Achieving Excellent Nonlinear Optics in α ‑Ba ₂ Ge ₄ S ₁₀

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.