STRUCTURAL AND MAGNETIC PROPERTIES OF IRIDIUM AND NIOBIUM PYROCHLORE OXIDES

Abstract

This thesis has two major components: a study of Bi\(_{2-x}\)Co\(_{x}\)Ir\(_{2}\)O\(_{7}\) and a study of Ca\(_{2}\)Nb\(_{2}\)O\(_{6}\)F. Solid oxide compounds with the pyrochlore structure (A2B2O7) are known for interesting and occasionally dramatic magnetic and electric properties due to the motif of corner-sharing tetrahedra that they can contain. Iridium is a very large element with relativistic effects, such as spin-orbit coupling. This thesis reports the metallic series Bi\(_{2-x}\)Co\(_{x}\)Ir\(_{2}\)O\(_{7}\) (x=0.5, 0.42, 0.35, 0.25, 0.1, 0). Existing structures predict that cobalt should substitute on the iridium site, but here cobalt substituted on the bismuth site. Bi\(_{2}\)Ir\(_{2}\)O\(_{7}\) was diamagnetic, and the cobalt doped compounds were paramagnetic. At the highest cobalt content (x=0.5, 0.42, 0.35), the compounds ordered as a spin glass, as shown by magnetic susceptibility data. Niobium strongly prefers a +5 oxidation state, but when it does change its electron count, it is known to have interesting magnetic properties. The chemical formula of Ca\(_{2}\)Nb\(_{2}\)O\(_{6}\)F suggests Nb\(_{4}\):5+. This thesis reports a pyrochlore in the Ca\(_{2}\)Nb\(_{2}\)O\(_{6}\)F family. The compound was weakly diamagnetic and appeared to have the Niobium in a predominantly 5+ oxidation (4d0) state. These properties suggested the presence of calcium vacancies and the possibility of fluorine interstitials. Aluminum and titanium doping altered the properties of the pyrochlore.