Synthesis, structure and properties of heavy metal pyrochlore oxides

Many interesting and often exotic magnetic phenomena have been observed over the<br/>past twenty years in the family of the heavy metal pyrochlore oxides, A2B2O6O’. In<br/>these systems, competing interactions are common and tend to emerge mainly at low<br/>temperatures. Magnetic properties are related to the contribution of d orbitals that transition<br/>metal (TM) offer and their interplay with the positive contribution coming from<br/>the cation sitting on the A site.<br/><br/>The work described in this thesis explores the effects of chemical substitution of the<br/>A site cation on the symmetry and the properties of a number of pyrochlores containing<br/>TM of the second and third transition row, such as Ir, Ru, Mo and Os. Depending if<br/>this site is fully occupied and on the radius size the behaviour of the resulting material<br/>can change significantly. The defect pyrochlore family AOs2O6 (A = Cs, Rb, K)<br/>was studied in detail following the attention that its members attracted for having been<br/>the first materials, belonging to this subgroup, to exhibit superconducting properties<br/>[1, 2, 3] and for the diversity in their properties. In these compounds, the smaller the<br/>ionic radius of an element, the weaker is the bond to the structure. As a result, the<br/>temperature dependence of their energy becomes quite different from normal lattice<br/>vibrations and gives interesting low temperature behaviours.<br/><br/>The effect of replacing the weak bonded potassium ions on the structures and superconducting<br/>properties of the AOs2O6 phases has also been studied. The ability to<br/>exchange the A-type in pyrochlores is well known and leads to applications for example<br/>in nuclear clean-up. Thus the ion exchange of K+ by smaller cations such as Li+<br/>or Na+ has been investigated with the aim of synthesising compounds which cannot be<br/>obtained by direct synthesis. New defect pyrochlore materials, AA’Os2O6 (A’ = Li+,<br/>Na+) have been synthesised together with hydrated derivatives, AA’OsO6.nH2O. Parent<br/>materials were obtained by ion exchange in non-aqueous solvents under strictly dry<br/>conditions. A Ba-based osmium oxide pyrochlore, Ba(Os2O6)2.H2O was also synthesised,<br/>for the first time, from the K-osmate starting material by ion-exchange methods<br/>in aqueous medium.<br/><br/>In this work, diffraction techniques, X- ray and neutron powder diffraction, were<br/>employed and found suitable for detecting the change in cation and anion location<br/>and general structure investigation.These structural descriptions are correlated with<br/>the measured superconducting properties and theoretical descriptions of variations in<br/>the Tc in these systems as a function of cation size and rattling.

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2010

Author dc:creator

• Galati, Rosa

Advisor dc:contributor.advisor

• Weller, Mark T.