New defect-crystal-chemical approach to non-Vegardianity and complex defect structure of fluorite-based MO2–LnO1.5 solid solutions (M4+ = Ce, Th; Ln3+ = lanthanide) part I: Model description and lattice-parameter data analysis

Lattice-parameter (a0(ss)) data of defect-fluorite M1 − yLnyO2 − y/2 solid solutions (ss) (M4+ = Ce, Th; Ln3+ = Y, Gd, Eu, Sm, Nd, La) exhibit markedly positive deviations from Vegardʹs law, systematically larger in M4+ = Th than Ce. A detailed analysis elaborates upon a previous a0(ss) model [1,2] and clarifies the ‘generalized non-Vegardian’ behavior of oxygen-vacancy (VO) type ‘distortionally dilated’ MO2–LnO1.5 solid solutions with non-random oxygen coordination around the cations of CN(Ln3+) ≠ CN(M4+). M4+ = Ce and Th are similarly modestly Ln3+–VO associative (CN(Ln3+) < CN(M4+)) for the smallest Ln3+ = Y and Gd, and shift to completely opposite non-randomness for the largest Ln3+ = Nd and La, being strongly Ce4+–VO associative (CN(Ce4+) ≪ CN(Ln3+)) with the former and extremely Ln3+–VO associative (CN(Ln3+) ≪ CN(Th4+)) with the latter. Because of the compatibility with the ion-packing model this can also be used to provide their metal–oxygen bond-length data. As a first paper in a series the author will describe random → non-random model extension and major a0(ss)-analysis results that include a comparison with the reported local-structure, thermodynamic and ionic-conductivity data from these systems. Several basic concepts and premises of the present model are also discussed, paving the way to a detailed local-structure analysis of the systems in Part II.