title: Lack of a threefold rotation axis in α-Fe₂O₃ and α-Cr₂O₃ crystals.
abstract: The crystal structure of α-Fe2O3 and α-Cr2O3 is usually described with the corundum-type trigonal crystal structure based on the space group R3¯c. There are, however, some observations of the magnetic ordering of both α-Fe2O3 and α-Cr2O3 that are incompatible with the trigonal symmetry. We show experimental evidence based on X-ray powder diffraction and supported by transmission electron microscopy that the symmetry of the crystal structure of both α-Fe2O3 and α-Cr2O3 is monoclinic and it is described with the space group C2/c (derived from R3¯c by removing the threefold rotation axis). The magnetic orderings of α-Fe2O3 and α-Cr2O3 are compatible with the magnetic space groups C2/c and C2/c', respectively. These findings are in agreement with the idea from Curie [(1894), J. Phys. 3, 393-415] that the dissymmetry of the magnetic ordering should be associated with a dissymmetry of the crystal structure.
authors.affiliation: Faculty of Physics, University of Warsaw, Pasteura 5, Warsaw PL 02-093, Poland.
authors.forename: Michał
authors.initials: M
authors.lastname: Stękiel
authors.affiliation: Faculty of Physics, University of Warsaw, Pasteura 5, Warsaw PL 02-093, Poland.
authors.forename: Radosław
authors.initials: R
authors.lastname: Przeniosło
authors.affiliation: Faculty of Physics, University of Warsaw, Pasteura 5, Warsaw PL 02-093, Poland.
authors.forename: Izabela
authors.initials: I
authors.lastname: Sosnowska
authors.affiliation: ESRF, The European Synchrotron, CEDEX 9, Grenoble F 38043, France.
authors.forename: Andrew
authors.initials: A
authors.lastname: Fitch
authors.affiliation: Conn Center for Renewable Energy Research, University of Louisville, KY 40292, USA.
authors.forename: Jacek B
authors.initials: JB
authors.lastname: Jasiński
authors.affiliation: Department of Chemistry, University of Manitoba, Winnipeg, MB, Canada R3T 2N2.
authors.forename: Joey A
authors.initials: JA
authors.lastname: Lussier
authors.affiliation: Department of Chemistry, University of Manitoba, Winnipeg, MB, Canada R3T 2N2.
authors.forename: Mario
authors.initials: M
authors.lastname: Bieringer
keywords: magnetic ordering; symmetry; threefold rotation axis