3D Transition Metal Ordering and Rietveld Stacking Fault Quantification in the New Oxychalcogenides La₂O₂Cu_2–4xCd_2xSe₂

A number of LnOCuCh (Ln = La–Nd, Bi; Ch = S, Se, Te) compounds have been reported in the literature built from alternating layers of fluorite-like [Ln 2 O 2 ] 2+ sheets and antifluorite-like [M 2 Se 2 ] 2– sheets, where M is in the +1 oxidation state leading to full occupancy of available MSe 4/2 tetrahedral sites. There is also a family of related LnOM 0.5 Se (Ln = La & Ce, M = Fe, Zn, Mn & Cd) compounds built from alternating layers of [Ln 2 O 2 ] 2+ sheets and [MSe 2 ] 2– sheets, where M is in the +2 oxidation state with half occupancy of available tetrahedral sites and complex ordering schemes in two dimensions. This paper reports a new family of compounds containing both +1 and +2 metal ions in the La 2 O 2 Cu 2–4 x Cd 2 x Se 2 family. We show how Cu 1+ and Cd 2+ ions segregate into distinct fully occupied and half occupied checkerboard-like layers respectively, leading to complex long-range superstructures in the third (stacking) dimension. To understand the structure and microstructure of these new materials we have developed and implemented a new methodology for studying low and high probability stacking faults using a Rietveld-compatible supercell approach capable of analyzing systems with thousands of layers. We believe this method will be widely applicable.