Disorder in Mg1-xNixAl2O4 Spinel and its Impact on the Response to Swift Heavy Ion Irradiation

Spinels, which are currently used in optical applications, have many proposed functions in the nuclear industry, such as a nuclear waste form due to its radiation resistance. Disordering plays a key role in the response of spinel to a wide range of irradiation conditions. The first part of this research project was on characterizing the type and degree of disorder that can be induced by chemical substitution in the magnesium nickel aluminate spinel solid solution series (Mg(1-x)Ni(x)Al2O4). Neutron total scattering has been used to characterize the structure at both the cation and anion sublattices. High-resolution pair distribution function analysis was applied to obtain information on the local structure of disorder. The second part of thesis describes how compositional-induced disorder impacts the radiation response to swift heavy ions (2.2 GeV Au). Complementary x-ray powder diffraction and small angle x-ray scattering was additionally used for the analysis of the radiation-induced modifications. The long-range structure of the unirradiated samples shows an increasing amount of cation disorder with Ni content as well as local anion disorder. Swift heavy ions create a significant amount of cation and anion disorder in all spinel compositions without evidence of amorphization up to a fluence of 6e12 per square centimeter. An ion-induced phase transformation from spinel to a metastable rock-salt phase was evidenced in the local structure in Ni containing spinels. The pre-existing disorder affects significantly the radiation response with a more efficient rock-salt phase formation for spinel compositions with a higher amount of cation disorder.