Synthesis, structure and properties of heavy metal pyrochlore oxides

Abstract

Many interesting and often exotic magnetic phenomena have been observed over the
past twenty years in the family of the heavy metal pyrochlore oxides, A2B2O6O’. In
these systems, competing interactions are common and tend to emerge mainly at low
temperatures. Magnetic properties are related to the contribution of d orbitals that transition
metal (TM) offer and their interplay with the positive contribution coming from
the cation sitting on the A site.

The work described in this thesis explores the effects of chemical substitution of the
A site cation on the symmetry and the properties of a number of pyrochlores containing
TM of the second and third transition row, such as Ir, Ru, Mo and Os. Depending if
this site is fully occupied and on the radius size the behaviour of the resulting material
can change significantly. The defect pyrochlore family AOs2O6 (A = Cs, Rb, K)
was studied in detail following the attention that its members attracted for having been
the first materials, belonging to this subgroup, to exhibit superconducting properties
[1, 2, 3] and for the diversity in their properties. In these compounds, the smaller the
ionic radius of an element, the weaker is the bond to the structure. As a result, the
temperature dependence of their energy becomes quite different from normal lattice
vibrations and gives interesting low temperature behaviours.

The effect of replacing the weak bonded potassium ions on the structures and superconducting
properties of the AOs2O6 phases has also been studied. The ability to
exchange the A-type in pyrochlores is well known and leads to applications for example
in nuclear clean-up. Thus the ion exchange of K+ by smaller cations such as Li+
or Na+ has been investigated with the aim of synthesising compounds which cannot be
obtained by direct synthesis. New defect pyrochlore materials, AA’Os2O6 (A’ = Li+,
Na+) have been synthesised together with hydrated derivatives, AA’OsO6.nH2O. Parent
materials were obtained by ion exchange in non-aqueous solvents under strictly dry
conditions. A Ba-based osmium oxide pyrochlore, Ba(Os2O6)2.H2O was also synthesised,
for the first time, from the K-osmate starting material by ion-exchange methods
in aqueous medium.

In this work, diffraction techniques, X- ray and neutron powder diffraction, were
employed and found suitable for detecting the change in cation and anion location
and general structure investigation.These structural descriptions are correlated with
the measured superconducting properties and theoretical descriptions of variations in
the Tc in these systems as a function of cation size and rattling.

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