EngineeringIntermediate Phases in Topotactic PhaseTransitions of Cobaltite Perovskites

Topotactic phase transitions in perovskite cobaltites (R1–xAxCoO3−δ; R = rare earth, A = alkaline-earth ion) play a key role in the operation of neuromorphic devices, where oxygen vacancy concentration (δ) modulates resistive switching. However, the emergence of an insulating intermediate Grenier phase (δ = 0.33) often creates an energetic barrier for electronic transport, leading to a nonmonotonic increase in the threshold voltage. We employ first-principles calculations to examine how different combinations of lanthanoids (R = La, Nd) and alkaline-earth metals (A = Ca, Sr, Ba) influence the stability of intermediate phases. Our results reveal that while smaller cation combinations such as (R, A) = (La, Sr) and (La, Ca) stabilize the insulating Grenier phase with tetrahedral CoO4 coordination, specific cation substitutions (R, A) = (Nd, Sr) and (La, Ba) shift the thermodynamic stability toward a staggered square pyramidal phase (δ = 0.25). The latter is metallic and may facilitate topotactic phase transitions from the perovskite to brownmillerite structure. Overall, our findings provide a design strategy to achieve energy-efficient, low-voltage switching in cobaltite-based memristive systems.