Effects of the synthesis procedure, doping and non-stoichiometry on the order–disorder transformation in Ln 2Ti 2O 7 (Ln = Tm–Lu) oxygen-ion conductors

Abstract

The ionic conductivity of the Ln 2Ti 2O 7 (Ln = Tm–Lu) pyrochlores is found to correlate with the degree of anti-site cation disordering in these materials. The highest ionic conductivity is exhibited by Ln 2Ti 2O 7 containing 3.53–4.5% Ln Ti + Ti Ln anti-structure pairs. This degree of cation disordering can be achieved by heat treatment just below the melting point of the material, by mechanical activation with subsequent high-temperature firing and by doping with small amounts of cations of different sizes (as, e.g., in the case of (Yb 0.91Sc 0.09) 2Ti 2O 7). The Ln Ti + Ti Ln anti-structure pairs seem to interact with oxygen vacancies. The defect structure of Lu 2+ x Ti 2− x O 7− x/2 (34.5–44 mol% Lu 2O 3) oxygen-ion conductors is investigated. The materials with x = 0.052 (34.5 mol% Lu 2O 3); 0.096 (35.5 mol% Lu 2O 3); 0.286 (40 mol% Lu 2O 3) contain the whole one type of anti-site defect, Lu Ti, and possess an ionic conductivity of about 10 − 3 S/cm at 740 °C. The conductivity of the materials containing Lu Ti defects is lower than that of the stoichiometric materials containing Lu Ti + Ti Lu anti-structure pairs. A strong correlation is found between the variable positional parameter X of O(2) (position 48f) in the structure of Lu 2+ x Ti 2− x O 7− x/2 ( x = 0, 0.052, 0.096, 0.286) nanoceramics and the I 311/ I 222 and I 331/ I 222 intensity ratios in their X-ray diffraction patterns.