Achieving Excellent Nonlinear Optics in α‑Ba₂Ge₄S₁₀ via Flux‑Induced Symmetry Breaking

Although noncentrosymmetry is an essential prerequisite for second-order nonlinear optical (NLO) effects, achieving it remains synthetically challenging. To date, the flux method has seldom been utilized to induce a transition from centrosymmetric to noncentrosymmetric structure. Herein, we successfully synthesized the noncentrosymmetric α-Ba₂Ge₄S₁₀ phase via flux-induced symmetry breaking from centrosymmetric β-Ba₂Ge₄S₁₀, enabling the growth of millimeter-sized single crystal (up to 10 × 4 × 4 mm³) using a sealed-system flux method. Structurally, the structure of α-Ba₂Ge₄S₁₀ consists of zero‑dimensional supertetrahedral [Ge₄S₁₀]^4- clusters charge-balanced by Ba²⁺ ions. Optically, α-Ba₂Ge₄S₁₀ displays a competitive phase‑matching second-harmonic generation response (1.2 × AgGaS₂ @2050 nm) and a broad infrared transparency range (2.5-12.8 µm). To our knowledge, this is the first NLO material based on T2-[Ge₄S₁₀] clusters. Moreover, the introduction of electropositive Ba²⁺ as a structural spacer in α-Ba₂Ge₄S₁₀ contributes to a wide optical band gap (3.15 eV) and a significantly enhanced laser‑induced damage threshold (14.8 × AgGaS₂), underscoring its promising NLO applicability. This study demonstrates that controlled phase transformation from centrosymmetric to noncentrosymmetric structures via the flux method offers a viable strategy for designing high‑performance NLO materials.