  Using a combination of density functional theory calculations and statistical
mechanics, we show that a wide range of intermediate compositions of ceria -
zirconia solid solutions are thermodynamically metastable with respect to phase
separation into Ce-rich and Zr-rich oxides. We estimate that the maximum
equilibrium concentration of Zr in CeO2 at 1373 K is ~2%, and therefore
equilibrated samples with higher Zr content are expected to exhibit
heterogeneity at the atomic scale. We also demonstrate that in the vicinity of
the (111) surface, cation redistribution at high temperatures will occur with
significant Ce enrichment of the surface, which we attribute to the more
covalent character of Zr-O bonds compared to Ce-O bonds. Although the kinetic
barriers for cation diffusion normally prevent the decomposition/segregation of
ceria-zirconia solid solutions in typical catalytic applications, the
separation behaviour described here can be expected to occur in modern
three-way catalytic converters, where very high temperatures are reached.
