ANb2(PS4)2(S2)2 (A = K, Rb) Synthesized by Metal Oxide–Boron–Chalcogen (MOBQ) Method: Featuring [Nb2S8(S2)2] Dimers and [PS4] Tetrahedra for Infrared Nonlinear Optical Properties

NbnSm clusters containing S–S and Nb–Nb bonds are promising as infrared nonlinear optical (IR NLO) functional motifs, yet their study in the thiophosphate system remains scarce. Here, ANb2(PS4)2(S2)2 (A = K (1), Rb (2)) were synthesized by the metal oxide–boron–chalcogen (MOBQ) solid-state routine, and their structures feature a three-dimensional double-helical tunnel framework costructed by alternately connected [Nb2S8(S2)2] dimers and [PS4] tetrahedra. Both compounds exhibit narrow band gaps (1.59 and 1.58 eV), large birefringences (0.20 and 0.21 @ 2.1 μm), second-harmonic generation (SHG) responses, and photocurrent responses. Theoretical calculations indicate that their weak SHG response originates from the multidirectional cancellation of dipole moment vectors at the microscopic scale. This work enriches the IR NLO research on Nb-based thiophosphates and clarifies the application potential and structural limitations of Nb-based sulfides in the NLO field.