Fluorite structural principles: disordered α-phase to ordered intermediate phases in praseodymia

Experimental observations of the structural changes effected during rapid cooling of alpha-phase praseodymium oxide of various compositions in their ambient oxygen pressure have been reported. These results are discussed in terms of the recently discovered structural principles for the anion-deficient, fluorite-related rare earth higher oxides. Two new polymorphs of the beta-phase of praseodymia were found in the experiments, and their structures are now predicted using the new structural principles. Furthermore, the structure of one of two new pi-phase polymorphs, PrnO2n-2m, that were discovered, pi(1) R16O30 (n=16, m=1), RO1.88, is predicted. The transformation of the established beta(1)-phase in the praseodymium oxides to the newly observed beta(3)-phase, found in the quenching experiments, is modeled. Finally the surfaces of beta(1)- and the alpha-phase are compared. It is observed that the superstructure of beta(1) is clearly reflected in its surface structure. (C) 1998 Elsevier Science S.A.