Hydrothermal synthesis and properties of Ce_1-xM_xO_2-δ (M= La, Bi, Sm, Pr, Tb) solid solutions

A systematic study of hydrothermally prepared Ce_1-xM_xO_2-δ (M= Sm, Bi, Pr, Tb; x= 0-0.30) solid solutions, promising materials for application in solid oxide fuel cells and oxygen membranes is presented. Ultrafine particles of uniform crystallite dimension, ∼ 20 nm can be formed in 30 min. under hydrothermal conditions (260°C, 10 MPa). The small particle size (20-50 nm) of the hydrothermally prepared materials allows sintering of the samples into highly dense ceramics at 900-1350°C, significantly lower temperatures than 1600-1650°C required for samples prepared by solid state techniques. The solubility limit of Bi₂O₃ in CeO₂ was determined to be around 20 mol. %. The maximum conductivity, σ_600°C ∼ 4.4×10^-3 S/cm with E_a= 1.01 eV, and σ600_°C= 5.7×10^-3S/cm with E_a≈ 0.9 eV was found at x= 0.20 and x= 0.17 for Bi and Sm, respectively. In the Ce-Pr/Tb oxide systems, in addition to the high oxide ion conductivity, electronic conductivity occurs through the hopping of small polarons by a thermally activated mechanism (electron hopping from the Pr³⁺/Tb³⁺ to a neighboring Pr⁴⁺/Tb⁴⁺ ion).