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

Abstract

We present a comparative study of the high-pressure behaviours of the nuclear waste immobilisation materials zirconolite-2 M, −4M, –3O, and −3T. The materials are studied under high-pressure conditions using synchrotron powder X-ray diffraction. For zirconolite-2 M we also performed density-functional theory calculations. A new triclinic crystal structure (space group P1¯), instead of the previously assigned monoclinic structure (space group C2/c) is proposed for zirconolite-2 M. 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-2 M and the related compound calzirtite. For the other three zirconolite structures (4 M, 3O, and 3 T) 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.