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

Abstract

A study of the crystallographic and magnetic structure of ${\text{SrCoO}}_{2.5}$ with a brownmillerite-type structure has been carried out from neutron powder-diffraction (NPD) measurements at temperatures ranging from 10 to 623 K, across the N\'eel temperature $({T}_{N}=537\text{ }\text{K})$ of this antiferromagnetic oxide. The study has been complemented with differential scanning calorimeter (DSC), $dc$ 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 ${\text{SrCoO}}_{2.5}$ brownmillerite. In $Ima2$ the crystallographic structure of ${\text{SrCoO}}_{2.5}$ is described as layers of corner-sharing $\text{Co}1{\text{O}}_{6}$ octahedra alternating along the $a$ axis with layers of vertex-sharing $\text{Co}2{\text{O}}_{4}$ tetrahedra, conforming chains running along the [0 0 1] direction. The magnetic structure below ${T}_{N}=537\text{ }\text{K}$ 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 $T=10\text{ }\text{K}$, the magnetic moment values for Co1 and Co2 atoms are $3.12(13){\ensuremath{\mu}}_{B}$ and $2.88(14){\ensuremath{\mu}}_{B}$, respectively, compatible with a ${\text{Co}}^{2+}\underset{}{L}$ state, where $\underset{}{L}$ 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 ${\text{SrCoO}}_{3\ensuremath{-}x}$ composition from high temperature, where cubic, ferromagnetic perovskites have been identified by diffraction methods.