A structural study of Ruddlesden-Popper phases Sr_3-xY_x(Fe₁₂₅Ni075)O_7-δ with x ≤ 0.75 by neutron powder diffraction and EXAFS/XANES spectroscopy

The structures of Ruddlesden-Popper n = 2 member phases Sr_3-xY_xFe₁25Ni0.75O_7-δ with 0 ≤ x ≤ 0.75 have been investigated using neutron powder diffraction 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 flow of N₂(g), with δ = 1.41-1.00, and samples subsequently annealed in air at 900°C, with δ = 0.44-0.59, were characterized. The as-prepared x = 0.75 phase has δ = 1, the O1 atom site is vacant, and the Fe³⁺/Ni²⁺ ions have a square pyramidal coordination. With decreasing x the O3 occupancy decreases nearly linearly to 81% for x = 0, while the O1 occupancy increases from 0 for x = 0.4 to 33% for x = 0. The air-annealed x = 0.75 sample has a δ value of 0.59 and the Fe³⁺/Fe⁴⁺/Ni²⁺/Ni³⁺ ions have both square pyramidal and octahedral coordination. With decreasing x, the δ value decreases to 0.45 for x = 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 Ni²⁺ ions and that the air-annealed samples contain both Fe³⁺/Fe⁴⁺ and Ni²⁺/Ni³⁺ ions.