Neutron powder diffraction study at high temperature of the Ruddlesden–Popper phase Sr3Fe2O6 + δ

The crystal and oxygen defect structure of the n = 2 Ruddlesden–Popper phase Sr3Fe2O6 + δ have been studied by in situ high temperature neutron powder diffraction in the temperature range 20 ≤ T ≤ 900 °C in air. The analysis of the neutron diffraction data revealed the presence of structural oxygen vacancies on both the O(1) sites linking the octahedra along the c axis and the O(3) sites in the FeO2 planes of the perovskite layers. The oxygen vacancies on the O(3) site increase with temperature up to ∼ 0.25 per formula unit at T = 900 °C. This result supports previously proposed oxygen ion diffusion mechanism in Sr3Fe2O6 + δ that involves the migration of vacancies from an O(3) site to an adjacent O(1) site. The total linear expansion along the c axis αc = 17.7(5) · 10− 6 K− 1 mainly affects the perovskite block while the width of the rock salt layers remains stable with temperature. The total volumetric expansion αV / 3 = 20(1) · 10− 6 K− 1 is around the average of the TEC values (14.8–27.1 K− 1) reported for the perovskite system La1 − xSrxCo1 − yFeyO3 − δ.