Crystallographic and magnetic structure ofSrCoO2.5brownmillerite: Neutron study coupled with band-structure calculations

Abstract

A study of the crystallographic and magnetic structure of SrCoO 2.5 with a brownmillerite-type structure has been carried out from neutron powder-diffraction ͑NPD͒ measurements at temperatures ranging from 10 to 623 K, across the Néel temperature ͑T N = 537 K͒ of this antiferromagnetic oxide. The study has been complemented with differential scanning calorimeter ͑DSC͒, dc susceptibility and magnetization measurements. Although the refinement of the crystal structure from NPD data is possible in the orthorhombic Pnma and Ima2 space groups, the support of ab-initio band-structure calculations has allowed us to select, without ambiguity, the Ima2 space group as the ground state for SrCoO 2.5 brownmillerite. In Ima2 the crystallographic structure of SrCoO 2.5 is described as layers of corner-sharing Co1O 6 octahedra alternating along the a axis with layers of vertex-sharing Co2O 4 tetrahedra, conforming chains running along the ͓0 0 1͔ direction. The magnetic structure below T N = 537 K is G-type with the magnetic moments directed along the c axis. This magnetic arrangement is stable from T N down to 10 K. At T = 10 K, the magnetic moment values for Co1 and Co2 atoms are 3.12͑13͒ B and 2.88͑14͒ B , respectively, compatible with a Co 2+ L គ state, where L គ stands for a ligand hole. The magnetic susceptibility curves show, below 200 K, a divergence of zero-field cooling and field cooling curves, displaying broad maxima which are interpreted as due to the presence of ferromagnetic clusters embedded into an antiferromagnetic matrix. These inhomogeneities are inherent to the synthesis process, by quenching microcrystalline samples of SrCoO 3−x composition from high temperature, where cubic, ferromagnetic perovskites have been identified by diffraction methods.

Crystallographic and Magnetic Structure of the Perovskite-Type Compound BaFeO2.5: Unrivaled Complexity in Oxygen Vacancy Ordering

“…In this article, we describe a detailed investigation of the crystallographic structure by means of a coupled Rietveld analysis of X-ray-and neutron-powder diffraction data, to clarify the real vacancy ordering and resulting iron coordination geometries for this compound, and we also interpret our Mössbauer spectroscopy data by comparison with that reported previously 27 . To our knowledge (and we have compared a large variety of such superstructures reported in reviews on this topic 21,22 ), the structure of BaFeO 2.5 reported here is the most complicated perovskite-type superstructure reported so far (largest primitive cell, number of ABX 2.5 units, number of different crystallographic sites). In addition, we report on the magnetic structure, which agrees well with the magnetic characterisation from SQUID measurements and we additionally describe the results of DFT based calculations as well as group theoretical considerations revealing the structural relationship to the cubic perovskite structure.…”

Symmetry‐Driven Atomic Rearrangement at a Brownmillerite–Perovskite Interface

“…While it can be difficult to identify the space group symmetry of brownmillerite SCO [20,32,33] , Figure 1 can provide a hint to the possible space group symmetry of our film. Muñoz et al determined from neutron diffraction coupled with theoretical calculations that the Ima2 space group, in which the tetrahedral chains maintain the same orientation in the alternating tetrahedral layers, is the most energetically stable .…”

“…Muñoz et al determined from neutron diffraction coupled with theoretical calculations that the Ima2 space group, in which the tetrahedral chains maintain the same orientation in the alternating tetrahedral layers, is the most energetically stable . [20] In contrast, Sullivan et al proposed the space group, Imma, in which the tetrahedral chains have different orientations in the alternating layers. [33] The question then becomes whether the tetrahedral chains maintain the same orientation or whether they are disordered.…”

“…The brownmillerite structure is derived from the perovskite phase (SrCoO 3−δ ) through the removal of 1/6 th of the oxygen atoms; thereby reducing the crystal symmetry from the highly symmetric cubic phase (space group Pm3 ̅ m) to a lower symmetry orthorhombic phase . [17][18][19][20] Interestingly, this structural phase transformation is accompanied by a change in properties from a ferromagnetic metal to an antiferromagnetic insulator as δ approaches 0.5. [21,22] While the lattice mismatch between SCO and STO is negligible when considering the average in-plane lattice parameter for SCO (a pc = 3.905 Å), compressive and tensile strains near 1% are calculated with respect to the individual a o and b o lattice parameters, inducing a non-uniform strain of the brownmillerite lattice.…”

Engineering the oxygen coordination in digital superlattices

“…The weaker intensities indicate that the 1:1 SL adopts a more perovskite-like structure in which the SrO layers separating the Co and Ti layers are spaced more evenly apart compared to the brownmillerite structure, in which the SrO interlayer distances separating the octahedral CoO 6 and tetrahedral CoO 4 layers differ by 0.87 Å. 24 The 2:1 SL shows the expected 1/3-order superlattice peaks, while the 50:50 alloy shows an absence of any superlattice peaks. In the case of the 1:1 SL and the 50:50 alloy, a small peak corresponding to CoO (2 0 0) is observed at L = 1.8 reciprocal lattice units (r.l.u.…”