Nanoscale Ordering in Oxygen Deficient Quintuple Perovskite
Sm2 ϵBa3+ϵFe5O15‐δ
revealed by
TEM
Authors: Lebedev Oleg , Turner Stuart , Cherepanov Vladimir , Raveau Bernard 
Published: December 2016

The introduction of two sorts of cations with different valence and size, such as Ba2+ or Sr2+ and Ln3+, in the A‐sites of transition metal perovskite oxides has generated numerous remarkable properties such as high Tc superconductivity, oxygen storage in cobalt based oxides for the realization of solid oxide fuel cell (SOFC) cathodes , CMR in manganates .

The investigation of the system Sm‐Ba‐Fe‐O in air has allowed an oxygen deficient perovskite Sm2‐εBa3+εFe5O15‐δ (δ=0.75, ε=0.125) to be synthesized. In contrast to the XRPD pattern which gives a cubic symmetry (ap= 3.934Å), the ED patterns of this phase (Fig.1a) show superstructure spots corresponding to
c
=
5ap
”. HRTEM study (Fig1b) revealed that this phase is nanoscale ordered with a quintuple tetragonal cell, “
ap ′ ap ′ 5ap
”. Bearing in mind that one cubic cell corresponds to the formula Sm0.375Ba0.625FeO2.85, these tetragonal nanostructures can be formulated as Sm2‐εBa3+εFe5O15‐δ (δ˜0.75, ε˜0.125). They consist of 5 SmO/BaO layers stacked alternately with 5 FeO2 layers along
c
. The HAADF‐STEM image (Fig.1c) of the Sm2‐εBa3+εFe5O15‐δ structure along the [100] is clearly established from the contrast segregation that the Ba2+ and Sm3+ cations are ordered in (001) layers along the c‐axis. One observes rows of bright dots perpendicular to
c
, which corresponds to three sorts of Sm or Ba cationic layers, judging from their intensity: pure Sm, mixed Ba/Sm and pure Ba layers. Thus the HAADF‐STEM image can be interpreted by the following periodic stacking sequence of the A cationic layers along the
c
axis: “Sm‐Ba‐Sm/Ba‐Sm/Ba‐Ba‐Sm”.

It appears from the ABF‐STEM images along [100] (Fig.1d) and [110] orientations (Fig.1e) of a single Sm2‐εBa3+εFe5O15‐δ domain, that the oxygen positions in all the layers are close to the ideal octahedral positions. However, a closer inspection of the images reveals that the oxygen columns in the equatorial positions close to the Sm layer deviate from their ideal octahedral position, and lie closer to the Sm3+ cations yielding a “zigzag” contrast along [100] and [110].

The “Sm‐Ba‐Sm/Ba‐Sm/Ba‐Ba‐Sm” chemical ordering is also confirmed by elemental EELS mapping (Fig.2a,b). The spatially resolved EELS data show that the O‐K edge spectra corresponding to the “FeO2” planes (labeled
A,B,C
) exhibit different intensity ratios of the two pre‐peaks to the O‐K edge, prepeak1/ prepeak2 at approximately 529/531 eV, depending on the nature of the surrounding “Sm,Ba” layers (Fig.2c). The O‐K fine structure in the Sm plane is very similar to that of plane A, whereas those of the Ba and Ba/Sm planes are similar to
B
and
C
planes respectively. A first observation is that pre‐peak 1 at ˜529 eV is less intense for oxygen anions close to Sm cations (SmO layers as well as A layers). According to the literature, the height of this pre‐peak is generally rather independent of the rare earth element and should be around the same height as pre‐peak 2. Pre‐peak 2, related to Fe3d eg – O2p hybridized states seems invariant in the structure, apart from the c plane where it is slightly subdued, accompanied by an increase of pre‐peak 1 below 530eV. Pre‐peak 1 can be attributed to a charge transfer from the eg to the t2g band of Fe (the eg band is usually empty for Fe3+). This increase of pre‐peak 1 related to Fe3d t2g – O2p hybridized states is also visible in the Ba/Sm mixed layers, and can be linked to the presence of oxygen vacancies in those planes. This peak is stronger in the C plane suggesting the presence of more vacancies in this plane.The spatially resolved EELS spectra of the Fe‐L2,3 edge are plotted in Fig. 2c. The Fe L3 and L2 “white lines” arise from transitions of 2p3/2 → 3d3/23d5/2 (L3) and 2p1/2 → 3d3/2 (L2) and are known to be sensitive to valency and coordination. Our data shows that the
A
and
B
FeO2 planes exhibit very similar Fe‐L2,3 edges, with an L3 peak maximum at 709.5 eV, and a pre‐peak to L3, even if faint at 708 eV. The energy position of the L3 maximum, together with the shape and positions of the L3 and L2 are then indicative of Fe3+ in an octahedral coordination. All the acquired Fe L3 edges are significantly broadened with respect to the plotted references for 6‐fold, 5‐fold and 4‐fold coordinated Fe3+. This broadening can be explained by a change in coordination of the Fe atoms. Bearing in mind that the measured oxygen stoichiometry is 14.25, instead of 15, this suggests that the iron coordination is mainly 6, i.e. octahedral, but may also be mixed with the presence of some FeO5 pyramids in those layers.

The nanoscale ordering of this perovskite explains its peculiar magnetic properties on the basis of antiferromagnetic interactions with spin blockade at the boundary between the nanodomains. The variation of electrical conductivity and oxygen content of this oxide versus temperature suggest potential SOFC applications.