Origins of radiation tolerance in complex materials

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Figure 6. Schematic highlights the relationship between the energetics of disordering and amorphization resistance as a function of the cation structure of the derivative compound, spinel or pyrochlore. In spinels that have cation vacancies relative to the basic rocksalt structure, amorphization resistance is proportional to the difficulty to disorder the compound. The kinetics of reordering, facilitated by the cation vacancies, is faster as the disordered phase becomes less favorable. These kinetics are absent in fluorite-­‐derivative compounds that have the same cation density as the basic fluorite structure. Energy builds up faster in compounds that have higher energies to disorder. These compounds are thus less resistant to amorphization.

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Knowing what makes some complex materials radiation tolerant is important for improving nuclear fuels and nuclear waste storage. A report in the journal Nature Communications provides new insight into the matter, generalizing the previous understanding and providing a new path for developing radiation-tolerant materials for use in extreme environments.

The research aims to understand at a fundamental level: 1) how materials respond to being irradiated, and 2) how that response depends on fundamental properties of the material, such as its crystal structure and crystal chemistry. The researchers examined oxide ceramics that have potential application for storing nuclear waste and the development of advanced nuclear fuels. The investigators discovered that fundamental differences in the structure of the material play a key role in how those materials respond to irradiation.

The relative ease in swapping cations is key to radiation tolerance. Using a combination of experimental characterization and a variety of simulation techniques, the Los Alamos researchers determined that different types of complex oxides have a fundamentally different response to irradiation. Decades of work on one class of complex oxides, pyrochlore (A2B2O7), have revealed that there are correlations between the ability of the cations (A and B) to swap position and the radiation tolerance of the material. Pyrochlores in which A and B can easily be swapped (such as when A=Er and B=Zr) are radiation tolerant.

In contrast, for pyrochlores in which this swapping is difficult (A=Er and B=Ti), the material quickly amorphizes under irradiation. This transformation from a crystalline to an amorphous structure is inherently bad for performance because it is accompanied by changes in volume and the ability of other species to leach from the material.

However, different oxides produced different results. The researchers examined another class of complex oxides, spinels (AB2O4), to determine if the same relationship holds. They irradiated three different chemistries of spinels and examined how the spinels responded to the irradiation damage.

The team found the opposite behavior from what was observed in pyrochlores: if the A and B cations are hard to swap in spinels, the material is more, not less, radiation tolerant. The investigators used atomistic simulation methods to examine these results. They attribute this difference to the fundamental manner in which the A and B atoms are arranged. There are holes in the cation structure in spinels that facilitate rearrangement back to the ordered structure. These holes do not exist in pyrochlore. Once the cations swap in pyrochlore, they are stuck there. Due to the holes, spinels can rearrange easily to recover the original structure. Thus, the fundamental difference is related to the holes – cation structural vacancies – that exist in one material but not the other.

The new insight from combined experimental and modeling effort generalizes the understanding that had been developed from the study of pyrochlores and provides new opportunities for identifying radiation-tolerant oxides for nuclear applications.

Reference: “Opposite Correlations between Cation Disordering and Amorphization Resistance in Spinels versus Pyrochlores,” Nature Communications 6, 8750 (2015); doi:10.1038/ncomms9750. Authors: Blas Uberuaga, Ming Tang, James Valdez, and Yongqiang Wang (Materials Science in Radiation and Dynamics Extremes, MST-8); and collaborators with Thermo-Calc Software Inc., Loughborough University, and the University of Tennessee.

The DOE Office of Science (Basic Energy Sciences, Materials Sciences and Engineering Division) funded this work, which supports the Laboratory’s Energy Security mission area and Materials for the Future science pillar. Researchers performed ion irradiations at the Ion Beam Materials Laboratory at Los Alamos, a DOE user resource supported by the DOE Office of Basic Energy Sciences and Office of Nuclear Energy, Laboratory Directed Research and Development (LDRD), and University of California lab-fee research. Technical contact: Blas Uberuaga