Systematic Study of the Phase Behavior of f-block Oxides Irradiated with Swift Heavy Ions.

Abstract

Particles with specific energies of ~1 MeV/u or greater interact with matter primarily through the excitation of electrons. Swift heavy ions, with high masses and velocities in this electronic excitation regime, are encountered by materials in the form of nuclear fission fragments, cosmic rays, and ion accelerator beams. The dense ionization resulting from ion-solid interactions in this regime produce diverse, material-dependent structural and chemical modifications in insulators. This dissertation reports investigation of swift heavy ion-induced phase and valence modifications to oxides of the f-block (lanthanide and actinide) elements. Dependence of these modifications on the variations in ionic radius (due to the lanthanide/actinide contraction) and accessible oxidation states (due to the effects of f¬-orbital localization) characteristic of elements in this system is discussed. Lanthanides typically exhibit a single trivalent oxidation state, such that they form sesquioxides. The phase systematics of these compounds varies across the lanthanide series due to the influence of lanthanide ionic radius on phase stability. In response to swift heavy ion irradiation, transformations to polymorphic and non-equilibrium phases were observed. These transformations were caused by atomic displacement and point defect formation during relaxation of an irradiation-induced electron-hole plasma, such that their composition-dependence could be attributed to the influence of cation size on the associated collective atomic reorganization mechanisms. In contrast to the lanthanides the light actinide elements have more itinerant ¬f-electrons and are typically multivalent. Swift heavy ion irradiation of several actinide and lanthanide dioxides resulted in no phase transformations, but caused cation valence reduction in those materials incorporating cations with stable trivalent oxidation states. Coupling between these chemical modifications and structural distortions, due to associated changes in the ionic radii and bonding of cations, were observed. In trioxides, this valence reduction caused changes in stoichiometry, with accompanying redox-coupled phase transformations. Ternary lanthanide stannate complex oxides exhibited both amorphization and disordering in response to irradiation. Their susceptibility to irradiation-induced amorphization decreased with the ionic radius of the lanthanide cation. Compounds that were resistant to amorphization instead exhibited disordering in the form of mixing of the two cations (Ln and Sn) onto a single sublattice, via cation antisite formation.