The order-disorder transition in ion-irradiated pyrochlore

A radiation-induced order-disorder transformation occurs in many A(2)B(2)O(7) pyrochlore structure-types by disordering of the A- and B-site cations, as well as anion vacancies. The ionic conductivity increases up to two orders of magnitude due to this order-disorder transformation. This irradiation-induced order-disorder transition has been examined in detail using ion beam irradiations and in situ transmission electron microscopy. Under ion irradiation, the ordered pyrochlore superstructure transforms to an anion-disordered pyrochlore prior to a final transformation to a cation-disordered defect-fluorite structure-type. The anion-disordered pyrochlore structure displays a partial ordering of the A- and B-site cations and complete disordering on the anion array-as evidenced by the disappearance of characteristic diffraction maxima resulting from ordering of the oxygen sublattice. These results suggest that anion disorder precedes cation disordering in the pyrochlore structure. (C) 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All fights reserved.