Rational\nPathways to Ordered Multianion Chalcogenides\nUsing Retrosynthetic Crystal Chemistry

Mixed-anion\nchalcogenides make up a versatile class of materials\nwith properties that can be fine-tuned for specific applications.\nWhile chalcogen anions (Q = S, Se, or Te) tend to form solid solutions\nin simple binary systems, ordering does occur in structures that have\nmore than one unique anion site. Here, we use crystallographic analysis\nand hard–soft acid–base principles to predict the Wyckoff\npositions that secondary chalcogens will occupy in a range of single-anion\nhosts. The analysis serves as a guide for generating ordered sulfoselenide,\nselenotelluride, and sulfotelluride products in a predictable manner.\nThe study focuses on ternary and quaternary systems with varying compositional\nand structural features, with an emphasis on 3D CsCu<sub>5</sub>Q<sub>3</sub> (<i>P</i>4<sub>2</sub>/<i>mnm</i>) and\n2D NaCuZrQ<sub>3</sub> (<i>Pnma</i>) crystal systems. The\nsamples were prepared by using high-temperature solid-state methods\nand structurally characterized by using single-crystal X-ray diffraction.\nOrdering in the mixed-anion products is attributed to the size and\nbonding variance across the chalcogen series. The degree of ordering\nvaries according to the distinctiveness of the local chemistry at\nthe anion sites and additionally the specific combination of chalcogens\nemployed in the reaction. Additionally, we find that mixed-chalcogen\nproducts can adopt phases that are distinct from their single-anion\nend members, including KCuZrTe<sub>2</sub>S (<i>P</i>2<sub>1</sub>/<i>m</i>), where an alternate arrangement of polyhedral\nbuilding blocks maximizes favorable bonding interactions. Density-functional\ntheory calculations corroborate the experimental findings and offer\nadditional insights into the ordering trends. We anticipate that the\nreverse-engineering approach, which is inspired by the retrosynthetic\nanalyses used in molecular chemistry, will help accelerate the discovery\nand understanding of heteroanionic phenomena.