First-Principles Study of Sr2-x La x FeO4 and Comparison with Sr2FeO4‑δ: The Nature of the Positively Charged Defect Determines the Character of the Electronic Structure.

Lanthanum doping of the first-order Ruddlesden-Popper (RP) phases Sr2-x La x FeO4 (0 ≤ x ≤ 1) is investigated by density functional theory (DFT) calculations. Excess volume and excess energy of this solid solution series are calculated. The evolution of the electronic structure with increasing La content is discussed based on the calculated Fe magnetic moment. It is found that for the same average Fe oxidation state, the electronic structure in Sr2-x La x FeO4 displays a more delocalized character than in corresponding Sr2FeO4‑δ composition. This is attributed to the fact that oxygen vacancies VO •• which are formed under Fe-O bond breaking (and which carry a doubly positive charge) represent a stronger local distortion than singly charged substitutional LaSr • dopants.