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A neutron powder diffraction study of the crystal and magnetic structures of ilvaite from 305 K to 5 K — a mixed valence iron silicate with an electronic transition |
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| Authors: | Subrata Ghose A W Hewat M Marezio |
| Institution: | 1. Department of Geological Sciences, AJ-20, University of Washington, 98195, Seattle, Washington, USA 2. Institut Max von Laue — Paul Langevin, BP 156X, 38042, Grenoble Cedex, France 3. Laboratoire de Crystallographie CNRS, BP 166X, 38042, Grenoble Cedex, France |
| Abstract: | The crystal and magnetic structures of ilvaite Ca(Fe2+, Fe3+)Fe2+Si2O7O(OH) have been obtained by profile refinement of high resolution neutron powder data from a natural sample from Seriphos, Greece. Below about 400 K an electronic transition from an itinerant to an ordered state is expected, with the structure changing from orthorhombic to monoclinic. The structure remains monoclinic P2 1/a down to 5 K, with Fe2+ almost completely ordered on one of the A-sites and Fe3+ on the other: the ordering may increase with decreasing temperature. The B-site contains Fe2+ plus a small amount of Mn2+ impurity. There are two magnetic transitions, at 116 K and 40 K: at 80 K the Fe2+ and Fe3+ spins on the A-sites along one infinite c-axis chain are parallel, and antiparallel to those along the adjoined edge-sharing centrosymmetrically related chain. The spin vectors are all perpendicular to the plane of these chains, i.e. almost parallel to the crystallographic b-axis. At 5 K, this order is maintained, but the Fe2+ spins on the B-sites order antiferromagnetically as well, again almost along the b-axis. These results explain the earlier Mössbauer and magnetisation measurements. |
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