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

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.We acknowledge beamtime at ALBA synchrotron (proposal 2019073634) and ILL (EASY access system) and Dr François Fauth and Dr Clemens Ritter, respectively, for their help during data collection. ICMAB authors acknowledge Spain's Agencia Estatal de Investigación Severo Ochoa Program for Centers of Excellence in R&D (CEX2019-000917-S) and funding through grant PID2020-113805GB-I00. ICTS-CNME at UCM is acknowledged for offering access to STEM microscopy and expertise.With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe