Abstract
There has been an increased focus on understanding the energetics of structures that can accommodate unconventional ordering (e.g., disordered periodic arrays that appear at different length-scales [1]). For example, the isometric pyrochlore structure [2], A2B2O7, forms a disordered, defect fluorite-structure, (A,B)4O7, or an aperiodic array, when exposed to extreme conditions [3-7]. The ability to accommodate disorder in its structure accounts for the tendency of some compositions to resist becoming aperiodic in high radiation fields or at high temperatures. Thus, these materials find application as host materials for immobilizing actinides, such as plutonium [4], fast ion conductors in solid oxide fuel cells [8, 9], and thermal barrier coatings for gas turbine jet engines [10, 11].
Despite the importance of the disordering process, there has been only a limited understanding of the role of local ordering on the energetic landscape, mainly due to the use of techniques (i.e., X-ray/electron diffraction) that can only characterize the average structure over a large number unit cells and are insensitive to disorder on the oxygen sublattice. We have used neutron pair distribution functions (PDFs) to show that the “disordered” fluorite structure consists of a locally-ordered, orthorhombic structural unit that is repeated by a pseudo-translational symmetry, such that the ordered, orthorhombic and disordered, isometric arrays coexist at different length-scales. This disordering mechanism is a more general phenomenon in complex oxides, as PDF analysis has revealed that inversion in the isometric spinel structure, AB2O4 à B(AB)O4, occurs by a similar process across the highly “randomized” B-site. This insight into order-disorder transformations induced intrinsically (chemical composition) or extrinsically (far from equilibrium conditions by high radiation fields) provides a new basis for understanding how modulated structures and correlated disorder at different length-scales affects the physical and transport properties of this important class of materials.
