Defect fluorite-type structures: modelling triclinic lanthanoid oxides*

Abstract

The co-ordination defect (c.d.) theory of fluorite-related structures has been compared and contrasted with the recently published phenomenological modular model. It is confirmed that the integrity and topology of the co-ordinated defect play a pivotal role in determining the structures of the higher lanthanoid oxides and that the c.d. diagrams derived from topological analysis enable both known and unknown model structures to be predicted. The predictive limitations of the phenomenological modular model are exposed and it is shown that fluorite modular sequences derived therefrom are already encapsulated in the {100}_F c.d. diagrams for the M_nO_2n–2 oxide phases. A new set of modules is proposed that is not only compositionally identical to that previously reported but which also possesses the cardinal advantage of incorporating the structure-determining topology of the co-ordination defect.