Polymorphism and high-temperature conductivity of Ln 2M 2O 7 (Ln = Sm ─Lu; M = Ti, Zr, Hf) pyrochlores

Abstract

We present a comparative analysis of the order ─disorder transitions in Ln 2(M 2 ─ x Ln x )O 7 ─ δ (Ln = Sm ─Lu; M = Ti, Zr, Hf; x = 0, 0.096) pyrochlore-like compounds and solid solutions existing in the Ln 2O 3 ─MO 2 systems. In the range ~ 600 ─1200 °C, Ln 2Ti 2O 7 (Ln = Sm ─Lu) and Ln 2Zr 2O 7 (Ln = Sm ─Gd) undergo ordering transitions, F⁎ → PI → P, which culminate in the formation of an ideal pyrochlore structure, P, existing between 1100 and 1300 °C. Above 1300 °C, Ln 2Ti 2O 7 (Ln = Gd ─Lu), Ln 2Zr 2O 7 (Ln = Sm ─Gd) and Ln 2Hf 2O 7 (Ln = Eu ─Tb) exist as oxygen-ion-conducting phases, PII, disordered in both the oxygen and cation sublattices. Ionic conductivity data for Ln 2(M 2 ─ x Ln x )O 7 ─ δ (Ln = Sm ─Lu; M = Ti, Zr, Hf; x = 0, 0.096) synthesized at 1600-1670 °C indicate that the highest conductivity in these systems is typically offered by nominally stoichiometric (Ln:M = 1:1), disordered Ln 2M 2O 7 (Ln = Sm ─Lu; M = Ti, Zr, Hf) pyrochlores containing anti-structure pairs (Ln M ' + M Ln •) and oxygen vacancies ( V O ••) on the 48 f (O2) site. The highest conductivity of Yb 2Ti 2O 7, in which the cations have the smallest radii among the lanthanides and Group IVa metals, seems to be due to the increased role of the geometric factor in the Ln 2Ti 2O 7 (Ln = Sm-Lu) pyrochlores with predominantly covalent metal ─oxygen bonding M-O (Ti-O). The ion transport parameters in these materials are determined primarily by the relationship between the sizes of the mobile oxygen ions and conduction channels.