Structural Transformation and Second-Harmonic-Generation Activity in Rare-Earth and d0 Transition-Metal Oxysulfides RE3NbS3O4 (RE = Ce, Sm, Gd, Dy).

Investigations of inorganic compounds with mixed anions have drawn much attention. Here, three oxysulfides, Sm3NbS3O4 (1), Gd3NbS3O4 (2), and Dy3NbS3O4 (3), are obtained by solid-state reactions. 1 and 2 crystallize in the polar space group Pna21, while 3 crystallizes in the centric space group Pnma. The anionic frameworks of 1 and 2 are built by isolated distorted [NbS2O4]7- octahedra, while [NbS3O4]9- is used for 3. 1 and 2 exhibit phase-matchable second-harmonic-generation (SHG) effects of about 0.3 and 0.4 × AGS at 2.1 μm. The [NbS2O4]7- octahedron was first used as a SHG-active motif for nonlinear-optical (NLO) materials. A systematic analysis of the transformation between these crystal structures, NLO performances, and magnetic behaviors, as well as first-principles theoretical studies, is presented. This work enriches the study on relatively rarely explored NLO-active metal oxysulfides.