Polysubstitution Induced Centrosymmetric-to-Noncentrosymmetric Structural Transformation and Nonlinear-Optical Behavior: The Case of Na 0.45 Ag 0.55 Ga 3 Se 5 .

Obtaining compounds with large nonlinear-optical (NLO) coefficients and wide band gaps is challenging due to their competitive requirements for chemical bonds. Herein, the first member with mixed cations on the A site in the A-M 3 -Q 5 or A-Ag-M 6 -Q 10 (A = alkali metal; M = Ga, In; Q = S, Se, Te) family, viz. Na 0.45 Ag 0.55 Ga 3 Se 5 (NAGSe), was obtained by a solid-state reaction. Its structure features [GaSe 4 ] tetrahedra built three-dimensional {[Ga 3 Se 5 ] - } ∞ network, with Na and Na/Ag cations located at the octahedral cavities. Noncentrosymmetric ( R 32) NAGSe can also be transformed from centrosymmetric RbGa 3 S 5 ( P 2 1 / c ) via multiple-site cosubstitution. NAGSe exhibits the highest NLO response (1.9 × AGS) in the A-Ag-M-Q family. Crystal structure analysis and theoretical calculations suggest that the NLO response is mainly contributed by the regularly arranged [GaSe 4 ] units. This work enriches the exploration of the undeveloped A-M 3 -Q 5 or A-Ag-M 6 -Q 10 family as potential infrared NLO materials.