Lu‑, Sm‑, and Gd-Doped Ceria: A Comparative Approach to Their Structural Properties

A room temperature
structural study has been performed through the whole compositional
range of the (Ce1–xLux)­O2–x/2 system
by synchrotron X-ray diffraction and μ-Raman spectroscopy. Samples
were synthesized by thermal treatment in air at 1373 K of coprecipitated
mixed oxalates. A CeO2-based solid solution with a fluorite-type
structure (F) was found to be stable up to x = 0.4,
while at higher Lu content a (F + C) biphasic region was observed,
with C being the cubic atomic arrangement typical of sesquioxides
of the heaviest rare earths. A comparative approach including also
results deriving from other (Ce1–xREx)­O2–x/2 systems (RE  Gd and Sm) allowed us to conclude that
the compositional extent of the F solid solution is a complex function
of RE3+ size and RE compressibility. On this basis, the
dependence of ionic conductivity on the RE identity was interpreted
as related both to the Ce4+/RE3+ size closeness
and to RE compressibility. Ce4+/RE3+ dimensional
issues were also revealed to rule the appearance of the hybrid structure
observed in the two aforementioned systems, consisting of the intimate
intergrowth of C microdomains within the F-based host lattice. Moreover,
a more extended definition of F-based solid solution, including also
the hybrid structure, is formulated; the latter is meant as a modification
of the former, occurring when mainly RE–vacancy aggregates
are incorporated into the host lattice in spite of isolated RE ions.
By μ-Raman spectroscopy it was possible to demonstrate that
the mechanism of oxygen vacancy formation is common to all the systems
studied, provided that the structure of the F-based solid solution,
also including the hybrid structure, is retained.