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

A V Shlyakhtina; K S Pigalskiy; D A Belov; N V Lyskov; E P Kharitonova; I V Kolbanev; A B Borunova; O K Karyagina; E M Sadovskaya; V A Sadykov; N F Eremeev

doi:10.1039/c7dt03912c

Proton and oxygen ion conductivity in the pyrochlore/fluorite family of Ln_2-xCa_xScMO_7-δ (Ln = La, Sm, Ho, Yb; M = Nb, Ta; x = 0, 0.05, 0.1) niobates and tantalates

Dalton Trans. 2018 Feb 13;47(7):2376-2392. doi: 10.1039/c7dt03912c.

Authors

A V Shlyakhtina¹, K S Pigalskiy, D A Belov, N V Lyskov, E P Kharitonova, I V Kolbanev, A B Borunova, O K Karyagina, E M Sadovskaya, V A Sadykov, N F Eremeev

Affiliation

¹ Semenov Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina 4, Moscow, 119991 Russia. annashl@inbox.ru annash@chph.ras.ru.

PMID: 29372920
DOI: 10.1039/c7dt03912c

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.