A concord of chemistry and crystallography

Researchers have released a theoretical approach to explain inconsistencies between crystallographic and chemical experimental data in the apparent transformation of a pyrochlore to defect fluorite in La₂Zr₂O₇. Their model can be extended to understand the ageing of a large class of complex oxides, such as spinels, with practical applications ranging from solid oxide fuel cells to the design and management of nuclear waste forms.

Published in the journal Scientific Reports, the research was co-authored by Professor Gordon Thorogood, a nuclear fuel cycle researcher from the Australian Nuclear Science and Technology Organisation (ANSTO). He explained that the study focused on a phase change to defect fluorite structure — a phase change that may, in fact, not be occurring.

“Matrix mathematics suggested that a summation of peaks in the crystallographic data will make it look like the peaks have disappeared,” said Professor Thorogood.

The diffraction patters of La₂Zr₂O₇ are in fact due to the interference of scattering waves between shifted pyrochlore nano-domains. According to Professor Thorogood, “The possibility has been viewed with surprise but considerable interest, as the physical mechanism responsible for the formation of a defect fluorite structure remains unclear.”

Professor Thorogood and his colleagues looked for a change in local symmetry that should have occurred with respect to decreasing grain size. Fine pyrochlore grains were sintered at different temperatures to probe order at different length scales.

Comparison of electron energy loss spectroscopy (EELS) spectra collected near the lanthanum edge of the zirconium indicated that local symmetry in the zirconium did not vary with respect to grain size. The study showed the valence state of the zirconium did not vary in small-grain pyrochlores.

From group theory calculations, the team determined that the defect fluorite results from an ensemble average of different perfectly ordered domains of specific size. The defect fluorite structure is achieved without invoking any simultaneous disordering of anions and cations at the atomic level and the result of intricate disorder due to a random distribution of fully ordered nano-domains. This removes the need for a new phase at the atomic scale.

Defect fluorite is similar to the mineral fluorite, with a single cation and anion site. The random mixing of oxygen and vacancies on a single site reduces the pyrochlore unit cell.

Understanding how damage occurs in a fluorite stricture is of interest in the nuclear fuel cycle, because fluorite is the stricture of uranium dioxide (UO₂) — the most common nuclear fuel. Radiation damage is thought to cause the change from pyrochlore to defect fluorite structure.

“With a phase change from pyrochlore to defect fluorite, you also have a volume change. But if that is not occurring, you need to understand why,” said Professor Thorogood.

“We would like to prevent swelling caused by damage to the fluorite matrices in nuclear fuel.

“Now that we have determined the position of the oxygen, we can move to radiation-induced damage studies.”