New defect-crystal-chemical approach to non-Vegardianity and complex defect structure of fluorite-based MO 2–LnO 1.5 solid solutions (M 4+ = Ce, Th; Ln 3+ = lanthanide) : Part II: Detailed local-structure and ionic-conductivity analysis

Abstract

In Part I a new lattice-parameter ( a 0(ss)) model for the entitled so-called defect-fluorite solid solutions (ss) was proposed; this clarified their markedly non-Vegardian a 0(ss) behavior as a macroscopic manifestation of their microscopic defect (V O)-related local structure, in particular the non-random oxygen coordination around the cations of CN(Ln 3+) ≠ CN(M 4+). In this Part II the proposed model was further applied to analyse their controversial detailed local-structure and ionic-conductivity behavior. The combination of the CN(Ln 3+, M 4+) data with ‘restricted (non-)randomness’ of defect-fluorite oxides can provide almost a full set of their mutually non-random cation (M 4+, Ln 3+) ↔ anion (O 2−, V O) coordination-structure data, such as oxygen 8, 7 and 6-fold coordinated cation (M 4+, Ln 3+) concentrations and dopant (Ln 3+) n(= 0–4)-fold coordinated anion (V O, O 2−) concentrations. This can also provide a new consistent description of ionic-conductivity maximum behavior in low- y range as a multiple V O ↔ O 2− site-exchange process in and between differently dopant (Ln 3+)-associated anion sites. Thus, together with the a 0(ss)-analysis results obtained in Part I, the present model is proposed as a new quantitative a 0(ss) model for these defect-fluorite oxides and as a new direct link to their complex defect (local) structure. Possible further elaborations and application extensions of the present model in the future are also discussed both from the basic theoretical and material sides.