Order-disorder phenomena in A2B2O7 pyrochlore oxides

The size difference between the cation species that occupy the A and B sites is believed to be the driving force fbr ordering in the pyrochlore superstructure. Indeed, substituting a larger tetravalent cation in solid solution in the B site ou a smaller trivalent cation in the A site progressively drives the structure to a disordered state in several systems, but not in cases cohere there is a strong covalent component to the bonding. Similar disordering behavior is found when the phases are heated. Pyrochlore is the only oxide in which simultaneous order-disorder transformations take place in the anion and cation arrays in the structure. Disorder evolves at independent rates in the cation and anion arrays despite the Coulombic interactions between them.