PNNL

Abstract

The Gd2(Ti1-yZry)2O7 pyrochlore series undergoes a structural phase transition from pyrochlore (Fd?3m) to defect fluorite (Fm?3m) that can be driven compositionally by increasing the Zr content or thermally by sintering Zr-rich compositions at temperatures above 1550 ?C. Our results demonstrate that ion-beam irradiation can also drive the structural phase transition for Zr-rich compositions. In an effort to understand the effects of composition and ion-beam irradiation on this phase transition, powder X-ray diffraction, polarized Raman, reflection infrared, and X-ray absorption spectroscopy experiments were conducted on Gd2(Ti1-yZry)2O7 pyrochlores prior to and following irradiation with 2 MeV Au2+ ions to a fluence of 5 ions/nm2. Analysis of the vibrational and X-ray absorption data suggests that the structural integrity of the pyrochlore structure is based on distorted corner-shared TiO6 or ZrO6 octahedra. The vibrational spectra indicate that both anion and cation disorder precede the compositionally-driven phase transition, while cation disorder appears to dominate the irradiation-driven transition. Analyses of the extended X-ray absorption fine structure of the Ti and Zr K-edges and the Gd LIII-edges reveal a significant change in the Gd local environment upon irradiation and with increasing Zr content. The Ti and Zr local environments are less affected by irradiation or compositional change, but show evidence of increasing disorder that can be attributed to rotations about shared polyhedral edges and corners.