Order-disorder in grossly non-stoichiometric SrFeO2.50 -: a simulation study

Configurational lattice energy techniques are used to investigate oxygen vacancy ordering and the order disorder transition in SrFeO2.50. Vacancy disorder is shown to present many new challenges, largely due to the extensive relaxation in such grossly non-stoichiometric systems. With large supercells it is not feasible to optimise each individual configuration. Efficient methods for choosing a small number of representative configurations are discussed. Oxygen vacancy-vacancy interactions are considerable in SrFeO2.50 and lead to the formation of preferred local structural entities. While the low-temperature structure consists of an ordered arrangement of octahedra and tetrahedra, the disordered high-temperature structure may be described as a mixture of tetrahedra, square pyramids and octahedra. Fe atoms with coordination numbers lower than four are negligible. The assumption of an ideal solution of oxygen vacancies in such systems, commonly made in standard thermodynamic treatments, is questionable.