Transmission Electron Microscopic Study of Pyrochlore to Defect-Fluorite Transition in Rare-Earth Pyrohafnates

A structural transition in rare earth pyrohafnates, Ln ₂Hf ₂O ₇ (Ln=Y, La, Pr, Nd, Tb, Dy, Yb and Lu), has been identified. Neutron diffraction showed that the structure transforms from well-ordered pyrochloric to fully fluoritic through the lanthanide series from La to Lu with a corresponding increase in the position parameter x of the 48f (Fd 3̄ m) oxygen site from 0.330 to 0.375. As evidenced by the selected area electron diffraction, La ₂Hf ₂O ₇, Pr ₂Hf ₂O ₇ and Nd ₂Hf ₂O ₇ exhibited a well-ordered pyrocholoric structure with the presence of intense superlattice spots, which became weak and diffuse (in Dy ₂Hf ₂O ₇ and Tb ₂Hf ₂O ₇) before disappearing completely as the series progressed towards the Lu end. High resolution electron microscopic studies showed the breakdown of the pyrochlore ordering in the form of antiphase domains resulting in diffused smoke-like superlattice spots in the case of Dy ₂Hf ₂O ₇ and Tb ₂Hf ₂O ₇.