From CuFeS to BaCuFeGeS: rational band gap engineering achieves large second-harmonic-generation together with high laser damage threshold.

A new germanium-based sulfide, Ba6Cu2FeGe4S16, achieves a band-gap broadening of more than 1 eV relative to CuFeS2. Remarkably, Ba6Cu2FeGe4S16 exhibits excellent comprehensive NLO performance (SHG, 1.5 × AgGaSe2; LDT, 2 × AgGaSe2), satisfying the essential requirements of mid-IR NLO candidates.