Oxide-ion diffusion in brownmillerite-type Ca₂AlMnO₅₊ from first-principles calculations

Area of Science:

• Materials Science
• Solid-State Chemistry
• Computational Materials Science

Background:

• Brownmillerite oxides are promising materials for various applications due to their ionic conductivity.
• Understanding oxide-ion diffusion is crucial for optimizing their performance in devices like solid oxide fuel cells.
• Ca2AlMnO5 and Ca2AlMnO5.5 exhibit reversible transformations via oxidation and reduction, influencing their diffusion characteristics.

Purpose of the Study:

• To systematically investigate oxide-ion diffusion pathways in brownmillerite oxides Ca2AlMnO5 and Ca2AlMnO5.5.
• To determine the migration energy barriers for oxide-ion movement in both reduced and oxidized phases.
• To elucidate the dimensionality of oxide-ion diffusion in these materials.

Main Methods:

• First-principles calculations were employed to model the oxide-ion diffusion.
• The nudged elastic band (NEB) method was utilized to determine migration energy barriers.
• Analysis focused on identifying preferred diffusion pathways and their associated energy costs.

Main Results:

• In the reduced Ca2AlMnO5, the lowest migration energy barrier (0.58 eV) is associated with vacancy-channel diffusion (1D).
• The oxidized Ca2AlMnO5.5 shows lower barriers (0.55 eV and 0.56 eV) for diffusion within AlO6 layers ([100] and [001] directions), indicating 2D diffusion.
• Diffusion perpendicular to the AlO6 layer in the oxidized phase (1.33 eV) is significantly hindered, suggesting limited [010] direction mobility.

Conclusions:

• Oxide-ion diffusion in Ca2AlMnO5 is predominantly one-dimensional along vacancy channels.
• Oxide-ion diffusion in Ca2AlMnO5.5 is two-dimensional within the AlO6 layers.
• The contrasting diffusion mechanisms in the reduced and oxidized phases offer insights for designing advanced oxide-ion conducting materials.