Speromagnetism in Iron Phosphate Glasses
Nanoscience
and
Materials
Prof Adrian C. Wright Prof Roger N. Sinclair Richard Haworth
Professor D.E. Day (University of Missouri, Rolla, U.S.A.)
The Fe2O3-P2O5 glass system is not only of great interest for scientific
reasons but also because it forms the basis of glasses which are being
considered for the storage of plutonium and/or other high-level nuclear wastes.
Fe2O3-P2O5 glasses with high Fe2O3 content are particularly interesting, in that
they exhibit short-range antiferromagnetic (speromagnetic) ordering at low
temperatures [1,2]. The atomic structure of Fe2O3-P2O5 glasses is being
investigated as a function of composition (30-44 mol. % Fe2O3), in the range
where the glasses have good chemical durability, using both steady-state reactor
and pulsed source neutron diffraction techniques, and the magnetic structure is
being studied by magnetic neutron diffraction. Measurements have also been
performed of the temperature dependence of the magnetic susceptibility at the
University of Warwick, in collaboration with Drs D. Holland and M.R. Lees [3],
and the magnetic excitations are being characterised by inelastic (magnetic)
neutron scattering.
The structure of Fe2O3-P2O5 glasses is complicated by the fact that they are
oxygen deficient and some of the iron is present as Fe2+ ions. Hence it will be
necessary to determine the Fe2+/Fe3+ ratio by Mössbauer spectroscopy. There are
two proposed structural models for Fe2O3-P2O5 glasses. One is based on
crystalline FePO4 (Fe2O3•P2O5 - a structural analogue of SiO2, exhibiting both
α- and β-quartz polymorphs), with regions containing excess P2O5. An alternative
model, for glasses containing significant concentrations of Fe2+, is based on
the crystal structure of Fe3(P2O7)2 in which the iron is present as (Fe3O12)16-
clusters comprising one Fe2+ and two Fe3+ ions. At present, the neutron
diffraction data favour the first model. However, recent small angle neutron
scattering measurements have shown that the glasses are heterogeneous on a scale
of ~700 Å.
References
[1] F.A. Wedgwood & A.C. Wright, J. Non-Cryst. Solids 21 (1976), 95.
[2] J.L. Shaw, Ph.D. Thesis (University of Reading, 2003).
[3] J.L. Shaw et al., J. Non-Cryst. Solids, in press.
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