A structural study of Ruddlesden–Popper phases Sr3−xYx(Fe1.25Ni0.75)O7−δ with x ≤ 0.75 by neutron powder diffraction and EXAFS/XANES spectroscopy

The structures of Ruddlesden – Popper n ¼ 2 member phases Sr 3 ? x Y x Fe 1.25 Ni 0.75 O 7 ? d with 0 # x # 0.75 have been investigated using neutron powder di ff raction and K-edge Fe and Ni EXAFS/XANES spectroscopy in order to gain information about the evolution of the oxygen vacancy distribution and Fe/Ni oxidation state with x . Both samples prepared at 1300 ? C under a fl ow of N 2 (g), with d ¼ 1.41 – 1.00, and samples subsequently annealed in air at 900 ? C, with d ¼ 0.44 – 0.59, were characterized. The as-prepared x ¼ 0.75 phase has d ¼ 1, the O1 atom site is vacant, and the Fe 3+ /Ni 2+ ions have a square pyramidal coordination. With decreasing x the O3 occupancy decreases nearly linearly to 81% for x ¼ 0, while the O1 occupancy increases from 0 for x ¼ 0.4 to 33% for x ¼ 0. The air-annealed x ¼ 0.75 sample has a d value of 0.59 and the Fe 3+ /Fe 4+ /Ni 2+ /Ni 3+ ions have both square pyramidal and octahedral coordination. With decreasing x , the d value decreases to 0.45 for x ¼ 0, implying an increase in the oxidation states of Fe/Ni ions. EXAFS/XANES data show that for the as-prepared samples the coordination changes are predominantly for Ni 2+ ions and that the air-annealed samples contain both Fe 3+ /Fe 4+