Chemistry and physics of layered oxychalcogenides containing an anti-cuprate type square lattice

There has been significant recent interest in layered solid-state materials containing an [ M 2 O] square lattice layer ( M = transition metal), particularly because [ M 2 O] is the anti-type of the [CuO 2 ] planes in the layered cuprate superconductors. In addition to the superconducting titanium oxypnictides, the [ M 2 O] anti-cuprate layer also occurs in a wide range of layered oxychalcogenide compounds with M spanning early (Ti, V) to later transition metals (Mn, Co, Fe). The chalcogenide in question – which sandwiches the anti-cuprate layer – may be S, Se or Te, and in combination with a wide range of intervening “spacer” layers, many different structural families have been investigated. This review surveys the structures and physical properties of all these oxychalcogenide materials and relates these properties to their common anti-cuprate square lattice [ M 2 O] layer. It is organised around the different oxidation states of the metal ion M , in order to explore the effects of the electronic configuration of M on the physical properties of each compound as a whole. A key part of the review highlights the use of soft-chemical modifications to alter physical properties of these materials, in the synthesis of novel van der Waals materials and other metastable compounds. Future avenues for these materials in the bulk, few- and single-layer limits are discussed. • Literature survey of materials containing the “anti-cuprate” square layer. • Includes metals, semiconductors and insulators with a range of magnetic properties. • Discussion of altermagnetism and the scope for future research in this area.