La-doped CeO2 materials have been widely investigated for potential
applications in different high-temperature electrochemical devices, such as fuel cells and
ceramic membranes for hydrogen production. However, the crystal structure is still
controversial, and different models based on fluorite, pyrochlore, and/or type-C
structures have been considered, depending on the lanthanum content and synthesis
method used. In this work, an exhaustive structural analysis of the Ce1−xLaxO2−x/2 system
(0.2 < x ≤ 0.7) is performed with different techniques. The average crystal structure,
studied by conventional X-ray diffraction, could be considered to be a disordered fluorite;
however, the local structure, examined by electron diffraction and Raman spectroscopy,
reveals a biphasic mixture of fluorite and C-type phases. The thermal and electrical
properties demonstrate that the materials with x ≥ 0.4 are oxide ion proton conductors in
an oxidizing atmosphere and mixed ionic electronic conductors in a reducing
atmosphere. The water uptake and proton conductivity increase gradually with the increase in La content, suggesting that
the formation of the C-type phase is responsible for the proton conduction in these materials.