The impact of three new quaternary sulfides on the current predictive tools for structure and composition of diamond-like materials

• Ag2FeSiS4, Li2FeGeS4, and Li2FeSnS4 possess a wurtz–kesterite structure (Pn).
• These materials and others do not comply with Pauling’s radius ratio rule.
• These materials and others do not comply with Pfitzner’s tetrahedral volume theory.
• Five radii sets are assessed for accuracy in predicting M–S bonds.
• A four-coordinate S2− radius of 1.63 Å is added to the Shannon radii set.

Iron-containing diamond-like materials Ag2FeSiS4, Li2FeSnS4, and Li2FeGeS4 were synthesized for the first time via high-temperature, solid-state synthesis and found to adopt the wurtz–kesterite structure, crystallizing in the noncentrosymmetric space group Pn. These materials are considered in the broader context of design principles for new cubic- and hexagonal-derived diamond-like materials. All three of these new compounds violate Pauling’s radius ratio rule and Pfitzner’s tetrahedral volume theory. An evaluation of the adherence of over 40 published quaternary diamond-like structures to Pauling’s radius ratio rule and Pfitzner’s tetrahedral volume theory reveals that tetrahedral structures can often be generated even though these ideals are violated. To assess the radius ratios in diamond-like structures, an appropriate radii set must be selected. Accordingly, five radii sets have been investigated for accuracy in predicting metal–sulfur bond distances in diamond-like materials. Furthermore, a crystal radius of 1.63 Å for four-coordinate S2− has been calculated using the metal–sulfur bond lengths of quaternary diamond-like materials and is proposed as an addition to the popular Shannon radii set.

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