Two variants of the 1/2[110]p(203)p crystallographic shear structure: The phasoid Sr0.61Pb0.18(Fe 0.75Mn0.25)O2.29

Article abstract of DOI:10.1021/ic900762s

For the composition (Sr_0.61 Pb_0.18)(Fe _0.75Mn_0.25)O_2.29, a new modulated crystallography shear structure, related to perovskite, has been synthesized and structurally characterized by transmission ele

Full text of DOI:10.1021/ic900762s

Inorg. Chem. 2009, 48, 8257–8262 8257
DOI: 10.1021/ic900762s
Two Variants of the 1/2[110]p(203)p Crystallographic Shear Structures:
The Phasoid Sr0.61Pb0.18(Fe0.75Mn0.25)O2.29
†
Christophe Lepoittevin,†,‡ Joke Hadermann,‡ Sylvie Malo,*,† Olivier Perez, Gustaaf Van Tendeloo,‡ and
ꢀ
Maryvonne Hervieu†
†
‡
ꢀ
Laboratoire CRISMAT-ENSICAEN, Bd du Marechal Juin-14050 CAEN cedex, France, and EMAT,
University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium
Received April 20, 2009
For the composition (Sr0.61Pb0.18)(Fe0.75Mn0.25)O2.29, a new modulated crystallographic shear structure, related to
perovskite, has been synthesized and structurally characterized by transmission electron microscopy. The structure
˚
can be described using a monoclinic supercell with cell parameters am = 27.595(2) A, bm = 3.8786(2) A, cm =
˚
˚
13.3453(9) A, and βm = 100.126(5)ꢀ, refined from powder X-ray diffraction data. The incommensurate crystal-
lographic shear phases require an alternative approach using the superspace formalism. This allows a unified
description of the incommensurate phases from a monoclinically distorted perovskite unit cell and a modulation
wave vector. The structure deduced from the high-resolution transmission electron microscopy and high-angle annular
dark-field-scanning transmission electron microscopy images is that of a 1/2[110]p(203)p crystallographic shear
structure. The structure follows the concept of a phasoid, with two coexisting variants with the same unit cell. The
difference is situated at the translational interface, with the local formation of double (phase 2) or single (phase 1)
tunnels, where the Pb cations are likely located.
Introduction
SrFeO3-y
coordination. Sr deficiency in SrxFeO3-y can be described by
the formation of complex [Fe2O2.5-ε ¥ double layers in the
,
3-12 in which iron atoms can adopt a IV, V, or VI
The “ABO3” perovskite structures are well-known to
accommodate a large variety of structural modifications.1,2
A huge amount of work has been devoted to the study of the
chemical and physical properties associated with their
flexibility in oxygen nonstoichiometry (ABO3-x), but also
in A-site cation deficiency (AxBO3), for which the octa-
hedral framework of the nonstoichiometric A-site cation
remains theoretically unchanged, down to x = 0 in the
ReO3-type structure. However, the intermediate value x=
0.75 is often considered as the limit of the “deficient
perovskites” (1 g x > 0.75).
]
perovskite matrix, up to the formation of an ordered phase
for x=2/3, corresponding to the composition Sr2/3FeO2.17 or
Sr4Fe6O13-δ
4D formalism16 showed that, along cB, one perovskite-type
slice [SrFeO3]¥ alternates with one complex [SrFe2O3-ε ¥
slice related to a triple rock salt-type block. In the intermedi-
ate complex [Fe2O2.5-ε ¥ double layer, iron cations exhibit
.
13-15 The structure refinements carried out in a
]
]
three types of coordination: tetragonal pyramids, trigonal
bipyramids, and monocapped tetrahedra. The intergrowth of
The Sr-based ferrites are exceptionally rich systems, espe-
cially by the existence of the oxygen-deficient perovskites
(8) Takano, M.; Okita, T.; Nakayama, N.; Bando, Y.; Takeda, Y.;
Yamamoto, O.; Goodenough, J. B. J. Solid State Chem. 1988, 73, 140–150.
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A. S.; Rozova, M. G.; Antipov, E. V.; Van Tendeloo, G. Chem. Mater. 2008,
20, 7188–7194.
*To whom correspondence should be addressed. E-mail: sylvie.malo@
ensicaen.fr.
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r
2009 American Chemical Society
Published on Web 07/24/2009
pubs.acs.org/IC

8258 Inorganic Chemistry, Vol. 48, No. 17, 2009
Lepoittevin et al.
one perovskite-type slice and one triple rock salt-type layer
can be associated with a 2201-type structure,17 and a large
family of new materials, denoted FeA-(n - 1)2(m - 1)m, has
been evidenced, with the members FeBi-2212 and FeTl-2234
associated with the formation of double and quadruple
perovskite slices, respectively.18,19 By doping the Sr site with
members and of their structure determination from transmis-
sion electron microscopy (TEM) can be found in ref 33.
In order to understand the role played by the different
cations in the formation of the crystallographic shear
structures in the Sr-rich part of the ferrites system, doping
of the B cation has been investigated for a cationic ratio A/B
in the range 2/3-4/5. For the composition (Sr0.61Pb0.18)-
(Fe0.75Mn0.25)O2.29, a new modulated CS structure has been
synthesized, characterized by a perovskite related structure
but different from the undoped terrace structure.21 It has
been studied by transmission electron microscopy, and its
transport properties were characterized. Its relation to the
different CS structures associated with A-cation-deficient
perovskites is discussed.
Pb2+ 20
a solid solution is obtained (calculated to the
,
equivalent composition Sr0.55Pb0.12FeO2.17), whereas for a
small lead excess (Sr0.54Pb0.17)FeO2.21, the formation of
complex crystallographic shear (CS) planes is observed, with
the stabilization of a “terrace structure”21 described from the
intergrowth of FePb-2201 and FePb-2212 structures. Besides
the major steric role of the 6s2 lone pair of the A=Bi3+ and
Pb2+ cations, the combined effects of the valence and size
(Ba2+ and Pb2+) are indeed known to favor the formation of
CS planes in similar layered ferrite structures.22-27
Experimental Section
Recent studies of the Pb-rich ferrites in the “perovskite”
PbFeO2.528-30 highlight the role of the lone pair cation and
evidence the existence of a large family of Pb-site-deficient
perovskite-related compounds Pb4m+3nFe4(m+n)O10m+9n
with different 1/2[110](h0l) CS structures. It was shown that
different CS structures can coexist in the same crystallite. The
The powder compound was prepared in a glovebox start-
ing from the oxides PbO, SrO, Fe2O3, and Mn2O3, in order to
stabilize trivalent Fe3þ and Mn3þ cations. SrO was initially
preparedbydecomposingSr(OH)2 8H2Oat1100ꢀCandwas
kept stored at this temperature. The precursors, weighed in
the stoichiometric ratio, areground toobtaina homogeneous
powder. The mixture was then pressed into bars, sealed in a
silica tube, and heated at 1100 ꢀC for 48 h at a heating rate of
1.5 ꢀC/min and slow-cooled at the same rate.
The X-ray powder diffraction (XRPD) analyses were
carried out at room temperature with a Philips diffractometer
using Cu KR radiation (λ=1.5418 A) in the range 10ꢀ e 2θ e
110ꢀ.
The structural study was carried out using TEM. A small
amount of sample was crushed in an agate mortar containing
n-butanol, and a droplet was deposited on a copper grid
covered with a holey carbon film. The electron diffraction
(ED) investigation was carried out with a JEOL 200 CX
microscope. The high-resolution TEM (HRTEM) was car-
ried out using a JEOL 4000EX operating at 400 kV. HRTEM
images were simulated using the Mac Tempas software.
Z-contrast images were obtained on a JEOL 3000F micro-
scope equipped with a scanning transmission electron micro-
scopy (STEM) unit and a high-angle annular dark-field
(HAADF) detector.
3
27
31
oxygen-deficient Pb2/3Sr1/3FeO2.5 and Pb0.5Ba0.5FeO2.5
frameworks have been proven to exhibit similar CS struc-
tures. The Pb-Mn-O compound Pb0.9MnO2.63, synthesized
at 880 ꢀC under 7.8 GPa, also exhibits a crystallographic
shear structure, of the type 1/2[110]p(704)p.32 All structures
are characterized by the formation of six-sided tunnels where
˚
two Pb2+ cations are located. However, in Pb0.9MnO2.63, the
√
interface induces a translation by 1/4ap 2 along [110]p be-
tween two perovskite blocks and an elongation of the tunnels
along [100]p, whereas in the Pb4m+3nFe4(m+n)O10m+9n struc-
tures, the perovskite blocks and the long diagonal of the
tunnels are aligned along [110]p. An overview of the reported
(17) Raveau, B.; Michel, C.; Hervieu, M.; Groult, D. Crystal Chemistry of
High Tc Superconducting Copper Oxides; Springer-Verlag: Berlin, 1991;
Springer Series in Materials Science 15.
ꢀ
(18) Grebille, D.; Lepoittevin, Ch.; Malo, S.; Perez, O.; Nguyen, N.;
Hervieu, M. J. Solid State Chem. 2006, 179(12), 3849–3859.
ꢀ
(19) Lepoittevin, C.; Malo, S.; Nguyen, N.; Hebert, S.; Van Tendeloo, G.;
The resistivity measurements were carried out using the
four-probes method on a physical properties measurement
system.
Hervieu, M. Chem. Mater. 2008, 20, 6468–6476.
(20) Lepoittevin, C.; Malo, S.; Van Tendeloo, G.; Hervieu, M. Solid State
Sci. 2009, 11, 595–607.
ꢀ
(21) Lepoittevin, C.; Malo, S.; Perez, O.; Nguyen, N.; Maignan, A;
Hervieu, M. Solid State Sci. 2006, 8, 1294–1301.
(22) Hervieu, M.; Michel, C.; Caldes, M. T.; Pham, A. Q.; Raveau, B. J.
Solid State Chem. 1993, 107, 117–126.
Results
The X-ray Powder diffraction (Figure 1) and electron
diffraction studies show that the synthesis resulted in a new
phase. The homogeneity of the sample and the cationic
composition were determined by combining the ED and
EDS analyses. The cationic ratios have been measured
on numerous crystallites, leading to an average cation
ratio Sr/Pb/Fe/Mn close to 17:5:21:7, that is, (Sr0.61Pb0.18)-
(Fe0.75Mn0.25)O2.29(ε calculated per perovskite unit; the
standard deviation is estimated to be 3% for the transition
elements and strontium and increases to 7% for lead.
The compound is an insulator at 300 K with a resistivity on
the order of 5 ꢀ 105 Ω cm. This value is consistent with the
behavior observed for the different phases related to the
strontium-rich phases FeA-(n - 1)2(m - 1)m of the different
(23) Hervieu, M.; Caldes, M. T.; Michel, C.; Pelloquin, D.; Raveau, B. J.
Solid State Chem. 1995, 118, 357–366.
ꢀ
(24) Hervieu, M.; Perez, O.; Groult, D.; Grebille, D.; Leligny, H.; Raveau,
B. J. Solid State Chem. 1997, 129, 214–222.
ꢀ
(25) Perez, O.; Leligny, H.; Baldinozzi, G.; Grebille, D.; Graafsma, H.;
ꢀ
Hervieu, M.; Labbe, Ph.; Groult, D. Phys. Rev. B 1997, 56, 5662–5672.
ꢀ
(26) Allix, M.; Perez, O.; Pelloquin, D.; Hervieu, M.; Raveau, B. J. Solid
State Chem. 2004, 177, 3187–3196.
(27) Raynova-Schwarten, V.; Massa, W.; Babel, D. Z. Anorg. Allg. Chem.
1997, 623, 1048–1054.
(28) Abakumov, A. M.; Hadermann, J.; Bals, S.; Nikolaev, I. V.; Anti-
pov, E.; Van Tendeloo, G. Angew. Chem., Int. Ed. 2006, 45, 6697–6700.
(29) Hadermann, J.; Abakumov, A. M.; Nikolaev, I. V.; Antipov, E. V.;
Van Tendeloo, G. Solid State Sci. 2008, 10, 382–389.
(30) Abakumov, A. M.; Hadermann, J.; Van Tendeloo, G.; Antipov,
E. V. J. Am. Ceram. Soc. 2008, 91(6), 1807–1813.
(31) Nikolaev, I. V.; D’Hondt, H.; Abakumov, A. M.; Hadermann, J.;
Balagurov, A. M.; Bobrikov, I. A.; Sheptyakov, D. V.; Pomjakushin, V.;
Pokholok, K. V.; Filimonov, D. S.; Van Tendeloo, G.; Antipov, E. V. Phys.
Rev. B 2008, 78, 024426-1-11.
(32) Bougerol, C.; Gorius, M. F.; Grey, I. E. J. Solid State Chem. 2002,
169, 131–138.
(33) Van Tendeloo, G.; Hadermann, J.; Abakumov, A. M. Antipov, E. V.
J. Mater. Chem. 2009, 19, 2660-2670.

Article
Inorganic Chemistry, Vol. 48, No. 17, 2009 8259
Figure 1. X-ray powder diffraction of (Sr17Pb5)(Fe21Mn7)O64, [(Sr0.61Pb0.18)(Fe0.75Mn0.25)O2.29; only the peaks corresponding to I/I0 > 2% are indexed,
I0 representing the highest intensity observed].
Figure 2. (a) The 1/2 [110](305)p structure of Pb0.9FeO2.428 and (b) the
32
.
1/2 [110](704)p structure of Pb0.9MnO2.63
systems (A=Ba, Pb, Bi, La),18,20 which are characterized by a
trivalent state of iron.
X-Ray and Electron Diffraction Techniques. The reci-
procal space was reconstructed by tilting around several
crystallographic axes. The ED patterns present large
similarities to those reported for CS structures related
to perovskite, such as the ferrite Pb0.9FeO2.428 (Figure 2a)
and the manganite Pb0.9MnO2.6332 (Figure 2b).
Out of these tilt series, a first example of a [010] ED
pattern is given in Figure 3a. It shows features that are
characteristic for such CS structures: a set of intense reflec-
tions, accompanied by weaker ones. An indexation, de-
Figure 3. (Sr0.61Pb0.18)(Fe0.75Mn0.25)O2.29. The intense reflections of the
[010] ED patterns are indexed in a perovskite monoclinic subcell (drawn
as a continuous line). (a) In this example, all of the reflections are indexed
√
˚
˚
in a C-type monoclinic supercell with am ≈ 27.6 A (≈ 5ap 2), bm ≈ 3.9 A,
˚
cm ≈ 14.0 A (≈ 13d(203)p), and β ≈ 101ꢀ (drawn in dotted line).
(b) Incommensurate pattern, associated with a modulation vector q =
0.0787a*p þ 0.01179c*p. (c) Indexation of the pattern reproduced in part (a)
considering the monoclinic supercell (red indices) or using four hklm
indices (blue indices) with a commensurate q vector q = 2/25a*p þ 3/25cp*.
The two basic vectors [203]p* and [101]*p of the subcell are drawn.
duced from these tilt series, in agreement with the following
˚
√
˚
monoclinic unit cell, am ≈ 27.6 A (≈ 5ap 2), bm ≈ 3.9 A,
˚
cm ≈ 14.0 A (≈ 13d(203)p), and β ≈ 101ꢀ, can be proposed.
The possible space groups compatible with the existence
conditions observed for the reflections are C2, Cm, or C2/m.
The cell parameters of the monoclinic supercell refined from
on the selected area, losses of the periodicity are observed.
Figure 3b illustrates this feature; along the [101]p direc-
tion, we observe a discontinuity in alignment of the reflec-
tions, which is not present in Figure 3a. A consequence of
this aperiodicity is the impossibility of finding a supercell
allowing a description of all diffraction patterns: a linear
˚
˚
XRPD patterns are am=27.595(2) A, bm=3.8786(2) A, cm=
˚
13.3453(9) A, and βm = 100.126(5)ꢀ. The corresponding
reciprocal unit cell is indicated in Figure 3a by six mono-
clinic cells drawn around the origin. However such an
interpretation is not totally satisfying because, depending

8260 Inorganic Chemistry, Vol. 48, No. 17, 2009
Lepoittevin et al.
combination of three basic vectors with integer coefficients
does not permit the indexation of all reflections. The
corresponding CS phases are clearly incommensurate, and
only the superspace formalism allows a rigorous description
of the patterns. Using an average monoclinic unit cell (ap ≈
˚
bp ≈ cp ≈ 3.9 A and βp ≈ 87ꢀ), identified considering the
intense reflections (see Figure 3b), and introducing the
modulation wave vector q = 0.0787a*p þ 0.01179cp*, each
reflection can be indexed as follows: ha*p þ kbp* þ lcp* þ mq,
with h, k, l, and m being integers (see Figure 3b). The
systematic existence condition observed for the main and
satellite reflections, h þ k þ l þ m=2n, is in agreement with
the centering vector [1/2 1/2 1/2 1/2] and is compatible with
the I 2/m(R0γ) superspace group. Such a description of the
reciprocal space can be successfully applied to the previous
˚
case described with the monoclinic supercell (am ≈ 27.6 A,
˚
˚
bm ≈ 3.9 A, cm ≈ 14.0 A, β ≈ 101ꢀ) but using a commensu-
rate q vector: q=2/25ap* þ 3/25c*p with the same average
monoclinic cell. Both indexations for this commensurate
phase are schematically illustrated in Figure 3c. The major
interest of the superspace approach is to provide a unified
description of the reciprocal space for the complete CS
structure family.
In the present contribution, we will mainly focus on the
commensurate phase, of which the diffraction pattern is
reproduced in Figure 3a.
Figure 4. (Sr0.61Pb0.18)(Fe0.75Mn0.25)O2.29 [010] HREM images of the
same region at different defocuses. High electron density zones are dark in
a and bright in b. The simulated images, calculated with the proposed
models, are superimposed onto the experimental images.
HRTEM and HAADF-STEM Observations. Two [010]
HRTEM images, carrying complementary information,
recorded in the course of a focus series are reproduced in
Figure 4. The images clearly illustrate the periodic for-
mation of crystallographic shear planes in the perovskite
structure, so that the present compound can be consid-
ered as a 1/2[110](203)p CS structure. In Figure 4a, the
high electron density zone positions are the darker ones
˚
(focus close to -190 A), whereas in Figure 4b these are the
˚
brighter zones (focus close to -20 A). The characteristic
variations of the contrast at the level of the shear planes
can be directly compared to those observed in the ferrite
Pb0.9FeO2.4.
28 An important characteristic is observed at
the interfaces: the alignment of the perovskites blocks and
the long diagonal of the tunnels along [101]p. This relative
arrangement of the perovskite blocks attests to an extra
displacement component denoted R1=ε[001]p, with ε ≈
1/3.29 Using the model of an ideal CS structure and based
upon the comparison with the structure and images of
Pb0.9FeO2.4, the dark dots on the HRTEM [010] image of
Figure 4a are interpreted as cation positions forming
perovskite blocks that are periodically translated in
(203)p slices, while the dark zones between these slices
and elongated parallel to [110]p can be associated with the
six-sided tunnels surrounded by edge-sharing tetragonal
pyramids. The HRTEM images show that the orientation
of the crystallographic shear planes remains the same
over large distances without suffering any deviation.
An interesting and important feature is the presence of
two types of A cations with a significant difference in Z in
the matrix (Pb=82 and Sr=38). HAADF STEM is the
appropriate technique to study the preferential distribu-
tion of Pb in the cages of the framework. In so-called
Z-contrast images, the intensity related to a column of
atoms is proportional to Zn (1<n<2). The [010] HAADF
STEM image (Figure 5a) exhibits rows of gray dots
Figure 5. (a) [010] HAADF STEM image. The gray dots are associated
with the projection of columns of A cations in the perovskite slices, while
the brighter dots correspond to the six-sided tunnels. (b) Local structure
obtained by overlaying the HRTEM image.
the projection of the columns of A cations in the perov-
skite slices (see the large yellow arrows). In between,
dumbbells of brighter dots (parallel to [101]p) and spaced
˚
by about 2.75 A in projection are associated with the six-
sided tunnels (labeled “H” hereinafter), and the bright-
ness of the contrast suggests that they are filled in
majority by lead cations (green dots in Figure 5a). The
STEM image of Figure 5a also shows the existence of
˚
spaced by ≈5.5 A along [101]p and [101]p, associated with

Article
Inorganic Chemistry, Vol. 48, No. 17, 2009 8261
five-sided tunnels (labeled “P” hereinafter) as well as
double “H” tunnels, along [302]p. Two single “P” tunnels
are separated by four less bright dots, forming a lozenge
(highlighted by a large ellipse), whereas two double “H”
tunnels are separated by two gray dots (highlighted by a
small ellipse). The combined information from the [010]
HRTEM and HAADF STEM images, together with
the help of the Pb0.9FeO2.4 (1/2[110]p(305)p)28 and
Pb0.9MnO2.63 (1/2[110]p(704)p)32 structure analyses, pro-
vides a guide for the interpretation of the contrast of the
present 1/2[110]p(203)p CS structure. The [010] HRTEM
image in Figure 4a, where the high electron density
positions are imaged as dark, has been used to draw a
schematic model of the local structure in Figure 5b. The
perovskite slice is clearly built up from octahedra peri-
odically translated along a (203)p slice. In between these
perovskite-type slices, the (Fe/Mn)Ox polyhedra result-
ing from the shearing mechanism (drawn in the form of
distorted pyramids shifted by 1/2[110]p) are corner-shar-
ing and form long tunnels or distorted cages, where the Pb
cations are most likely located; the Sr and possibly a
minority of Pb cations occupy the perovskite cages.
The HAADF STEM images offer an understanding of
the ordering arrangement, along cB, of the different dis-
torted pyramids and, consequently, tunnels (“P” and
“H”) they manage at the level of the translational inter-
faces. Along the [302]p interface, the single and double
“H” tunnels are distributed in alternating short sequences
of a few species. Along [101]p, single “P” tunnels tend to
be aligned with single “P” tunnels at the adjacent interface
or aligned with one of the double “H” tunnels (see vertical
line in Figure 5a). The HAADF STEM images evidence
that the perovskite blocks and the tunnels in the transla-
tion interfaces order according to two different patterns
Figure 6. (a) HAADF STEM image showing the two types of patterns
with exactly the same monoclinic cell. (b) Idealized models of the phases 1
and 2.
perovskite slices is similar to that in the model 2,
√
(1 and 2 in Figure 6a), which have exactly the same
˚
√
(5ap 2/2) along [101]p and 1/2bBp. Two single “H” tun-
nels are separated by two B4O14 units. The junction
between two B4O14 units creates pentagonal tunnels
generating, together with the distorted perovskite cages,
the lozenges observed in the STEM images. The theore-
tical composition of this cell 1 is Pb4Sr18(Fe,Mn)28O64,
accounting for the same convention on the tunnels and
cages’ occupancy. Note that the two formulations involve
a trivalent state of iron, in agreement with our conditions
of synthesis.
monoclinic cell in 3D space (am ≈ 5ap 2, bm ≈ 3.9 A,
cm ≈ 13d(203)p, and β ≈ 101ꢀ) and the same Cm space
group, in agreement with the refined parameters am =
˚
˚
˚
27.595(2) A, bm = 3.8786(2) A, cm = 13.3453(9) A, and
βm ≈ 100ꢀ126(5).
The idealized models inferred from the above images
are drawn in Figure 6b. One of these models, denoted
phase 2, is characterized by the presence of double “H”
tunnels. The perovskite slice is built up of blocks of seven
corner-sharing octahedra, translated along [302]p to form
Structure Analysis: A Phasoid? The models of the phases
1 and 2 (Figure 6b) deserve a thorough analysis because of
the unusual existence of their common monoclinic cell.
They are compared in Figure 7. The “perovskite blocks”
only differ by one oxygen atom, making the seventh
octahedron a pyramid (Figure 7a), and the translation
interface has a common part, that is, one B2O8 unit, one
tunnel, and one B4O14 pyramid (Figure 7b).
The two frameworks 2 and 1 are built up as follows:
Framework “2”: perovskite slices made of blocks of
seven corner-sharing octahedra. Translation interface:
two B4O14 units and one B2O8 unit forming double
“H” tunnels (Figure 6b)
the perovskite (203)p slice. The adjacent perovskite slice is
√
shifted by (5ap 2/2) along [101]p and 1/2 bBp. The six-
sided tunnels are surrounded by two octahedra, belong-
ing to two adjacent perovskite slices, bordered by two
structural B2O8 units made of two edge-sharing pyramids
(B=Fe, Mn). These units are translated along [302]p so
that the junction of two structural B2O8 units forms a
block of four edge-sharing B4O14 pyramids. A theoretical
composition, Pb8Sr16(Fe,Mn)26O63, can be proposed for
the structure 2 considering, with a simplifying aim, that
Pb atoms are only located in the double “H” tunnels and
that the pentagonal tunnels and perovskite cages are
occupied by Sr atoms. The other model is characterized
by the presence of single “H” tunnels and is denoted phase
1. The perovskite slice is built up of blocks of six corner-
sharing octahedra, and the seventh octahedron is now a
pyramid that is part of a B4O14 unit of the translation
interface. Along [101], the translation of the adjacent
Framework “1”: Perovskite slices made of blocks of
six corner-sharing octahedraþ1 pyramid. Translation
interface: two B4O14 units forming single tunnels
The common features of the two models are illustrated
in Figure 7c. The region, highlighted by the yellow

8262 Inorganic Chemistry, Vol. 48, No. 17, 2009
Lepoittevin et al.
Figure 8. HAADF STEM image showing the coexistence of nano-
phases “1” and “2” (numbers refer to the structure models).
Concluding Remarks
This study reveals the existence of anion-deficient perov-
skite compounds in the Sr-rich part of the diagram. The
investigation of the mechanism generating crystallographic
shear structures in the Sr-rich ferrites confirms the major role
played by the two cation ratios, A/B and Pb/Sr. Starting
from the upper limit of the solid solution Sr0.55Pb0.12FeO2.17
(A/B=(Sr þ Pb)/(Fe) ≈ 0.67 and Pb/Sr þ Pb=0.18) with the
structure type FeA-(n - 1)2(m - 1)m,20 it appears that a small
excess of lead (Pb/Sr þ Pb g 0.2) induces the formation of
complex CS 1/2[110](hkl)p structures21 and that the stabiliza-
tion of the six-sidedtunnels requires a higher A/B cation ratio.
An interesting point in the present material is the existence
of two structures, having a similar 3D unit cell but character-
ized by the presence of either single or double six-sided
tunnels, denoted “H”. These structures can be considered
as two ordered nanostates of an average macroscopic struc-
ture (Sr16Pb4)(Fe þ Mn)26M4O64, with M = Pb or Sr and
(Fe, Mn), and therefore the material can be considered a
phasoid. The possible occupancy of the M sites by one of the
four cations can be compared to different structural mechan-
isms observed in the terrace structures,24,26,21 at the junction
between the mixed rock salt-type [(Sr,A)O] layers (A is a lone
pair cation, Bi3þ or Pb2þ) and the [FeO2-x] layers.
Figure 7. Comparison of models 1 and 2: (a) in the “perovskite blocks”
and (b) in the translational interface. (c) The difference lies in a critical
zone (yellow rectangle) responsible for the two ordered nanophases.
(d) The global structure can be written as (Sr16Pb4)(FeþMn)26M4O64
.
rectangle, is critical for the formation of the two models.
If this zone is occupied by one octahedron (i.e., no oxygen
vacancy) and two Pb ions (Figure 7b), the structure is that
of model 2; if it is occupied by two pyramids (i.e., oxygen
vacancy) and one Sr(Pb) cation, the structure is that of
model 1. The above models have the same amount of
cations, that is Pb þ Sr þ (Fe þ Mn)=50; the difference
lies in the fact that one Fe pyramidal site (M=Fe) and one
Pb site (M = likely Pb or Sr) can be exchanged in the
highlighted zone. Considering the general formula
Sr16Pb4(Fe þ Mn)26M4O64, the nature of the M cation
is associated with one or the other structure in the
common framework “(Sr16Pb4)(FeþMn)26” (Figure 7d)
built of the phases 1 and 2.
Finally, it is important to outline that the two models
presented herein are associated with a sole monoclinic cell,
with the commensurate q vector q = 2/25a*p þ 3/25cp*, and
allow the structural mechanism of the average structure to be
explained. However, keeping in mind that the ED investiga-
tion has evidenced local losses of the periodicity, the origins of
the incommensurate modulated structures must be explained
through the mechanism of the phasoid formation. The study
of these nonstoichiometry mechanisms is in progress.
These characteristics of closely related structures rather
obey the concept of a phasoid, as described by Magneli.34
This is illustrated in the STEM image (Figure 8) where the
coexistence of both nanophases 1 and 2 is highlighted.
Acknowledgment. The authors acknowledge financial
support from the European Union under the Framework
6 program under a contract for an Integrated Infrastruc-
ture Initiative, reference 026019 ESTEEM.
(34) Magneli, A. Microsc. Microanal. Microstruct. 1990, 1, 299–302.