Relationship between the Structure and Transport Properties in the Ce_1–xLa_xO_2–x/2 System

Abstract

La-doped CeO 2 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 Ce 1−x La x O 2−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.

Oxygen Vacancy Distribution in Yttrium-Doped Ceria from 89Y–89Y Correlations via Dynamic Nuclear Polarization Solid-State NMR

“…Our results point toward a gradual transition from distorted fluorite to distorted C-type structure . This is in agreement with the continuous bulk model, which does not exclude superstructural motifs, observed in the diffraction data of the highly doped sample (Figure S1).…”

Reversible Phase Transformations in Novel Ce‐Substituted Perovskite Oxide Composites for Solar Thermochemical Redox Splitting of CO2

“…The pyrochlore Ce 2+ x La 2‐ x O 7‐ δ ( Fd‐3m ) phase is known to act as a mixed oxide and electronic conductor in reducing atmospheres [ 37 ] and can possibly serve as a buffer layer to suppress cation interdiffusion. [ 38,39 ] This renders the phase very attractive for thermochemical CO 2 splitting, so that we proceeded with detailed structural analyses and performance tests.…”

“…[ 39,47 ] However, the crystal structure of these materials still remains controversial, and two models based on disordered defect fluorite ( Fm‐3m ) and pyrochlore ( Fd‐3m ) type are mainly considered, depending on the concentration of B‐site cations and the ionic radii ratio of the A and B cations ( r A / r B ). [ 37,48,49 ] The general composition of the identified pyrochlore phase is Ce 1‐ x La x O 2− x /2 (where x = 0.35, 0.40) with La 3+ at the A‐site and Ce 4+ at the B‐site and r(La 3+ )/r(Ce 4+ ) = 1.3, resulting in a large lattice distortion. Neutron diffraction experiments of the LSC25M75 composite revealed the presence of a disordered pyrochlore Ce 2.6 La 1.4 O 7‐ δ ( Fd‐3m ) structure, which is analogous to a 2 × 2 × 2 superstructure of the disordered defect fluorite Ce 0.65 La 0.35 O 2‐ δ ( Fm‐3m ) type with 1/8 of the oxygen positions on the 8a site being vacant.…”

“…According to the literature, the Ce 0.60 La 0.40 O 2‐ δ phase is known to act as a buffer layer to prevent cation interdiffusion between perovskite phase particles during high temperature reaction conditions, by selectively permitting oxygen ion migration. [ 37,50 ] Furthermore, these pyrochlore ( Fd‐3m ) phases are widely used as a high temperature resistant materials and can possibly act as anti‐sintering agents. [ 38,39 ] Sintering of particles can exert a detrimental effect on the CO 2 splitting ability of materials during long term thermochemical redox cycling experiments.…”

Tuning of Shape, Defects, and Disorder in Lanthanum-Doped Ceria Nanoparticles: Implications for High-Temperature Catalysis

“…4,5 Doping with lanthanum has been less explored because the larger difference in ionic radius (La 3+ , 1.16 Å) leads to higher lattice distortions in the fluorite structure and a lower increase in conductivity as compared to Gd and Sm. 6,7 Recently, ceria−lanthana solid solutions (La 2 Ce 2 O 7 ) have attracted great attention as oxide catalysts for oxidative coupling of methane (OCM) reactions. 8,9 Lanthana has a very high solubility in the ceria fluorite structure, and more than half of Ce atoms can be replaced by La still preserving the fluorite structure.…”