B-cation arrangements in double perovskites

We characterize experimentally and theoretically the promising new solid oxide fuel cell electrode material Sr2Fe1.5Mo0.5O6−δ (SFMO). Rietveld refinement of powder neutron diffraction data has determined that the crystal structure of this material is distorted from the ideal cubic simple perovskite, instead belonging to the orthorhombic space group Pnma. The refinement revealed the presence of oxygen vacancies in the as-synthesized material, resulting in a composition of Sr2Fe1.5Mo0.5O5.90(2) (δ = 0.10(2)). DFT+U theory predicts essentially the same concentration of oxygen vacancies. Theoretical analysis of the electronic structure allows us to elucidate the origin of this nonstoichiometry and the attendant mixed ion–electron conductor character so important for intermediate temperature fuel cell operation. The ease with which SFMO forms oxygen vacancies and allows for facile bulk oxide ion diffusivity is directly related to a strong hybridization of the Fe d and O p states, which is also responsible for ...

Unveiling Structure–Property Relationships in Sr2Fe1.5Mo0.5O6−δ, an Electrode Material for Symmetric Solid Oxide Fuel Cells

Perovskite-related compounds in the series A2FeMO6(A Ca, Sr, Ba; M Nb, Ta, Sb) have been prepared using the standard techniques of solid state chemistry and characterized by X-ray powder diffraction, magnetometry and Mossbauer spectroscopy. All but the 6H hexagonal compound Ba2FeSbO6 adopt cubic or pseudo-cubic crystal structures and show a magnetic susceptibility maximum at ca. 25 K. Field cooled and zero-field cooled susceptibility data show a hysteresis below this maximum. The Mossbauer spectra recorded at 4.2 K comprise a single magnetic hyperfine pattern typical of Fe3+, but with a high degree of line broadening. The spectra show a relaxational (nonBrillouin) collapse above 20 K. The data are described in terms of spin-glass behaviour and the factors controlling the magnetic properties are discussed.

Investigation of magnetic frustration in A2FeMO6(A Ca, Sr, Ba; M Nb, Ta, Sb) by magnetometry and Mössbauer spectroscopy

The crystal structure of the perovskite Sr
 2
FeTaO
 6

has been refined by simultaneous analysis of X-ray and neutron powder
diffraction data collected at 280 K; space group Pbnm,
a=5.6204(3), b=5.6161(3), c=7.9266(3) A.
The structure is of the GdFeO
 3
 type, with a disordered
distribution of Fe and Ta over the six-coordinate cation sites. The
structure of Sr
 2
FeSbO
 6
 has been refined in a similar
manner; space group P2
 1
 /n,
a=5.6132(5), b=5.5973(5), c=7.9036(7) A,
β=90.01(1)°. The two crystallographically distinct
six-coordinate sites in Sr
 2
FeSbO
 6
 are occupied in a
partially ordered manner [0.795(6):0.205(6)] by Fe and Sb atoms. Neutron
diffraction data collected from Sr
 2
FeTaO
 6
 at 1.5 K
show no evidence of long-range magnetic ordering and, in the light of
previous susceptibility and Mossbauer measurements, it is concluded
that Sr
 2
FeTaO
 6
 is a spin glass below 23 K. Neutron
diffraction data collected from Sr
 2
FeSbO
 6
 at 1.5 K
include magnetic Bragg peaks characteristic of a type I magnetic structure
with an average ordered moment of 3.06(9) µ
 B
 per Fe
atom on the Fe-dominated octahedral site, and no significant ordered
moment on the second site. The magnetic Bragg scattering decreases to zero
in the temperature interval 1.5≤T/K≤37(2). It is concluded
that the partial cation ordering leads to the coexistence of a
magnetically ordered spin system and a spin-glass
system.

Neutron diffraction study of the influence of structural disorderon the magnetic properties of Sr2FeMO6 (M=Ta,Sb)

The influence of the lattice strain on the kinetics of the oxygen reduction reaction (ORR) was investigated at the surface of Nd2NiO4+δ (NNO). Nanoscale dense NNO thin films with tensile, compressive and no strain along the c-axis were fabricated by pulsed laser deposition on single-crystalline Y0.08Zr0.92O2 substrates. The ORR kinetics on the NNO thin film cathodes was investigated by electrochemical impedance spectroscopy at 360–420 °C in air. The oxygen exchange kinetics on the NNO films with tensile strain along the c-axis was found to be 2–10 times faster than that on the films with compressive strain along the c-axis. A larger concentration of oxygen interstitials (δ) is found in the tensile NNO films compared to the films with no strain or compressive strain, deduced from the measured chemical capacitance. This is consistent with the increase in the distance between the NdO rock-salt layers observed by transmission electron microscopy. The surface structure of the nonstrained and tensile strained f...

M.P. Attfield

Papers

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