Dopant solubility in ceria: alloy thermodynamics combined with the DFT+U calculations

Abstract

Tb-doped CeO 2 (ceria) is a promising mixed conductor for oxygen permeation membranes and reversible oxygen sorbents. To predict solubility of Tb ions in ceria for a wide range of concentrations, density functional theory (DFT+U) calculations with two different values of Hubbard U-parameter on Tb and Ce ions were combined with alloy thermodynamics and the Concentration Wave approach. It is shown that, to predict properties of disordered solid solutions at finite temperatures, the energy parameters in the mixing energies can be extracted from the DFT+U calculations performed at T=0 K for two ordered configurations of the dopant in the supercells. The unlimited solubility of Tb 4+ in CeO 2 in the quasi-binary cross-section CeO 2-TbO 2 is predicted in the temperature range where both stoichiometric TbO 2 and CeO 2 reveal fluoride structures (above 700 o C).

First principles calculations on CeO2 doped with Tb3+ ions

“…S3). Actually, the occupation of Fermi energy by O 2p states is even more pronounced for an increased Tb concentration in CeO2 [7]. In the electrical conductivity measurements [5] the p-type conductivity was identified at Tb concentration around 50% .Tb2O3 has an additional band3 at -4.25 eV ( fig.…”

“…In contrast, the Tb impurity can be in both 3+ and 4+ oxidation states and charge compensation by oxygen vacancies is not here so obvious. Another difference is that the Gd 3+ shows a limited solubility in ceria [6] unlike unlimited solubility of Tb 4+ [7]. Large electronic conductivity due to a mixed valence state of Tb impurity makes enhanced ceria performance in mixed conductive membranes for oxygen separation [8,9].…”