Pyrochlore-defect fluorite phase transitions and stability in the Y2Sn2-xZrxO7 system

Abstract

The Y2Sn2-xZrxO7 pyrochlore series undergoes a phase transformation from a cubic pyrochlore structure type (Fd3m) to defect fluorite (Fm3m) actuated by an increase in Zr content, coupled with thermal annealing above 1500 °C. X-ray diffraction analysis reveals the onset of a pyrochlore to defect fluorite transition at Y2Sn0.8Zr1.2O7 with the loss of long range ordering. This is confirmed further by selected area electron diffraction (SAED) illustrating shorter range ordering in the defect fluorite phase incommensurate with unit cell size. This transformation however, occurs at a much higher Zr content than that predicted by classical radius ratio models. The diffuse scattering features observed in electron diffraction patterns of defect fluorite phases indicate some form of shorter range ordering involving compositional-displacive structural modulation. The behaviour of these materials during irradiation will be discussed and linked with the observed structural parameters (diffuse scattering, unit cell size).