Pressure-induced magnetic behavior in Ca₂Mn₂O₅-type A₂B₂O₅ oxides

Brownmillerite oxides with the chemical formula A₂B₂O₅ are derived from stoichiometric ABO₃ perovskites and exhibit ordered oxygen vacancies (OOVs). The OOVs transform the octahedral BO₆ units into different BO_6-x polyhedra, which allows for large changes in the crystal field split orbital structure and subsequent changes in functional properties. The magnetic order of transition metal oxides is governed by the correlation between these orbitals and corresponding electronic configurations. For example, Sr₂Mn₂O₅ (SMO; d⁴) in the Ca₂Mn₂O₅-type square pyramidal network yields E-type antiferromagnetic (AFM-E) order, consistent with the Goodenough-Kanamori rules, whereas Sr₂Fe₂O₅ (SFO; d⁵) exhibits AFM-G order within the same structure. Here, we investigate with first principles calculations the effect of hydrostatic pressure on the Ca₂Mn₂O₅-type structure, which drives contrasting behavior in SMO and SFO. We show the AFM-E order in SMO is further stabilized under pressure while SFO exhibits a magnetic transition to a FM state at ~24 GPa arising from a spin crossover. Lastly, we evaluate the pressure effect induced by biaxial strain in OOV structures and discuss the feasibility of such phase transition in thin film geometries.