Direct Atomic Observation in Powdered 4H-Ba 0.8 Sr 0.2 Mn 0.4 Fe 0.6 O 2.7 María Hernando, † Laura Miranda, † Aurea Varela, † Khalid Boulahya, † Sorin Lazar, ‡ Derek C. Sinclair, § Jose ́ M. Gonza ́ lez-Calbet,* ,† and Marina Parras* ,† † Departamento de Química Inorga ́ nica, Facultad de Químicas, Universidad Complutense, Campus de Excelencia Internacional Moncloa, E-28040-Madrid, Spain ‡ FEI Company, 5600 KA Eindhoven, The Netherlands § Department of Materials Science & Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, United Kingdom * S Supporting Information ABSTRACT: A new hexagonal polytype in the BaMn 1‑x Fe x O 3‑δ system has been stabilized. Powdered Ba 0.8 Sr 0.2 Mn IV 0.4 Fe III 0.6 O 2.70 crystallizes in the 4H hexagonal polytype (space group P6 3 /mmc) according to X-ray diffraction. HAADF images and chemical maps with atomic resolution have been obtained by combining Cs-corrected electron microscopy and EELS spectroscopy. The structure is formed by dimers of face-sharing octahedra linked by corners. EELS data show a random distribution of the transition metals ions identified by Fe and Mn- L2,3 chemical maps. A systematic difference in contrast observed in the O-K signal mapping suggests that anion deficiency is randomly located along the hexagonal layers in agreement with ND data. The magnetic structure consists of ferromagnetic sheets with the magnetic moments aligned along the x-axis and coupled antiferromagnetically along the c-axis. KEYWORDS: corrected aberration electron microscopy, energy electron loss spectroscopy, hexagonal perovskites ■ INTRODUCTION Prompted by the need to control properties of technological promise such as ferroelectricity in hexagonal perovskites, considerable work on the compositional variations of these polytypes has been carried out in order to establish the most adequate relationship between structural type and electric and magnetic properties to get useful devices. The paramount role played by the ratio of hexagonal and cubic layers defining the stacking sequence of a given polytype was early recognized, and efforts devoted to search hexagonal perovskites containing different cubic/hexagonal stacking sequences have led to developing dielectric resonators at microwave frequencies with high permittivity and moderate losses. 1,2 The wide variety of arrangements of corner/face sharing octahedral building blocks in ABO 3 perovskite-like compounds is an excellent example of the diversity and flexibility of perovskite-based crystal structures. In fact, the stacking sequence of the AO 3 layers in ABO 3 perovskites can be cubic or hexagonal leading to the 3C- and 2H-ABO 3 structural types. These two structures represent the two “extreme” polytypes of the perovskite structure. The first one is based only on cubic close packed (...ccp...) AO 3 layers. The structure consists of a 3D array of corner sharing BO 6 octahedra. This structure is stable if the structural tolerance factor t is close to 1 (t = d A‑O /√2d B-O , where d A‑O and d B-O represent the average cation-oxygen interatomic distances of the A- and B-sites, respectively). 3 When the A cationic size is large enough, t becomes greater than unity leading to perovskites based on hexagonal close packing of AO 3 layers which give rise to infinite 1D chains of face-sharing octahedra parallel to the c-axis. This structure, adopted by BaMnO 3 , is known as the 2H-type 4 (two layer hexagonal cell). Between these two extremes, a wide variety of perovskite based structures can be obtained by combination of different cubic (c) and hexagonal (h) stacking layer sequences, leading to several polytypes containing both corner- and face- sharing octahedra. These hexagonal polytypes are denoted by the symbols nH or nR, where n stands for the number of layers and H or R for the hexagonal or rhombohedral symmetry of the unit cell. Keeping manganese at the B perovskite site, the tolerance factor can be modified by substitutions at the A sublattice in order to modify the d(A-O) distance. A representative example is 4H-SrMnO 3 , 5 an intermediate polytype between 2H-BaMnO 3 and 3C-CaMnO 3 with 50% hexagonal and 50% cubic stacking: ...chch..., a sequence which according to Negas 6 can be stabilized in the Ba 1‑x Sr x MnO 3 system for x = 0.5 and x = 0.9. Oxygen deficiency in the 2H structural type is usually accommodated by the introduction of cubic deficient BaO 3‑δ layers leading to the stabilization of different hexagonal polytypes. The introduction of anionic vacancies modifies the layer stacking sequence since, obviously, the Mn 4+ →Mn 3+ reduction process increases the B-O length and, therefore, Received: November 7, 2012 Revised: January 21, 2013 Published: January 23, 2013 Article pubs.acs.org/cm © 2013 American Chemical Society 548 dx.doi.org/10.1021/cm303609r | Chem. Mater. 2013, 25, 548-554