From Orthorhombic Cu8GeS6 to Cubic Cu6HgGeS6: Second-Harmonic Generation and Photocurrent Responses

A new noncentrosymmetric sulfide, Cu6HgGeS6 (CHGS), has been successfully synthesized via a heterovalent substitution strategy based on argyrodite-type Cu8GeS6 (CGS). Benefiting from the substitution of Cu+ with Hg2+ cation, the structure changes from orthorhombic Pmn21 to cubic P213. CHGS exhibits obvious nonlinear optical (NLO) activity, while CGS not. The combination of theoretical calculation and structure analysis indicates that the introduction of Hg2+ ions leads to a structural transformation, in which the trigonal pyramid [CuS3] units provide the main contribution to the NLO performance to a certain extent. Additionally, CHGS exhibits a relatively large photocurrent response (2.5 μA/cm2) among chalcogenides. This case of heterovalent substitution induced structural transformation and enhanced NLO properties provides a good example to design new NLO materials.