Probing magnetite with X-ray standing waves : A high-resolution Fe K-edge spectroscopy study

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Title: Probing magnetite with X-ray standing waves : A high-resolution Fe K-edge spectroscopy study
Alternative title: Magnetiitin luotausta seisovilla röntgenaalloilla : Korkearesoluutioinen raudan K-reunan spektroskopiatutkimus
Author: Kivelä, Feliks
Other contributor: Helsingin yliopisto, Matemaattis-luonnontieteellinen tiedekunta
University of Helsinki, Faculty of Science
Helsingfors universitet, Matematisk-naturvetenskapliga fakulteten
Publisher: Helsingin yliopisto
Date: 2022
Language: eng
Thesis level: master's thesis
Degree program: Materiaalitutkimuksen maisteriohjelma (Materials Research)
Master 's Programme in Materials Research
Magisterprogrammet i materialforskning
Specialisation: Laskennallinen materiaalifysiikka
Computational Materials Physics
Abstract: The crystal structure of magnetite (Fe3O4) involves Fe2+ ions in sites with octahedral (Oh) symmetry and Fe2+ and Fe3+ ions in sites with tetrahedral (Td) symmetry. Magnetite exhibits several interesting physical phenomena, such as the Verwey transition, in which the roles of the different Fe sites are an active subject of research. In the X-ray standing wave (XSW) technique, incoming and diffracted X-ray beams interfere inside a crystal, creating a standing wave with the periodicity of the diffracting atomic lattice. The phase of the wave, i.e. whether the nodes are located on the lattice planes or between them, can be adjusted by finely tuning the diffraction angle. Changing the phase in this way makes it possible to selectively vary the contributions of different atoms and absorption types (dipole versus quadrupole) to the measured total absorption spectrum. Iron K-edge absorption spectra in magnetite were studied in the presence of an XSW in an experiment conducted at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. This thesis presents an analysis of the data gathered during the experiment, with the goal of decomposing the experimentally measured pre-edge peak into its constituent components. The methods used in the analysis include principal component analysis and fitting predicted absorption peaks calculated with the Quanty software to the experimental data. The results show the dipole and quadrupole contributions of the tetrahedral sites responding to changes in the phase of the XSW in opposite ways in a manner consistent with theoretical predictions.
Subject: Magnetite
Resonant inelastic X-ray scattering
X-ray absorption spectroscopy
X-ray standing waves

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