Modulating Nonlinear Optical Performances in α/β-SnGa₂GeS₆ Polymorphs via Atomic Coordination Engineering

Property modulation based on structural transformation has emerged as a crucial strategy for developing high-performance nonlinear optical (NLO) crystals. Among them, polymorphic systems with structural diversity and fascinating performances have attracted growing attention in the exploration of NLO materials. In this work, adjusting the coordination environment (from SnS₅ to SnS₃) of the lone-pair Sn²⁺ cation enables the discovery of another new phase, β-SnGa₂GeS₆, which crystallizes in the Cc group that is different from the previously reported α-phase (Fdd2). Detailed characterization reveals distinct NLO properties between the α- and β-polymorphs, including NLO response (α: 17.8 pm/V vs β: 13.7 pm/V) and birefringence (α: 0.151 vs β: 0.126). Theoretical calculations confirm that the reduced NLO response in the β-phase arises from the canceled polarization of the [SnS₃] units, as evidenced by dipole moment analysis. This study highlights the influence of atomic coordination engineering in polymorphic systems for structural transition and tunable NLO performances. Such established "phase transition → structural transformation → NLO property" relationship in polymorphs provides a design paradigm for discovering next-generation NLO materials.