Valence state of europium and samarium in Ln2Hf2O7 (Ln = Eu,Sm) based oxygen ion conductors

Ln₂(Hf_2-xLn_x)O_7-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 °C, respectively. According to the ESR data, the Eu cations on the Hf site in the Hf_1-xEu_xO₆ octahedra in pyrochlore Eu₂(Hf_2-xEu_x)O_7-x/2 (x = 0.1) are most readily oxidized and reduced. Oxidation at 840 °C for 24h in air reduces the total conductivity of the Ln₂(Hf_2-xLn_x)O_7-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 Ln²⁺ ions as a result of the oxidation. The anomalous low-frequency behavior of the permittivity of the Eu₂(Hf_2-xEu_x)O_7-x/2 (x = 0.1) at ~800 °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.