Strong Kleinman-Forbidden Second Harmonic Generation in Chiral Sulfide: La₄InSbS₉

A new chiral sulfide family, Ln4InSbS9 (Ln = La, Pr, Nd), with its own structure type in space group P41212 or its enantiomorph P43212 has been synthesized by solid-state reaction. Remarkably, the La member shows the strongest Kleinman-forbidden second harmonic generation to date, with an intensity 1.5 times that of commercial AgGaS2 at a laser wavelength of 2.05 μm, and exhibits type-I phase-matchable behavior. Density functional theory calculations and ab initio molecular dynamics simulations suggest that lattice vibrations may be responsible for the origin and magnitude of the strong SHG effect.