On the Atomistic Interactions That Direct Ion Conductivity and Defect Segregation in the Bulk and Surface of Samarium-Doped Ceria: A Genetic Algorithm Study

Abstract

We study the (111) surface of 10.3%, 14.3%, and 18.5% samarium-doped ceria (SDC) using a genetic algorithm (GA) to search for the most energetically stable configurations. In all cases, both Sm ions and oxygen vacancies segregate to the surface, which is similar to experimental findings for 5.3% SDC. 1 Importantly, at the optimal doping level of SDC (∼11%), where conductivity is maximal, defect segregation is limited such that vacancies remain 6 Å apart and pairs of vacancies do not form. At higher concentrations, pairs of vacancies are present, which likely contributes to the observed decrease in ionic conductivity. We also investigate the low-energy bulk structure of SDC from 10.3% to 18.5%, at the DFT+U level of theory, which has not been previously reported. The DFT+U energetics allow us to gain further insight on the fundamental interactions that influence ionic conductivity and defect segregation, as well as to confirm the insight reported from classical simulations. 2 The low-energy configurations found by our GA search enable future studies of SDC at experimentally relevant concentrations and identify the important interactions at the bulk and surface of the fuel cell electrolyte.

Mechanisms of enhanced sulfur tolerance on samarium (Sm)-doped cerium oxide (CeO2) from first principles

“…Here a Sm dopant was located on the top surface, rather than on the subsurface since the Sm dopant tends to segregate on the CeO 2 surface, rather than migrate into the bulk. 25,26 The geometries were optimized until the forces were reduced below 0.05 eV Å À1 . A Hubbard-U term, U = 5 eV, was added to properly describe the localized nature of the 4f electrons of Ce.…”

Association of defects in doped non-stoichiometric ceria from first principles

“…24 A value of 5 eV for the 4f-orbitals of cerium was chosen according to earlier studies. 17,[25][26][27][28][29] In case of the dopants no U-parameter was necessary since the f-electrons were kept frozen in the core. 28,29 The lattice parameter of the unit cell was fixed at 5.49 Å according to our previous calculations.…”

“…17,[25][26][27][28][29] In case of the dopants no U-parameter was necessary since the f-electrons were kept frozen in the core. 28,29 The lattice parameter of the unit cell was fixed at 5.49 Å according to our previous calculations. 17 For Ce ÀCe  Ce distances were shortened in the initial structure by about 0.11 Å and 0.05 Å, respectively.…”

A combined DFT + U and Monte Carlo study on rare earth doped ceria

“…The same authors also found a maximum in ionic conductivity at approximately 20 atom% for SDC. 3 Although a lot of experimental [4][5][6] as well as theoretical research [7][8][9][10][11][12][13][14][15][16][17][18][19] was conducted in recent years, the exact reason for the maximum remains incompletely explained. One explanation for the decrease in conductivity at high rare earth fractions is the attraction of oxygen vacancies to the trivalent rare earth ions which leads to the trapping of oxygen vacancies and lowered oxygen ion mobility.…”