First principles calculations on order and disorder in La₂Ce₂O₇ and Nd₂Ce₂O₇

In this paper, we highlight the connection between the local structure and collective dynamics of the defective fluorites La₂Ce₂O₇ and Nd₂Ce₂O₇. The local and average structure is explored by investigating a large number of different structural models and snapshots from Born-Oppenheimer Molecular dynamics calculations. Both compounds show a strong preference for local oxygen vacancy order similar to that found in the C-type structure. This suggests that previous studies, where Nd₂Ce₂O₇ and La₂Ce₂O₇ are viewed as disordered defective fluorites, or as a pyrochlore for the latter, did not capture the nature of local order in the disordered phase. We observe more collective chains of migrating oxygen in Nd₂Ce₂O₇- a manifestation of a stronger preference for a dynamic local oxygen vacancy order - than in La₂Ce₂O₇. The stronger preference for 〈210〉 vacancy-vacancy alignments can explain why long range ordering is identified by distinct C-type like superlattice peaks in neutron diffraction patterns for Nd₂Ce₂O₇ whereas they appear to be almost invisible in La₂Ce₂O₇.