PNNL

Abstract

The structural and electronic properties of Gd2(Ti1-yZry)2O7 (y=0-1) pyrochlores following 2.0 MeV Au2+ ion-beam irradiation (~5.0x1014 Au2+/cm2) have been investigated by Ti 2p and O 1s near-edge x-ray absorption fine structure (NEXAFS). The irradiation of Gd2(Ti1-yZry)2O7 leads to the phase transformation from the ordered pyrochlore structure (Fd3m) to the defect fluorite structure (Fm3m) regardless of Zr concentration. Irradiated Gd2(Ti1-yZry)2O7 with y=0.5 are amorphous although significant short-range order is present. Contrasting to this behavior, compositions with y=0.75 retain crystallinity in the defect fluorite structure following irradiation. The local structures of Zr4+ in the irradiated Gd2(Ti1-yZry)2O7 with y=0.75 determined by NEXAFS are the same as in cubic fluorite-structured yttria-stabilized zirconia (Y-ZrO2), thereby providing conclusive evidence for the phase transformation. The TiO6 octahedra present in Gd2(Ti1-yZry)2O7 are completely modified by ion-beam irradiation to TiOx polyhedra, and the Ti coordination is increased to eight with longer Ti-O bond distances. The similarity between cation sites and the degree of disorder in Gd2Zr2O7 facilitate the rearrangement and relaxation of Gd, Zr, and O ions/defects. This inhibits amorphization during the ion-beam induced phase transition to radiation-resistant defect fluorite structure contrasting to the ordered Gd2Ti2O7. Thus, during the ion-beam induced phase transition, the Gd2(Ti1-yZry)2O7 with y=0.5 becomes amorphous whereas compositions with y=0.75 retain crystalline structure.