Simulation of Al/Fe disorder in Ca2FexAl2−xO5

A method is described which is capable of simulating disorder across a range of composition. It is used to study the disorder of iron and aluminium on the octahedral and tetrahedral cation sites in the Brownmillerite phase Ca₂Fe_xAl_2−xO₅. The arrangements with the lowest lattice energies are those that maximize the number of Fe³⁺ on octahedral sites throughout the composition range. An exchange of one Fe³⁺ with one Al³⁺, which results in a decrease in the number of iron ions on octahedral sites, increases the lattice energy by an amount that is dependent on x but is independent of the number of exchanges. The exchange of trivalent cations also results in an expansion in the b lattice direction, accompanied by a decrease in the a and c directions across the full range of composition.