Probing disorder in isometric pyrochlore and related complex oxides

Abstract

There has been an increased focus on understanding the energetics of structures with unconventional ordering (for example, correlated disorder that is heterogeneous across different length scales). In particular, compounds with the isometric pyrochlore structure, A2B2O7, can adopt a disordered, isometric fluorite-type structure, (A, B)4O7, under extreme conditions. Despite the importance of the disordering process there exists only a limited understanding of the role of local ordering on the energy landscape. We have used neutron total scattering to show that disordered fluorite (induced intrinsically by composition/stoichiometry or at far-from-equilibrium conditions produced by high-energy radiation) consists of a local orthorhombic structural unit that is repeated by a pseudo-translational symmetry, such that orthorhombic and isometric arrays coexist at different length scales. We also show that inversion in isometric spinel occurs by a similar process. This insight provides a new basis for understanding order-to-disorder transformations important for applications such as plutonium immobilization, fast ion conduction, and thermal barrier coatings.

Role of composition, bond covalency, and short-range order in the disordering of stannate pyrochlores by swift heavy ion irradiation

“…Surprisingly, the spectra of these materials at the highest ion fluence achieved, 3×10 This is consistent with the observation of Shamblin et al 55 that the local ordering of disordered pyrochlores is independent of the disordering mechanism.…”

“…It is an ideal probe for the structural characterization of disordered and amorphous materials because it does not require structural periodicity. In contrast, XRD measurements probe long-range structure, which is absent in the irradiation-induced amorphous phase and differs substantially from the short-range structure in the disordered phase 55 . Fig.…”

“…These calculations assumed a quasi-random distribution of cations and anions on their respective sublattices in a defect-fluorite structure when simulating the disordered phase. However, Shamblin et al 55 have more recently reported that disordered pyrochlores instead adopt a local weberite-type structure 38 , wherein half the A-and B-site cations retain the eightfold and sixfold coordination, respectively, that they exhibit in the initial pyrochlore phase. Modulation of these orthorhombic structural units yields the long-range defect-fluorite ordering evident in XRD measurements.…”

Chemical ordering in substituted fluorite oxides: a computational investigation of Ho2Zr2O7 and RE2Th2O7 (RE=Ho, Y, Gd, Nd, La)

“…In a very recent contribution, pair distribution function (PDF) analysis based on neutron total scattering measurements for the RE-substituted fluorite oxide system Ho 2 Zr 2 O 7 has provided new insights into the nature of structural ordering20. In this study, the diffraction patterns for the samples studied displayed only peaks characteristic of the fluorite structure, indicative of the stability of a disordered fluorite solid-solution phase, consistent with previous reports (see, e.g., Stanek et al 21.…”

Universal geometric frustration in pyrochlores

“…Additional disorder, such as A / B site-mixing is not uncommon 5–8 , and can lead to the weberite structure 9 , unless optimal cation ordering occurs. Lastly, polyhedral distortions can also occur, where the tetrahedral corners move in or out.…”