A Composition-Matched Intermediate Reveals Site-Specific Symmetry Evolution with Retained Second-Harmonic Generation in Supertetrahedral Sulfides

Discrete [M4Sn4S17]10– supertetrahedral sulfides have an established structural chemistry, yet the site-specific associations among cation substitution, symmetry, and nonlinear optical properties remain unclear. Here, independently synthesized Cs10Zn4Sn4S17 serves as a composition-matched middle member rather than a traditional reaction intermediate in the sequence Rb10Zn4Sn4S17 → Cs10Zn4Sn4S17 → Cs10Cd4Sn4S17. This comparison separates A-site Rb+/Cs+ substitution from tetrahedral M-site Zn2+/Cd2+ substitution. The previously reported Cs10Cd4Sn4S17 is the Cd-containing structural end point, whose optical properties are evaluated here. Rb+/Cs+ substitution expands the lattice while cubic symmetry and optical isotropy are retained. In contrast, the Zn2+/Cd2+ comparison shows an increase in the M–S/Sn–S mean bond-length mismatch from 0.0325 to 0.1198 Å, an increase in crystallographically independent sites from 8 to 23, and tetragonal symmetry. For Cs10Cd4Sn4S17, the calculated birefringence is 0.044 at a 1392 nm fundamental wavelength, and the calculated refractive indices satisfy a geometrical Type-I phase-matching condition. The optical absorption-edge estimates remain 3.38 and 3.23 eV for the two Cs compounds, and their relative powder SHG responses are 0.6 and 0.8 × AgGaS2. These observations support a site-resolved association among substitution, symmetry, optical anisotropy, and retained second-order nonlinearity.