Evolution of Oxygen–Ion and Proton Conductivity in Ca-Doped Ln₂Zr₂O₇ (Ln = Sm, Gd), Located Near Pyrochlore–Fluorite Phase Boundary

• Main Authors: A.V. Shlyakhtina, J.C.C. Abrantes, E. Gomes, N.V. Lyskov, E.Yu. Konysheva, S.A. Chernyak, E.P. Kharitonova, O.K. Karyagina, I.V. Kolbanev, L.G. Shcherbakova
• Format: Article
• Language: English
• Published: MDPI AG 2019-08-01
• Series: Materials
• Subjects: pyrochlore; pyrochlore–fluorite morphotropic phase region; proton conductivity; oxygen—ion conductivity; grain-boundary conductivity; thermogravimetry; Rietveld refinement
• Online Access: https://www.mdpi.com/1996-1944/12/15/2452
• ISSN: 1996-1944

Sm_2−xCa_xZr₂O_7−x/2 (x = 0, 0.05, 0.1) and Gd_2−xCa_xZr₂O_7−x/2 (x = 0.05, 0.1) mixed oxides in a pyrochlore−fluorite morphotropic phase region were prepared via the mechanical activation of oxide mixtures, followed by annealing at 1600 °C. The structure of the solid solutions was studied by X-ray diffraction and refined by the Rietveld method, water content was determined by thermogravimetry (TG), their bulk and grain-boundary conductivity was determined by impedance spectroscopy in dry and wet air (100−900 °C), and their total conductivity was measured as a function of oxygen partial pressure in the temperature range: 700−950 °C. The Sm_2−xCa_xZr₂O_7−x/2 (x = 0.05, 0.1) pyrochlore solid solutions, lying near the morphotropic phase boundary, have proton conductivity contribution both in the grain bulk and on grain boundaries below 600 °C, and pure oxygen−ion conductivity above 700 °C. The 500 °C proton conductivity contribution of Sm_2−xCa_xZr₂O_7−x/2 (x = 0.05, 0.1) is ~ 1 × 10^−4 S/cm. The fluorite-like Gd_2−xCa_xZr₂O_7−x/2 (x = 0.1) solid solution has oxygen-ion bulk conductivity in entire temperature range studied, whereas proton transport contributes to its grain-boundary conductivity below 700 °C. As a result, of the morphotropic phase transition from pyrochlore Sm_2−xCa_xZr₂O_7−x/2 (x = 0.05, 0.1) to fluorite-like Gd_2−xCa_xZr₂O_7−x/2 (x = 0.05, 0.1), the bulk proton conductivity disappears and oxygen-ion conductivity decreases. The loss of bulk proton conductivity of Gd_2−xCa_xZr₂O_7−x/2 (x = 0.05, 0.1) can be associated with the fluorite structure formation. It is important to note that the degree of Ca substitution in such solid solutions (Ln_2−xCa_x)Zr₂O_{7−δ} (Ln = Sm, Gd) is low, x < 0.1. In both series, grain-boundary conductivity usually exceeds bulk conductivity. The high grain-boundary proton conductivity of Ln_2−xCa_xZr₂O_7−x/2 (Ln = Sm, Gd; x = 0.1) is attributable to the formation of an intergranular CaZrO₃-based cubic perovskite phase doped with Sm or Gd in Zr sublattice.