Proton and oxygen ion conductivity in the pyrochlore/fluorite family of Ln2−xCaxScMO7−δ (Ln = La, Sm, Ho, Yb; M = Nb, Ta; x = 0, 0.05, 0.1) niobates and tantalates

Abstract

The tolerance factor is a good criterion to understand the structural transitions in Ln_2−xCa_xScMO_7−δ (Ln = La, Sm, Ho, Yb; M = Nb, Ta; x = 0, 0.05, 0.1). Decreasing the Ln ionic radius in Ln₂ScNb(Ta)O₇ leads to a morphotropic transition from a pyrochlore to a fluorite-like structure. Ca²⁺-doping leads to a pyrochlore-to-fluorite transition in Ln_2−xCa_xScMO_7−δ (Ln = La, Sm) and a fluorite-to-pyrochlore transition in Ho_2−xCa_xScNbO_7−δ. Proton contribution to the total conductivity was observed for Ln_2−xCa_xScNb(Ta)O_7−δ (Ln = La, Sm; x = 0, 0.05, 0.1) 3+/5+ pyrochlores and the maximum proton contribution was shown by Sm_1.9Ca_0.1ScMO_6.95 (M = Nb, Ta), which are located at the boundary between pyrochlores and fluorites (comparative study of electrical conduction and oxygen diffusion). Proton conduction of Sm_1.9Ca_0.1ScNbO_6.95 and Sm_1.9Ca_0.1ScTaO_6.95 pyrochlores persists up to 800 and 850 °C, respectively. The conductivity of fluorite-like Ho_2−xCa_xScNbO_7−δ (x = 0, 0.05) and Yb₂ScNbO₇ is dominated by the oxygen ion transport, in accordance with their energy activation values 1.09–1.19 eV. The dielectric permittivity and TG studies were used for the investigation of oxygen vacancy dynamics and water incorporation into the Ln_2−xCa_xScNb(Ta)O_7−δ (Ln = La, Sm, Ho, Yb; x = 0, 0.05, 0.1) lattice. It is shown that oxygen vacancy-related dielectric relaxation in the range of 550–650 °C (ambient air), typical of pyrochlores and fluorites with pure oxygen ion conductivity, decreases and disappears for proton-conducting oxides.