Order-disorder phenomena in A(2)B(2)O(7) pyrochlore oxides

The size difference between the cation species that occupy the A and B site s is believed to be the driving force fbr ordering in the pyrochlore supers tructure. Indeed, substituting a larger tetravalent cation in solid solutio n in the B site ou a smaller trivalent cation in the A site progressively d rives the structure to a disordered state in several systems, but not in ca ses cohere there is a strong covalent component to the bonding. Similar dis ordering behavior is found when the phases are heated. Pyrochlore is the on ly oxide in which simultaneous order-disorder transformations take place in the anion and cation arrays in the structure. Disorder evolves at independ ent rates in the cation and anion arrays despite the Coulombic interactions between them.