From K4SrP2S8 to Na4SrP2S8: Alkali-Metal Cation Substitution Induces Noncentrosymmetric Structure and Moderate Second Harmonic Generation

Abstract Designing sulfide compounds with noncentrosymmetric structures is an important approach for the development of functional materials. We present the successful preparation of an unreported quaternary sulfide, Na4SrP2S8, through high-temperature solid-state synthesis. Na4SrP2S8 crystallizes in the noncentrosymmetric tetragonal space group P4̅n2. Alongside a wide direct band gap of 3.78 eV, the material shows a moderate phase-matchable second-harmonic generation response reaching 0.58 × AgGaS2─a combination that ensures excellent resistance to two-photon absorption. First-principles calculations reveal that the nonlinear optical (NLO) performance is predominantly governed by the synergistic covalent interactions within the high-dipole-moment [PS4] and [NaS4] units (specifically the S–P and Na–S bonds), while the Sr-related ionic interactions contribute negligibly. Overall, this study validates alkali-metal cation substitution as a promising strategy for constructing noncentrosymmetric chalcogenides, and also provides a promising structural prototype for high-performance infrared NLO materials.