High Energy Heavy Ion-Induced Structural Modifications in Binary Oxides.

Abstract

The objective of this work was to determine the relation between materials properties and their effect on the structural response of binary oxides to high energy heavy ion irradiation. The Group 14 oxides offered an ideal system of study due to the gradual change in materials properties from SiO2 to PbO2, while their electronic configurations remain consistent; this series facilitated the association of specific materials properties with their effect on radiation response. SnO2 and PbO2 were investigated experimentally in order to complete the body of data for this system. For comparative purposes, Ta2O5 was investigated under the same conditions due to the contrast in physical and chemical characteristics it offers, as well as its unusually large and complicated unit cell. SnO2, PbO2, and Ta2O5 were irradiated by 2.2 GeV 197Au ions (11.1 MeV/u) at room temperature. Samples were analyzed with synchrotron X-ray diffraction, Raman spectroscopy, transmission electron microscopy, small-angle X-ray scattering, and X-ray photoelectron spectroscopy. Irradiation of SnO2 led to the formation of a crystalline SnO phase with trace quantities of metallic Sn, indicating the loss of oxygen and cation reduction during irradiation. Irradiation of PbO2 resulted in the formation of seven distinct structures with compositions of Pb2O3, Pb3O4, PbO, and Pb. Gradual cation reduction was measured. Irradiation of Ta2O5 induced amorphous ion tracks with core-shell morphologies. Oxygen loss was evidenced, increasing with fluence to an estimated final stoichiometry of Ta2O4.2. Using the Group 14 oxide system, the following relations were made: (i) increased susceptibility to amorphization has been attributed to high enthalpy of formation, bandgap, electrical resistivity, and cation electronegativity (relative to those resistant to amorphization), as well as relatively low bond ionicity and bond lengths; (ii) increased susceptibility to oxygen loss during irradiation has been attributed to relatively low bond dissociation energy, bandgap, and electrical resistivity, as well as relatively large bond lengths; (iii) increased susceptibility to cation reduction has been attributed to relatively high bond ionicity as well as low enthalpy of formation, melting temperature, resistivity, and cation electronegativity. Materials property value thresholds are presented for all properties that show correlations to each radiation effect.