Cation site preferences in layered oxide chalcogenides, synthesis, structures and magnetic ordering in Sr3-xCaxFe2O5Cu2 Ch 2 (Ch = S, Se; x = 1, 2)

Highlights: • Novel iron oxide chalcogenides with Ca²⁺/Sr²⁺ ordering synthesised. • The presence of differently sized cations dictates the structure adopted. • Sr²⁺ cations prefer the larger alkaline earth metal site; Ca²⁺ favour the smaller site. • Long range antiferromagnetic ordering persists well above room temperature. • Magnetic ordering model consists of tilted moments which reorientate upon heating. Solid solutions between the known compounds Ca₂FeO₃CuCh and Sr₂FeO₃CuCh (Ch=S, Se) in which there are two fairly similar sites (8 and 9 coordinate) for the alkaline earth cations are not attainable under standard high temperature solid state syntheses under thermodynamic control. Instead compounds with greater condensation of FeO₅ square pyramids form as these afford one 8-coordinate site and one 12-coordinate site for the alkaline earths which is better suited to the size-mismatched cations in the compounds Sr_3-xCa_xFe₂O₅Cu₂Ch₂ (Ch=S, Se; x=1, 2). Sr₂CaFe₂O₅Cu₂S₂, SrCa₂Fe₂O₅Cu₂S₂, Sr₂CaFe₂O₅Cu₂Se₂ and SrCa₂Fe₂O₅Cu₂Se₂ all crystallise in the tetragonal space group I4/mmm with two formula units in the unit cell with the crystal structure first described for Sr₃Fe₂O₅Cu₂S₂. Oxide slabs composed of vertex-sharing FeO₅ square pyramids are separated by Cu₂Ch₂ anti-fluorite-type layers. The larger Sr²⁺ ions have a strong preference for the 12-coordinate site in the oxide slabs, while Ca²⁺ cations dominate the 8-coordinate sites separating the oxide and chalcogenide slabs. Powder neutron diffraction reveals that all the compounds display antiferromagnetic long range ordering of the Fe³⁺ moments with ordering temperatures well above room temperature and exceeding 526K in the case of Ca₂SrFe₂O₅Cu₂Se₂.