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.