Sr₆ Cd₂ Sb₆ O₇ S₁₀ : Strong SHG Response Activated by Highly Polarizable Sb/O/S Groups

A new nonlinear optical (NLO) oxysulfide, Sr₆ Cd₂ Sb₆ O₇ S₁₀ , which contains the functional groups [SbO_x S_5-x ]^7- (x=0, 1) with a 5s² electron configuration, is synthesized by a solid-state reaction. This compound displays a phase-matchable second harmonic generation (SHG) response four times stronger than AgGaS₂ (AGS) under laser irradiation at 2.09 μm. Single-crystal-based optical measurements reveal a SHG intensity that can be tuned by temperature and novel photoluminescence properties. Theoretical analyses demonstrate that tetragonal [SbOS₄ ]^7- and [SbS₅ ]^7- pyramids make the predominant contribution to the enhanced SHG effect. Among those, the [SbOS₄ ]^7- units with mixed anions make a larger contribution. This work proposes that oxysulfide groups with an ns² electron configuration can serve as new functional building units in NLO materials and opens a new avenue for the design of other optoelectronic materials.