AgEuPS4: Unlocking the CentrosymmetricShackle of Rare-Earth Thiophosphates via Microscopic Distortion Cascadingfor Enhanced Second-Harmonic Generation Response

Abstract Breaking the rigid centrosymmetric (CS) shackles of rare-earth thiophosphates remains a significant challenge for designing high-performance mid-to-far-infrared (MFIR) nonlinear optical (NLO) materials. Herein, using the CS LiEuPS4 (I41/acd) as the structural template, a noncentrosymmetric (NCS) rare-earth thiophosphate AgEuPS4 (Ama2) can be successfully synthesized via the flux-assisted metal oxide–boron–chalcogen (MOBQ) solid-state method. This structural transformation is driven by the “microscopic distortion cascading” strategy. Quantitative distortion index evaluation validates that the introduction of soft d10-configured Ag+ cations induces severely deformed [AgS4] tetrahedra, which function as highly efficient “microscopic chemical levers” to propagate intense localized lattice strains to neighboring [EuS8] and [PS4] units. AgEuPS4 achieves an exceptional balance among critical NLO performances: it delivers a strong phase-matchable NLO response of 2.3 × AgGaS2, an enhanced laser-induced damage threshold (2.0 × AgGaS2), and a moderate experimental birefringence of 0.101@546 nm. Theoretical analyses reveal that the macroscopic asymmetry originates from the cooperative superposition of local polar vectors. This work provides an efficient avenue for discovering outstanding NCS functional materials through precision engineering of local lattice strains.