Helium Diffusion and Accumulation in Gd2Ti2O7 and Gd2Zr2O7

The effects of helium accumulation on bubble formation and mechanical properties, as well as the fundamentals of helium diffusion in pyrochlores, are experimentally investigated in Gd₂Ti₂O₇ [gadolinium titanate] and Gd₂Zr₂O₇ [gadolinium zirconate]. We find that helium accumulation results in bubble formation at concentrations of 6 at.% in pre-damaged Gd₂Ti₂O₇ and 4.6 at.% in pre-damaged Gd₂Zr₂O₇. Lattice parameter, residual stress, and hardness changes due to helium accumulation were investigated in Gd₂Zr₂O₇, which remains crystalline after irradiation. Predicting the long-term behavior of helium requires a solid understanding of helium migration behavior in the pyrochlore lattice. Thermal helium desorption measurements suggest that helium binds to vacant lattice sites at room temperature. Helium-vacancy clusters likely formed in the sample implanted with higher helium implantation energy, which is likely a combination of the initial defect structure and implantation depth. A deeper implantation allows helium to desorb from small vacancy sites and re-trap into helium-vacancy clusters that form at high temperature. Voronoi volume calculations provide estimations of the lowest energy helium interstitial configurations, which were found to be the octahedral site in Gd₂Ti₂O₇ and the tetrahedral site in both pyrochlore and defect-fluorite Gd₂Zr₂O₇. Finally, based on helium trapping information, the critical concentration required for bubble formation was estimated and compared to the experimental transmission electron microscopy results.