  Ceria has proven to be an excellent ion-transport and ion-exchange material
when used in polycrystalline form and with a high-concentration of aliovalent
doped cations. Despite its widespread application, the impact of atomic-scale
defects in this material are scarcely studied and poorly understood. In this
article, using first-principles simulations, we provide a fundamental
understanding of the atomic-structure, thermodynamic stability and electronic
properties of undoped grain-boundaries (GBs) and alkaline-earth metal (AEM)
doped GBs in ceria. Using density-functional theory simulations, with a GGA+U
functional, we find the $\Sigma$3 (111)/[$\bar{1}$01] GB is thermodynamically
more stable than the $\Sigma$3 (121)/[$\bar{1}$01] GB due to the larger atomic
coherency in the $\Sigma$3 (111)/[$\bar{1}$01] GB plane. We dope the GBs with
$\sim$20% [M]$_{GB}$ (M=Be, Mg, Ca, Sr, and Ba) and find that the GB energies
have a parabolic dependence on the size of solutes, the interfacial strain and
the packing density of the GB. We see a stabilization of the GBs upon Ca, Sr
and Ba doping whereas Be and Mg render them thermodynamically unstable. The
electronic density of states reveal that no defect states are present in or
above the band gap of the AEM doped ceria, which is highly conducive to
maintain low electronic mobility in this ionic conductor. The electronic
properties, unlike the thermodynamic stability, exhibit complex
inter-dependence on the structure and chemistry of the host and the solutes.
This work makes advances in the atomic-scale understanding of aliovalent cation
doped ceria GBs serving as an anchor to future studies that can focus on
understanding and improving ionic-transport.
