What is it about?
Both alpha-Fe2O3 and alpha-Cr2O3 crystals have a special direction [110] which is perpendicular to the hexagonal [001] axis. The rotation by 180 deg. around this special direction [110] is a symmetry operation of the crystal. The monoclinic distortion of alpha-Fe2O3 is described with "positive sign" while that of and alpha-Cr2O3 is described with "negative sign".
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Why is it important?
These observations may indicate the existence of some kinds of symmetry in alpha-Fe2O3 and alpha-Cr2O3 crystals which are not yet understood nowadays.
Perspectives
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Radoslaw Przenioslo
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This page is a summary of: Lack of a threefold rotation axis in α-Fe2O3and α-Cr2O3crystals, Acta Crystallographica Section B Structural Science Crystal Engineering and Materials, March 2015, International Union of Crystallography,
DOI: 10.1107/s205252061500342x.
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