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

• Published in: Dalton transactions : an international journal of inorganic chemistry 2018-02, Vol.47 (7), p.2376-2392
• Main Authors: Shlyakhtina, A. V, Pigalskiy, K. S, Belov, D. A, Lyskov, N. V, Kharitonova, E. P, Kolbanev, I. V, Borunova, A. B, Karyagina, O. K, Sadovskaya, E. M, Sadykov, V. A, Eremeev, N. F
• Format: Article
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/c7dt03912c

The tolerance factor is a good criterion to understand the structural transitions in Ln 2− x Ca x ScMO 7−δ (Ln = La, Sm, Ho, Yb; M = Nb, Ta; x = 0, 0.05, 0.1). Decreasing the Ln ionic radius in Ln 2 ScNb(Ta)O 7 leads to a morphotropic transition from a pyrochlore to a fluorite-like structure. Ca 2+ -doping leads to a pyrochlore-to-fluorite transition in Ln 2− x Ca x ScMO 7−δ (Ln = La, Sm) and a fluorite-to-pyrochlore transition in Ho 2− x Ca x ScNbO 7−δ . Proton contribution to the total conductivity was observed for Ln 2− x Ca x ScNb(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 1.9 Ca 0.1 ScMO 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 1.9 Ca 0.1 ScNbO 6.95 and Sm 1.9 Ca 0.1 ScTaO 6.95 pyrochlores persists up to 800 and 850 °C, respectively. The conductivity of fluorite-like Ho 2− x Ca x ScNbO 7−δ ( x = 0, 0.05) and Yb 2 ScNbO 7 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− x Ca x ScNb(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. The tolerance factor is a good criterion to understand the structural transitions in Ln 2− x Ca x ScMO 7−δ (M = Nb, Ta).