A^llHgM^IVS₄ (A^ll?= Sr, Ba, M^IV?= Si, Ge): A Series of Materials with Large Second Harmonic Generation Response and Wide Band Gaps

Wide band gaps and large second harmonic generation (SHG) response are two crucial yet contradictory parameters for nonlinear optical (NLO) crystals. Herein, through exploring the influence of anion groups on structural crystallization dimension and space group, a series of new NLO Hg-based chalcogenides A(II)HgM(IV)S(4) (A(II) = Sr, Ba, M-IV = Si, Ge) with wide band gaps and large SHG responses have been successfully designed and synthesized. The structures of these compounds consist of 2D [HgMS4](2-) (M = Si and Ge) layers composed of distorted (HgS4 and MS4) tetrahedra arranged alternately, and the layers are separated by eight-coordinated A(2+) (A = Sr and Ba) cations. Their band gaps and SHG responses meet the basic requirements (SHG > 1 x AgGaS2; Eg > 2.33 eV). Especially, SrHgSiS4 and SrHgGeS4 exhibit both large SHG effects (2 and 2.4 x AgGaS2) and wide band gaps (3.06 and 2.97 eV), respectively, indicating they are promising IR NLO crystals. This work opens a new avenue for new NLO material design to achieve the balance between strong SHG response and large band gaps.