Study of the Crystal Chemistry of the n=2 Ruddlesden-Popper Phases Sr3FeMO6+δ (M = Fe, Co, and Ni) Using in Situ High Temperature Neutron Powder Diffraction

L.V. MOGNI; F.D. PRADO; G.J. CUELLO; A. CANEIRO

CHEMISTRY OF MATERIALS

AMER CHEMICAL SOC

Año: 2009 vol. 21 p. 2614 - 2623

0897-4756

Effects of the substitution of Fe by Co or Ni on the crystal structure of the n = 2 Ruddlesden-Popper phase Sr3Fe2O6+δ have been studied by in situ high temperature neutron powder diffraction(NPD) in the temperature range 20 C e T e 900 C. The analysis of neutron diffraction dataconfirmed that for a given temperature the oxygen non-stoichiometry increases when Fe is replacedby Co or Ni. Oxygen vacancies were detected at the crystal sites O(1), connecting the octahedra alongthe c-axis, and O(3) in the FeO2 planes of the perovskite layers. The substitution of Fe by Co orNi increases the oxygen vacancy concentration at the O(3) crystal site, which were detected even whenthe total oxygen content was higher than 6.00. This result supports a diffusion mechanism involvingthe migration of oxide ions from the O(3) site to its adjacent empty O(1) sites and vice versa. Theaverage linear expansion Rave ≈ 1/3 RVol calculated from NPD data increases with the substitution ofFe by Co or Ni (19.9, 25.1, and 22.710-6K-1 forM=Fe, Co and Ni, respectively). The expansionalong the c-axis (Rc) affects mainly the perovskite block while the width of the rock-salt block remainsconstant. The crystal structure expansion is discussed considering the thermal and chemicalcontributions, along with the effect of the increasing delocalization of charge carriers when Fe isreplaced with Co or Ni