First-Principles Study on the Linear and Nonlinear Optical Properties of Thiovanadates Modulated by Transition Metals (Cu/Ag)

Thiovanadates are promising candidates for infrared (IR) nonlinear optical (NLO) applications. Here, density functional theory calculations reveal the structure–property relationships and microscopic mechanisms of A3–nMnVS4 (A = K, Rb; M = Cu, Ag; n = 1, 2). Specifically, structural dimensionality determines the optical anisotropy (birefringence): the large birefringence of 1D K2CuVS4 (0.283 @ 1064 nm) stems from the constructive vector superposition of polarizability tensors, driven by aligned tetrahedral chains and strong Cu-3d/S-3p hybridization (stronger than that of Ag analogues). Meanwhile, the strong second-harmonic-generation (SHG) response in 2D ACu2VS4 (d24 = 12.12 pm/V for KCu2VS4) is attributed to the synergistic polarization of two inequivalent Cu sites, whose local geometric distortions optimize virtual-electron-transition channels. Alkali cations (K+/Rb+) act as electrostatic scaffolds, fine-tuning active moiety densities via cell-volume effects. This work provides clear physical guidelines for designing high-performance IR NLO materials via dimensional and anionic unit engineering.