Assessing the local structure and quantifying defects in Ca4Fe9O17 combining STEM and FAULTS

Abstract

Defects in crystalline structures play a vital role in their properties, so their proper characterization is essential to understanding and improving the behaviour of the materials. In this work, their presence in Ca₄Fe₉O₁₇ has been analysed. Its structure exhibits three different iron coordination topologies and can be described as layers of corner-sharing FeO₅ bipyramids stacked along the c axis together with layers of edge-sharing FeO₆ octahedra, both being linked by FeO₄ tetrahedra. The relative position of the FeO₄ tetrahedra generates three possible stacking directions, which results in stacking faults when more than one is combined. Structural refinement using the Rietveld method in X-ray and neutron powder diffraction data was not possible due to significant mismatches between the observed and calculated integrated intensities for several peaks resulting from this atomic disorder. Selected area electron diffraction (SAED) and high-resolution scanning transmission electron microscopy (HR-STEM) images confirm the local defective nature of the material. The FAULTS software enabled a successful refinement of the structure considering a high concentration of planar defects, conferred by the existence of three possible stacking directions in the crystal structure, all of them confined in the basal plane.