Study of the Crystal Chemistry of the n = 2 Ruddlesden−Popper Phases Sr₃FeMO_6+δ (M = Fe, Co, and Ni) Using in Situ High Temperature Neutron Powder Diffraction

Abstract

Effects of the substitution of Fe by Co or Ni on the crystal structure of the n = 2 RuddlesdenPopper phase Sr 3 Fe 2 O 6+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 data confirmed that for a given temperature the oxygen non-stoichiometry increases when Fe is replaced by Co or Ni. Oxygen vacancies were detected at the crystal sites O(1), connecting the octahedra along the c-axis, and O(3) in the FeO 2 planes of the perovskite layers. The substitution of Fe by Co or Ni increases the oxygen vacancy concentration at the O(3) crystal site, which were detected even when the total oxygen content was higher than 6.00. This result supports a diffusion mechanism involving the migration of oxide ions from the O(3) site to its adjacent empty O(1) sites and vice versa. The average linear expansion R ave ≈ 1 / 3 R Vol calculated from NPD data increases with the substitution of Fe by Co or Ni (19.9, 25.1, and 22.7 Â 10 -6 K -1 for M = Fe, Co and Ni, respectively). The expansion along the c-axis (R c ) affects mainly the perovskite block while the width of the rock-salt block remains constant. The crystal structure expansion is discussed considering the thermal and chemical contributions, along with the effect of the increasing delocalization of charge carriers when Fe is replaced with Co or Ni.

Crystal structure and high-temperature properties of the Ruddlesden–Popper phases Sr3−Y (Fe1.25Ni0.75)O7− (0≤x≤0.75)

“…For x r0.3, an average value of $ 5.5 was obtained for the total O content per formula unit. These findings agree with observations for similar systems [13,16,27], as well as with an O content of 5.5 derived from TG recordings in 4%H 2 /Ar atmosphere for as-prepared x ¼0.25 (see below).…”

“…However, the conductivity of such compounds dramatically decreases above 400 1C, because of loss of oxygen, which makes them unsuitable for use in IT-SOFC [13]. The substitution of Fe by Ni (Sr 3 À x La x Fe 2 À y Ni y O 7 À δ , with 0 rx r0.3 and 0 ryr1.0) [14,15] and Co (Sr 3 Fe 2 À x Co x O 7 À δ with 0 rxr 1) [16,17] results in an increase of the electrical conductivity. The average linear thermal expansion coefficients (TEC) of the substituted phases are, however, too high (25.1 and 22.7 ppm K À 1 for Co and Ni, respectively) [16], to be compatible with the TEC of typically 12-14 ppm K À 1 of the ceria-based electrolytes currently used in IT-SOFC [18].…”

Oxygen nonstoichiometry, chemical expansion, mixed conductivity, and anodic behavior of Mo-substituted Sr3Fe2O7-δ

“…An attractive combination of properties important for these applications, namely a relatively high conductivity, moderate thermal expansion coefficients (TECs), and substantial stability in reducing atmospheres, is known for the Ruddlesden-Popper (RP) type Sr 3 Fe 2 O 7-δ [5][6][7][8][9][10][11][12]. The intergrowth structure of this ferrite ( Fig.…”

“…1) is built of alternating perovskite-like (SrFeO 3-δ/2 ) 2 and rock-salt SrO layers, and has three different anion positions [5,10]. At low temperatures, oxygen vacancies are predominantly located in the apical O1 sites (according to the site notation from [10][11][12]). The most likely mechanism of anion diffusion in these conditions involves consecutive O3→ O1→ O3 jumps along the octahedra edges; the limiting factor is related to low concentration of the O3 vacancies available for anion transfer [7,8].…”

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

“…6 The ideal stoichiometry of an n ¼ 2 RP phase is A 3 B 2 O 7 , where A is an alkaline-earth or rare-earth element, B is a transition metal element, and the structure contains two ABO 3 perovskite layers that alternate with single AO rock-salt layers, For real n ¼ 2 RP phases, the O1 site may be partially or totally vacant and the O3 site may also be partially vacant, while no evidence for O2 vacancies has been found. [8][9][10][11][12][13] The O content per formula unit, 7 À d, may be as low as 5.45.9 For oxygen-decient phases the probable B atom coordination polyhedra are, in varying proportions, octahedra, square pyramids, trigonal bipyramids and tetrahedra, Fig. 2.…”