Effects of Irradiation-induced Structural Disordering in Zirconate Pyrochlores at High Pressures

Abstract:

Subsequent effects of high pressure on zirconate pyrochlores (A₂Zr₂O₇, where A = Nd, Er, and Sm) are investigated after the materials had been disordered from swift heavy ion irradiation. The motivations behind the study are that: 1) disordering induced by ion irradiation reduces the activation energy for anion vacancy migration, and the enhanced oxygen diffusion alters the pressure transition point of the material; and 2) a subsequent effect of pressure on ion-irradiated materials provides a unique way of creating a non-equilibrium states of matter, which can potentially yield a novel phase.

The study finds that the critical pressure transition point (crystalline to amorphous) of zirconate pyrochlores decreases by ~ 15 ± 4% after they had been disordered. This can be attributed to the bigger unit-cell volume of the ordered-pyrochlore structure, which allows more free space for compression. Ion irradiation also increases the coordination number around the B-site cation, which enhances the phase transition with increased pressure. For Er₂Zr₂O₇, where the material initially has a disordered-fluorite structure from its small cation ionic radii ratio, r_A/r_B = 1.33, a reverse outcome is noticed i.e., the pressure transition point increases by ~ 5%. The increased compressibility may be due to the defects induced by swift heavy ions. A novel X-phase appears with high pressure in irradiated Nd₂Zr₂O₇. The phase is determined to be similar to the Nd₂O₃ phase, and is comparable to that found in the previous study where an energetic ion irradiation was performed simultaneously with high pressure experiment. Although the X-phase is not quenchable, this study demonstrates that such a phase is accessible even without having to perform a simultaneous irradiation and pressure experiment.