Structural modifications in pyrochlores caused by ions in the electronic stopping regime

Abstract

Damage caused by swift heavy ions with three different electronic stopping powers (120 MeV Au⁹⁺ (22 keV/nm), 90 MeV I⁷⁺ (17 keV/nm) and 70 MeV Ni⁵⁺ (11 keV/nm)) in three pyrochlores (Gd₂Zr₂O₇, Nd₂Zr₂O₇ and Gd₂Ti₂O₇) with progressively increasing radius ratios (r_A/r_B of A₂B₂O₇) of cations is reported. Since I⁺⁷ is one of the ions used, these measurements also simulate fission fragment damage in pyrochlores and identify a potential host lattice for inert matrix fuel. X-ray diffraction on the irradiated materials indicate amorphization in Gd₂Ti₂O₇ at the lowest S_e used, and a transition to anion-deficient fluorite (Fm3̅m) structure with about 1% decrease in volume in Nd₂Zr₂O₇ and Gd₂Zr₂O₇ at higher S_e. In Nd₂Zr₂O₇ this is followed by amorphization and Gd₂Zr₂O₇ does not amorphize even at the highest S_e employed. Raman analysis of Gd₂Zr₂O₇ and Nd₂Zr₂O₇, indicate an increase of Zr coordination number after irradiation and the bands become broader due to disordering. Analysis of the results shows that the radiation susceptibility of these pyrochlores in the electronic stopping regime strongly depends on the radius ratio of A to B cations and hence on the energy required for formation of cation antisites and anion Frenkel pairs, similar to their susceptibility in the nuclear stopping regime.