ge 2 S 3 O 10 with an exceptionally wide band gap of 4.70 eV due to from the unique anion-ordered frameworks comprising 1D 1 ¥ [(Ga 3/5 Ge 2/5 ) 3 S 3 O 3 ] triangular tubes and 0D (Ga 3/5 Ge 2/5 ) 2 O 7 dimers of corner-sharing (Ga/Ge)S 2 O 2 and (Ga/Ge)O 4 tetrahedra, respectively. Second harmonic generation (SHG) measurements revealed that La 3 Ga 3 Ge 2 S 3 O 10 was phase matchable with twice the SHG response of KH 2 PO 4 . The results of theoretical calculations suggest that the strong SHG response is mainly attributable to the S-3 p and O-2 p orbitals in the occupied states. The anion-directed band-gap engineering may give insights into the application of NLO oxychalcogenides in the UV regions.