Effect of Different Fabrication Avenues of Pyrochlore Ceramics Toward Order–disorder Transitions

The mineral pyrochlore is denoted by the formula (Na,Ca)2Nb2O6(OH,F), which is part of the pyrochlore supergroup. In general, the formula for a pyrochlore is written as A2B2O7, but is sometimes written as A2B2X7 to account for the anion site being occupied by fluorine atoms and to highlight its relationship to the fluorite AF2 structure. The pyrochlore structure can accommodate a deficiency in anions and these defect pyrochlores are written as A2B2X6, or sometimes AB2X6. Moving from A2B2O7 to AB2X6, the systems become more disordered in nature to accommodate the inherent defects. The increase in disorder might cause the pyrochlore to go through an order–disorder transition from pyrochlore to the defect fluorite structure or transition all the way into an amorphous state, depending on the radius ratio, as calculated from values specified by Shannon (1976) and defect formation energies. This chapter focuses on the control of disorder, mainly in relation to valence in pyrochlores, as achieved via various synthesis routes, beginning with the more common examples currently in the literature, such as the pyrochlores (A2B2O7) with an A cation of 3+ and a B cation of valency 4+, and then on to the disordered defect pyrochlores such as those composed of B5 + cations, through to the highly disordered high-entropy pyrochlores that are currently gaining in interest.