A comparative study of the high-pressure structural stability of zirconolite materials for nuclear waste immobilisation

TL;DR

This study compares the high-pressure structural stability of various zirconolite materials used for nuclear waste immobilisation, revealing a new triclinic structure for zirconolite-2M and phase transition behaviors under pressure.

Contribution

It introduces a new triclinic structure for zirconolite-2M and provides the first high-pressure analysis of zirconolite-4M, 3O, and 3T, expanding understanding of their stability.

Findings

Zirconolite-2M has a new triclinic structure (zirconolite-2TR).

Zirconolite-2TR undergoes a phase transition at 14.7 GPa.

No pressure-induced phase transitions observed in 4M, 3O, and 3T.

Abstract

We present a comparative study of the high-pressure behaviours of the nuclear waste immobilisation materials zirconolite-2M, -4M, -3O, and -3T. The materials are studied under high-pressure conditions using synchrotron powder X-ray diffraction. For zirconolite-2M we also performed density-functional theory calculations. A new triclinic crystal structure (space group P-1), instead of the previously assigned monoclinic structure (space group C2/c) is proposed for zirconolite-2M. We named the triclinic structure as zirconolite-2TR. We also found that zirconolite-2TR undergoes a phase transition at 14.7 GPa to a monoclinic structure described by space group C2/c, which is different than the high-pressure structure previously proposed in the literature. These results are discussed in comparison with previous studies on zirconolite-2M and the related compound calzirtite. For the other three zirconolite structures (4M, 3O, and 3T) this is the first high-pressure study, and we find no evidence for pressure induced phase transitions in any of them. The linear compressibility of the studied compounds, as well as a room-temperature pressure-volume equation of state, are also presented and discussed.