Valence State of Europium and Samarium in Ln2Hf2O7 (ln = Eu, Sm) Based Oxygen Ion Conductors

Ln(2)(Hf(2-x)Ln(x))O7-x/2 (Ln = Sm, Eu; x = 0.1) pyrochlores have been prepared via mechanical activation of oxide mixtures, followed by heat treatment for 4h at 1450 and 1600 degrees C, respectively. According to the ESR data, the Eu cations on the Hf site in the Hf1-xEuxO6 octahedra in pyrochlore Eu-2(Hf2-xEux)O7-x/2 (x = 0.1) are most readily oxidized and reduced. Oxidation at 840 degrees C for 24h in air reduces the total conductivity of the Ln(2)(Hf(2-x)Ln(x))O7-x/2 (Ln = Sm, Eu; x = 0.1) by a factor of 2.5-6, due to the decrease in the concentrations of oxygen vacancies and Ln(2+) ions as a result of the oxidation. The anomalous low-frequency behavior of the permittivity of the Eu-2(Hf2-xEux)O7-x/2 (x = 0.1) at similar to 800 degrees C can be understood in terms of the changes in the oxygen sublattice of the pyrochlore structure as a result of the oxidation of divalent europium and partial filling of oxygen vacancies at this temperature.