Topochemical synthesis of ternary metal sulfides including new LaAgS₂

Ternary sulfides are increasingly sought after for applications as battery materials, photovoltaics and photocatalysts as a result of their small band gaps and good electrical conductivities but their synthesis is often more challenging than that of oxides. We report here the low temperature synthesis of new sulfide LaAgS₂ by an anionic redox topochemical reaction, alongside the synthesis of LaCuS₂ by mechanochemical topochemical reaction. Our computational work indicates that reaction enthalpies to form LaAgS₂, with structure closely related to that of LaCuS₂, are relatively small, consistent with our synthetic work. Structural analysis suggests this is due to the larger size of the Ag⁺ ion compared with Cu⁺. Diffuse reflectance measurements indicate that both sulfides have band gaps ∼2.1(2) eV.