Abstract

Lattice-parameter (a₀(ss)) data of defect-fluorite M_1 − yLn_yO_2 − y/2 solid solutions (ss) (M⁴⁺ = Ce, Th; Ln³⁺ = Y, Gd, Eu, Sm, Nd, La) exhibit markedly positive deviations from Vegard's 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 (V_O) type ‘distortionally dilated’ MO₂–LnO_1.5 solid solutions with non-random oxygen coordination around the cations of CN(Ln³⁺) ≠ CN(M⁴⁺). M⁴⁺ = Ce and Th are similarly modestly Ln³⁺–V_O 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⁺–V_O associative (CN(Ce⁴⁺)  CN(Ln³⁺)) with the former and extremely Ln³⁺–V_O 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.