Competing Magnetic Structures and the Evolution of Copper Ion/ Vacancy Ordering with Composition in the Manganite Oxide Chalcogenides Sr 2 MnO 2 Cu 1.5 (S 1−x Se x ) 2 Paul Adamson, † Joke Hadermann, b Catherine F. Smura, † Oliver J. Rutt, † Geoffrey Hyett, † David G. Free, † and Simon J. Clarke †, * † Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QR, U.K. b Electron Microscopy for Materials Science (EMAT), University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium * S Supporting Information ABSTRACT: The series Sr 2 MnO 2 Cu 1.5 (S 1−x Se x ) 2 (0 ≤ x ≤ 1) contains mixed-valent Mn ions (Mn 2+ /Mn 3+ ) in MnO 2 sheets which are separated by copper-deficient antifluorite-type Cu 2−δ Ch 2 layers with δ ∼ 0.5. The compounds crystallize in the structure type first described for Sr 2 Mn 3 Sb 2 O 2 and are described in the I4/mmm space group at ambient temperatures. Below about 250 K, ordering between Cu + ions and tetrahedral vacancies occurs which is long-range and close to complete in the sulfide-containing end member of the series Sr 2 MnO 2 Cu 1.5 S 2 but which occurs over shorter length scales as the selenide content increases. The superstructure is an orthorhombic 2√2a × √2a × c expansion in Ibam of the room temperature cell. For x > 0.3 there are no superstructure reflections evident in the X-ray or neutron diffraction patterns, and the I4/mmm description is valid for the average structure at all temperatures. However, in the pure selenide end member, Sr 2 MnO 2 Cu 1.5 Se 2 , diffuse scattering in electron diffractograms and modulation in high resolution lattice image profiles may arise from short-range Cu/vacancy order. All members of the series exhibit long-range magnetic order. In the sulfide-rich end member and in compounds with x < 0.1 in the formula Sr 2 MnO 2 Cu 1.5 (S 1−x Se x ) 2 , which show well developed superstructures due to long-range Cu/vacancy order, the magnetic structure has a ( 1 / 4 1 / 4 0) propagation vector in which ferromagnetic zigzag chains of Mn moments in the MnO 2 sheets are coupled antiferromagnetically in an arrangement described as the CE-type magnetic structure and found in many mixed-valent perovskite and Ruddlesden−Popper type oxide manganites. In these cases the magnetic cell is an a × 2b × c expansion of the low temperature Ibam structural cell. For x ≥ 0.2 in the formula Sr 2 MnO 2 Cu 1.5 (S 1−x Se x ) 2 the magnetic structure has a (0 0 0) propagation vector and is similar to the A-type structure, also commonly adopted by some perovskite-related manganites, in which the Mn moments in the MnO 2 sheets are coupled ferromagnetically and long-range antiferromagnetic order results from antiferromagnetic coupling between planes. In the region of the transition between the two different structural and magnetic long-range ordering schemes (0.1 < x < 0.2) the two magnetic structures coexist in the same sample. The evolution of the competition between magnetic ordering schemes and the length scale of the structural order with composition in Sr 2 MnO 2 Cu 1.5 (S 1−x Se x ) 2 suggest that the changes in magnetic and structural order are related consequences of the introduction of chemical disorder. KEYWORDS: oxychalcogenide, manganite, magnetic order, vacancy order ■ INTRODUCTION Oxide chalcogenides are a relatively under-investigated class of solid state compound compared with oxides and other solids containing only one type of anion. Due to the different sizes and coordination requirements of the oxide and the heavier chalcogenide anions, oxide chalcogenides tend to adopt layered structures. 1 In the oxide sulfides A 2 MO 2 Cu 2 S 2 (A = electro- positive metal; M = transition metal, known for A = Sr: M = Mn, 2 Co, 3 Ni, 4 Cu, 4 and Zn; 2 A = Ba: M = Co, 3 which were first described, along with a series of related compounds, by Zhu and Hor and co-workers, 2 the more polarizable sulfide anion bonds to Cu + ions in (Cu 2 S 2 ) 2− antifluorite-type layers and the less polarizable oxide anion bonds to the divalent M 2+ ion in square planar (MO 2 ) 2− layers which are two-dimensional fragments of the perovskite structure. The two layer types stack alternately with A 2+ cations in between (Figure 1). The structure type was first reported for Sr 2 Mn 3 Sb 2 O 2 . 5 Oxide chalcogenides and oxide pnictides with these layered structures provide a counterpoint to important perovskite-related oxide phases such as the three-dimensional cubic perovskites AMO 3−δ and the layered Ruddlesden−Popper (A n+1 M n O 3n+1 ) phases. It is well-established 6 that the copper chalcogenide layers present in the oxide sulfides can readily accept holes in the antibonding states at the top of a valence band that is composed of well- Received: May 15, 2012 Revised: June 15, 2012 Published: June 19, 2012 Article pubs.acs.org/cm © 2012 American Chemical Society 2802 dx.doi.org/10.1021/cm301486v | Chem. Mater. 2012, 24, 2802−2816