Response of Gd2Ti2O7 and La2Ti2O7 to swift-heavy ion irradiation and annealing

Abstract

Swift heavy ion (2 GeV ¹⁸¹Ta) irradiation-induced amorphization and temperature-induced recrystallization of cubic pyrochlore Gd₂Ti₂O₇(View the MathML source) are compared with the response of a compositionally-similar material with a monoclinic-layered perovskite-type structure, La₂Ti₂O₇ (P2₁). The averaged electronic energy loss, dE/dx, was 37 keV/nm and 35 keV/nm in Gd₂Ti₂O₇and La₂Ti₂O₇, respectively. Systematic analysis of the structural modifications was completed using transmission electron microscopy, synchrotron X-ray diffraction, Raman spectroscopy, and small-angle X-ray scattering. Increasing ion-induced amorphization with increasing ion fluence was evident in the X-ray diffraction patterns of both compositions by a reduction in the intensity of the diffraction maxima concurrent with the growth in intensity of a broad diffuse scattering halo. Transmission electron microscopy analysis showed complete amorphization within ion tracks (diameter: ∼10 nm) for the perovskite-type material; whereas a concentric, core–shell morphology was evident in the ion tracks of the pyrochlore, with an outer shell of disordered yet still crystalline material with the fluorite structure surrounding an amorphous track core (diameter: ∼9 nm). The radiation response of both titanate oxides with the same stoichiometry can be understood in terms of differences in their structures and compositions. While the radiation damage susceptibility of pyrochlore A₂B₂O7 materials decreases as a function of the cation radius ratio rA/rB, the current study correlates this behavior with the stability field of monoclinic structures, where rLa/rTi > rGd/rTi. Isochronal annealing experiments of the irradiated materials showed complete recrystallization of La₂Ti₂O₇ at 775 °C and of Gd₂Ti₂O₇ at 850 °C. The annealing behavior is discussed in terms of enhanced damage recovery in La₂Ti₂O7, compared to the pyrochlore compounds Gd₂Ti₂O₇. The difference in the recrystallization behavior may be related to structural constraints, i.e., reconstructing a low symmetry versus a high symmetry phase.