First-Principles Thermodynamics of Coherent Interfaces in Samarium-Doped Ceria Nanoscale Superlattices

Nanoscale superlattices of samarium-doped ceria layers with varying doping levels have been recently proposed as a novel fuel cell electrolyte. We calculate the equilibrium composition profile across the coherent $\{100\}$ interfaces present in this system using lattice-gas Monte Carlo simulations with long-range interactions determined from electrostatics and short-range interactions obtained from ab initio calculations. These simulations reveal the formation of a diffuse, nonmonotonic, and surprisingly wide (11 nm at 400 K) interface composition profile, despite the absence of space charge regions.

Figure 1

(a) Coupled-sublattice lattice-gas model describing the samarium-doped ceria system on the fluorite lattice. (b) Effective cluster interactions ($J\dots$).

Figure 2

Equilibrium composition profile. The solid line (with the ordinate on the left axis) gives $x$ in the formula ${\mathrm{Ce}}_x{\mathrm{Sm}}_{1-x}{\mathrm{O}}_y$, while the dotted line (with the ordinate on the right axis) gives $y$. The vertical scale of each curve is selected such that, when the two curves overlap, there is no net charge in that region in space. The profiles represent average compositions from 160 snapshots from the Monte Carlo simulations. The error bar represents $2\sigma$ confidence intervals for the mean composition (nearly identical for Ce and O on the scales of the curves). One lattice unit is $5.44\AA$. The horizontal range of the plot covers half the period of the superlattice.