The tolerance factor is a good criterion to understand the structural transitions in LnCaScMO (Ln = La, Sm, Ho, Yb; M = Nb, Ta; x = 0, 0.05, 0.1). Decreasing the Ln ionic radius in LnScNb(Ta)O leads to a morphotropic transition from a pyrochlore to a fluorite-like structure. Ca-doping leads to a pyrochlore-to-fluorite transition in LnCaScMO (Ln = La, Sm) and a fluorite-to-pyrochlore transition in HoCaScNbO. Proton contribution to the total conductivity was observed for LnCaScNb(Ta)O (Ln = La, Sm; x = 0, 0.05, 0.1) 3+/5+ pyrochlores and the maximum proton contribution was shown by SmCaScMO (M = Nb, Ta), which are located at the boundary between pyrochlores and fluorites (comparative study of electrical conduction and oxygen diffusion). Proton conduction of SmCaScNbO and SmCaScTaO pyrochlores persists up to 800 and 850 °C, respectively. The conductivity of fluorite-like HoCaScNbO (x = 0, 0.05) and YbScNbO 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 LnCaScNb(Ta)O (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.

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