Polymorphism and high-temperature conductivity of Ln2M2O7 (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 2 O 3 ─ MO 2 systems. In the range ~ 600 ─ 1200 °C, Ln 2 Ti 2 O 7 (Ln = Sm ─ Lu) and Ln 2 Zr 2 O 7 (Ln = Sm ─ Gd) undergo ordering transitions, F ⁎ → P I → P , which culminate in the formation of an ideal pyrochlore structure, P , existing between 1100 and 1300 °C. Above 1300 °C, Ln 2 Ti 2 O 7 (Ln = Gd ─ Lu), Ln 2 Zr 2 O 7 (Ln = Sm ─ Gd) and Ln 2 Hf 2 O 7 (Ln = Eu ─ Tb) exist as oxygen-ion-conducting phases, P II, 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 2 M 2 O 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 2 Ti 2 O 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 2 Ti 2 O 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.