Decoding Variants of Pyrite, Arsenopyrite, and Marcasite Using an Electron Counting Rule.

Pyrite (P), marcasite (M), and arsenopyrite (A) are of significant historical and contemporary interest. Compounds adopting these structure types are studied for their crystallographic properties and potential applications in energy storage and conversion. Despite numerous investigations since the 1970s, understanding the chemical behaviour of pyrite, marcasite, and arsenopyrite remains limited. This study proposes a new structure determining electron (SDE) rule to systematise these compounds without relying on formal charge assignments. The SDE rule predicts structure types based on the distribution of non-localised electrons around transition metals, providing a framework for identifying and categorising related compounds. We observe good agreement with literature data and furthermore synthesised nine new ternary compounds within the Pt/Ir-Ge-As/Sb systems, demonstrating the applicability of SDE. Our findings reveal that these compounds can be seen as layer configurations of P, M and A, enhancing our understanding of their chemical diversity. This work not only categorises existing compounds, but also paves the way for future exploration of new materials, highlighting the structural potential of P, M, A-related compounds beyond traditional frameworks. Preliminary results indicate that the type of layers influences physical properties, such as electrical conductivity, warranting further investigation into the relationship between structure and function in these compounds.