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 <i>Pmn</i>2₁ to cubic <i>P</i>2₁3. 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 <i>μ</i>A/cm²) among chalcogenides. This case of heterovalent substitution induced structural transformation and enhanced NLO properties provides a good example to design new NLO materials.