Structural disorder has a profound effect on a material’s physical properties. Transport properties are strongly influenced by crystallographic defects which act as scattering centres for quasiparticles, phonons, photons, and so on. For example, structural disorder alters electron transport in graphene, photon transport in silicon photonic superlattices, flux pinning in superconductors, and heat transport properties in thermoelectric materials, and thus is used in diverse technological applications. For pyrochlore materials in particular, structural disorder increases catalytic activity in Li–O₂ batteries, increases ionic conductivity to levels comparable to yttria-stabilized zirconia, and alters radiation resistance, affecting its ability to immobilize radionuclides from spent nuclear fuel. Key to these applications is a comprehensive understanding of the origin of the order/disorder transformation.

—Shamblin et al.

Three UT researchers—Maik Lang, assistant professor of nuclear engineering; Haidong Zhou, assistant professor of physics; and Jacob Shamblin, a graduate research assistant in nuclear engineering and physics—studied pyrochlore and spinel. The materials, each consisting of two or more positively charged metal ions and oxygen, are used in a wide range of energy applications.

For their study, the team used state-of-the-art neutron characterization techniques to gain a detailed understanding and new insights into the nature of the atomic motifs in these materials.

The complex oxides we analyzed in this study—pyrochlore and spinel—have been investigated for decades by different researchers. When subjected to extreme environments such as high temperatures or high-energy radiation, many of these compounds partially lose their long-range crystal structure, and the multiple cations were thought to randomly exchange crystal sites.

—Maik Lang

The multidisciplinary research team and the unique capabilities of ORNL’s Spallation Neutron Source (earlier post), a DOE Office of Science User Facility, helped the team discover a novel atomic disordering mechanism in these materials.

With the help of SNS instrument scientists Mikhail Feygenson and Joerg Neuefeind, Lang and his team used the Nanoscale-Ordered Materials Diffractometer (NOMAD) for an in-depth look at the local crystal structure of their samples—a NOMAD first for neutron scattering experiments on ion-irradiated materials.

Neutrons are indispensable for this type of study because they can accurately detect the position of oxygen atoms present in materials.

Using neutrons to measure samples of such small sizes would have been difficult, if not impossible, just a few years ago. However, with the combination of the high neutron flux of SNS and the wide detector coverage of the NOMAD instrument, scientists can look at very small samples, which are typically the domain of X-ray scattering experiments.

—Mikhail Feygenson

Data analysis from NOMAD revealed the cations and oxygens in the materials are not randomly arranged at the atomic level but only appear so when sampling over longer scales, a key discovery.

Lang said the heterogeneous disorder was unexpected but seems to be a general phenomenon for many other materials functioning in harsh conditions. He said the new insight into disorder is fundamental to controlling oxygen mobility and phonon transport in complex oxides, a critical aspect for technological applications.

By gaining a better understanding of such materials, the team could help improve and control performance across a range of technologies—containment and immobilization of nuclear waste being a prime example.

This ability to accommodate atomic disorder in their structure accounts for the tendency of some compositions to resist becoming fully amorphous under irradiation or at high temperatures. Such materials find application as host materials for immobilizing actinides, such as plutonium.

Predicting transport of radionuclides is important for their safe use as nuclear waste forms and requires a detailed knowledge of how the atomic structure responds to self-irradiation.

—Maik Lang

Lang said the team’s data will provide much needed atomic-scale information from the initial local defect structure to the long-range observable material modifications so that waste form properties and degradation can be accurately simulated.

Other researchers on the team included Cameron Tracy and Rodney Ewing of Stanford University, Fuxiang Zhang of the University of Michigan, and Sarah Finkeldei and Dirk Bosbach of the Forschungszentrum Jülich Institute of Energy and Climate Research in Germany.

The work was supported as part of the Materials Science of Actinides, an Energy Frontier Research Center funded by DOE’s Office of Science.

Resources

• Jacob Shamblin, Mikhail Feygenson, Joerg Neuefeind, Cameron L. Tracy, Fuxiang Zhang, Sarah Finkeldei, Dirk Bosbach, Haidong Zhou, Rodney C. Ewing & Maik Lang (2016) “Probing disorder in isometric pyrochlore and related complex oxides” Nature Materials doi: 10.1038/nmat4581