Nonlinear optical (NLO) crystals are essential materials for laser frequency conversion. However, commercially available infrared (IR) NLO crystals such as AgGaS₂ and AgGaSe₂ suffer from either low laser-induced damage thresholds (LIDT) or significant two-photon absorption, limiting their applicability in high-power laser systems.

Although noncentrosymmetry is an essential prerequisite for second-order nonlinear optical effects, achieving it remains challenging. To date, the flux method has seldom been utilized to induce a transition from centrosymmetric to noncentrosymmetric structure.

In a study published in Small, a research team led by Profs. GUO Guocong and LIU Binwen from the Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences developed a strategy for synthesizing high-performance IR NLO materials via flux-induced symmetry breaking.

Researchers synthesized the noncentrosymmetric phase α-Ba₂Ge₄S₁₀ of centrosymmetric phase β-Ba₂Ge₄S₁₀ using a lower-temperature flux-growth method under a sealed system. They found that a variety of fluxes could yield the target product. By optimizing growth conditions with a NaI flux, they obtained millimeter-sized single crystals up to 10 × 4 × 4 mm³.

Structural analysis revealed that α-Ba₂Ge₄S₁₀ crystallizes in the noncentrosymmetric space group I-42d, featuring a zero-dimensional T2-supertetrahedral [Ge₄S₁₀]^4– clusters linked by Ba²⁺ cations. Compared with the β phase, α-Ba₂Ge₄S₁₀ exhibits significantly larger distortions in both the [Ge₄S₁₀]^4– clusters and the Ba²⁺ coordination environments.

Besides, along the crystallographic c-axis, the Ge–S framework in the α phase displays a pronounced twisted configuration, while that in the β phase remains nearly parallel. Bond strain index and global instability index calculations indicated that α-Ba₂Ge₄S₁₀ is a metastable phase, which rationalizes its formation under the lower-temperature flux conditions.

The α-Ba₂Ge₄S₁₀ possesses a wide optical bandgap of 3.15 eV, substantially larger than those of commercial AgGaS₂ and AgGaSe₂. Under 2050 nm laser excitation, its powder second-harmonic generation response reached 1.2 times that of AgGaS₂, with phase-matchable behavior. The single-crystal IR transmission ranged from 2.5 to 12.8 μm.

Moreover, the LIDT of α-Ba₂Ge₄S₁₀ is as high as 442.7 MW/cm², approximately 14.8 times that of AgGaS₂. First-principles calculations revealed that the microscopic origin of the second-harmonic generation effect is mainly attributable to strong covalent interactions between S and Ge atoms within the [Ge₄S₁₀] clusters.

This study demonstrates the first example of an NLO material based on T2-[Ge₄S₁₀] clusters, and achieves a centrosymmetric to noncentrosymmetric phase transition via a flux method. It offers a promising way for designing advanced functional materials.