Configurational Thermodynamics of the CeO2-Gd2O3 System : A Combined DFT, Cluster Expansion and Monte Carlo Approach to Bulk and Interfaces

Abstract: In this thesis, we study the configurational thermodynamics of Ce1-xGdxO2-x/2 x ≤ 1 or CGO. We apply a combined Density Functional Theory (DFT), cluster expansion and Monte Carlo (MC) approach in which the configurational energy of CGO is described by means of the Ising-type Hamiltonian. The interactions are determined by the cluster expansion of total energies calculated with DFT for a set of various cation–anion configurations. This allows one to perform on- the-fly calculations of the configurational energies in MC simulations of cation and anion ordering.The cluster interactions are essentially electrostatic and long-range, and describe the configurational energetics in the entire range of concentrations rather well.The phase diagram obtained in the MC simulations allows one to rationalise existing experimental data and is largely in agreement with that. We observe the phase separation into pure oxides, in equilibrium, below ca. 1000 K for x ≤ 1 (however it is kinetically hindered as diffusion of cations below ca. 1500 K is slow). We also observe the C-type oxygen–vacancy ordering in the x = 0.3–1 range below ca. 1200–3300 K (C phase), and a largely disordered, fluorite (F) phase in the x ≤ 0.3 range and above the ordering temperature.The DFT supercell calculations of the F and C phase configurations obtained in MC simulations allow us to study the effect of concentration and ordering on the lattice relaxations, lattice parameter and elastic moduli, providing insights into relation between preparation conditions, structure and properties.The bulk cluster interactions appear to be applicable also to coherent CeO2/C- type Gd2O3 interfaces, hence we examine a configurational energy landscape of the oxygen vacancy migration therein.This combined approach can be applied to study configurational thermodynamics of similar materials as well as the influence of configurational state on ionic conductivity and other properties.