Achieving Excellent Nonlinear Optics in α ‑Ba ₂ Ge ₄ S ₁₀ via Flux‑Induced Symmetry Breaking

ABSTRACT 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 ‐Ba ₂ Ge ₄ S ₁₀ phase via flux‐induced symmetry breaking from centrosymmetric β ‐Ba ₂ Ge ₄ S ₁₀ , enabling the growth of millimeter‐sized single crystal (up to 10 × 4 × 4 mm ³ ) using a sealed‐system flux method. Structurally, the structure of α ‐Ba ₂ Ge ₄ S ₁₀ consists of zero‑dimensional supertetrahedral [Ge ₄ S ₁₀ ] ^4– clusters charge‐balanced by Ba ²⁺ ions. Optically, α ‐Ba ₂ Ge ₄ S ₁₀ displays a competitive phase‑matching second‐harmonic generation response (1.2 × AgGaS ₂ @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‐[Ge ₄ S ₁₀ ] clusters. Moreover, the introduction of electropositive Ba ²⁺ as a structural spacer in α ‐Ba ₂ Ge ₄ S ₁₀ contributes to a wide optical band gap (3.15 eV) and a significantly enhanced laser‑induced damage threshold (14.8 × AgGaS ₂ ), 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.