Synthesis and crystal structure of the novel Pb5Sb 2MnO11 compound

Article abstract of DOI:10.1016/j.jssc.2004.04.047

The new Pb₅Sb₂MnO₁₁ compound was synthesized using a solid-state reaction in an evacuated sealed silica tube at 650°C. The crystal structure was determined ab initio using a combination of X-ray powder diffraction, electron diffraction and high-resolution electron microscopy (a=9.0660(8)A, b=11.489(1)A, c=10.9426(9)A, S.G. Cmcm, R_I=0.045, R_P=0.059). The Pb₅Sb ₂MnO₁₁ crystal structure represents a new structure type and it can be considered as quasi-one-dimensional, built up of chains running along the c-axis and consisting of alternating Mn⁺²O₇ capped trigonal prisms and Sb₂O₁₀ pairs of edge sharing Sb⁺⁵O₆ octahedra. The chains are joined together by Pb atoms located between the chains. The Pb⁺² cations have virtually identical coordination environments with a clear influence of the lone electron pair occupying one vertex of the PbO₅E octahedra. Electronic structure calculations and electron localization function distribution analysis were performed to define the nature of the structural peculiarities. Pb ₅Sb₂MnO₁₁ exhibits paramagnetic behavior down to T=5K with Weiss constant being nearly equal to zero that implies lack of cooperative magnetic interactions.

Full text of DOI:10.1016/j.jssc.2004.04.047

ARTICLE IN PRESS
Journal of Solid State Chemistry 177 (2004) 2855–2861
Synthesis and crystal structure of the novel Pb5Sb2MnO11 compound
Artem M. Abakumov,a,* Marina G. Rozova,a Pavel S. Chizhov,a Evgeny V. Antipov,a
Joke Hadermann,b and Gustaaf Van Tendeloob
a Department of Chemistry, Moscow State University, Leninskie Gory, Moscow 119992, Russia
b EMAT, University of Antwerp (RUCA), Groenenborgerlaan 171, B-2020 Antwerp, Belgium
Received 8 January 2004; received in revised form 21 April 2004; accepted 25 April 2004
Abstract
The new Pb5Sb2MnO11 compound was synthesized using a solid-state reaction in an evacuated sealed silica tube at 650ꢀC. The
crystal structure was determined ab initio using a combination of X-ray powder diffraction, electron diffraction and high-resolution
electron microscopy (a ¼ 9:0660ð8Þ A, b ¼ 11:489ð1Þ A, c ¼ 10:9426ð9Þ A, S.G. Cmcm, RI ¼ 0:045; RP ¼ 0:059). The Pb5Sb2MnO11
crystal structure represents a new structure type and it can be considered as quasi-one-dimensional, built up of chains running along
the c-axis and consisting of alternating Mn+2O7 capped trigonal prisms and Sb2O10 pairs of edge sharing Sb+5O6 octahedra. The
chains are joined together by Pb atoms located between the chains. The Pb+2 cations have virtually identical coordination
environments with a clear influence of the lone electron pair occupying one vertex of the PbO5E octahedra. Electronic structure
calculations and electron localization function distribution analysis were performed to define the nature of the structural
peculiarities. Pb5Sb2MnO11 exhibits paramagnetic behavior down to T ¼ 5 K with Weiss constant being nearly equal to zero that
implies lack of cooperative magnetic interactions.
r 2004 Elsevier Inc. All rights reserved.
Keywords: Pb5Sb2MnO11; Preparation; Crystal structure; Electron microscopy
1. Introduction
2. Experimental
Recently, we reported the synthesis and structural
investigation of (Sb1ÀxPbx)2(Mn1ÀySby)O4 (0:0pxp
0:608; 0:0pyp0:372) solid solutions (Abakumov et al.,
submitted to J. Solid State Chem., 2003) with the
Sb2MnO4-derived structure [1,2]. Upon variation of the
cation composition above the upper limit of the
homogeneity range (i.e., for large Pb content), a new
phase with a C-centered orthorhombic unit cell was
observed as an admixture. In this contribution we report
the preparation, crystal structure and chemical bonding
analysis for this Pb5Sb2MnO11 compound which pos-
sesses an original structure and represents a new
structure type.
The samples were prepared in evacuated sealed silica
tubes using solid-state reaction of various mixtures of
different starting materials. Commercial (Reakhim,
‘‘chemically pure’’ purity grade, AlfaAesar, 99.99%)
PbO, Pb3O4, P bO, Sb2O3, MnO, Mn2O3 and MnO2
2
were used as initial reagents. The amounts of initial
reagents taken according to the required cation ratio,
oxygen content and overall sample weight of 0.5–0.6 g,
were intimately mixed under acetone and pressed into
pellets. The pellets were placed into alumina crucibles to
prevent interaction with the ampoule walls and sealed
under dynamic vacuum of 10À3 mbar into silica tubes
with a diameter of 12 mm and a length of 60 mm. The
samples were annealed at 650ꢀC for 50–150 h with
intermediate regrinding and then finally furnace cooled.
Phase analysis and cell parameter determination were
performed using X-ray powder diffraction with a
focusing Guinier-camera FR-552 (CuKa1-radiation, Ge
was used as an internal standard) and a Philips X’pert
*Corresponding author. Fax: +7-95-939-47-88.
E-mail address: abakumov@icr.chem.msu.ru (A.M. Abakumov).
0022-4596/$ - see front matter r 2004 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2004.04.047

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diffractometer (CuKa-radiation, reflection geometry,
proportional counter). X-ray powder diffraction data
for crystal structure determination were collected on a
STADI-Pdiffractometer (Cu Ka1-radiation, curved Ge
monochromator, transmission mode, linear PSD). CSD
program package was used for ab initio structure
determination and the Rietveld refinement from powder
X-ray diffraction data [3]. The profile reliability factors
were calculated after background intensity subtraction.
Samples for electron microscopy were made by
grinding the powder sample in ethanol and depositing
it on a holey carbon grid. Electron diffraction (ED)
patterns and EDX spectra were obtained using a Philips
CM20 electron microscope. EDX analysis was per-
formed with Pb(Ma), Sb(La) and Mn(Ka) lines, using
Pb2MnO4 and Sb2MnO4 as standards. High-resolution
electron microscopy (HREM) observations were made
on a JEOL 4000EX instrument, having a point-to-point
resolution of 1.7 A. Image simulations were carried out
with the MacTempas software.
of Pb5Sb2MnO11 was indexed on an orthorhombic
lattice with cell parameters
a ¼ 9:0660ð8ÞA,
b=11.489(1)A, c ¼ 10:9426ð9ÞA. The observed h k l :
h þ k ¼ 2n and h 0 l : h; l ¼ 2n reflection conditions
suggest Cmcm as the highest space group for this
compound. Syntheses starting from combinations
of other oxides of constituent metals (e.g.,
PbO2þ3Pb3O4þ2MnO þ 2Sb2O3 or 3PbO2+7PbO+
Mn2O3+2Sb2O3) that provide the same total stoi-
chiometry also allow to prepare samples with
Pb5Sb2MnO11 as a main constituent, but with a larger
amount of admixture phases (up to 7–9% of the
pyrochlore, PbO and Mn3O4). Deviation from the
stoichiometric composition by decreasing the Pb content
by 10 at% or increasing the oxygen content up to
Pb5Sb2MnO11.2 result in a drastic increase of the
amount of impurity phases and yield the pyrochlore
phase as the main phase in the samples. Increasing the
Mn content (‘‘Pb4.5Sb2Mn2O12’’ sample), however,
allows to suppress the formation of the pyrochlore
admixture, but results in the appearance of 3–4% of
Mn3O4 and an unknown phase. The unit cell parameters
of Pb5Sb2MnO11 remain the same within the range of
two standard deviations for all samples, which indicates
that the compound composition does not depend on the
choice of the initial reagents and synthesis conditions.
The cation composition of the Pb5Sb2MnO11 phase was
confirmed by EDX analysis in the transmission
electron microscope performed at 12 points on four
different crystallites. The cation ratio Pb:Sb:Mn=
5.03(7):1.98(7):0.99(3) was found to be very close to
the nominal one.
Magnetic susceptibility measurements were per-
formed with a Quantum Design SQUID magnetometer
(MPMS-XL) in a 1000 Oe applied field in the tempera-
ture range of 5–385 K.
Electronic structure calculation and electron localiza-
tion function (ELF) analysis were carried out with the
TB-LMTO-ASA program [4]. The Barth–Hedin LDA
functional [5] with additional Langreth–Mehl–Hu non-
local part was used for exchange-correlation treating in
the SCF calculation. The k-space integration was
performed by the standard tetrahedron method using
14 k-points within the irreducible Brillouin zone. The
basis set consisted of 6s; 6p-orbitals for Pb, 5s; 5p-
orbitals for Sb, 4s; 4p; 3d-orbitals for Mn and 2p-
orbitals for O. 6d; 5f -orbitals for Pb, 5d; 4f -orbitals for
Sb, 3s; 3d-orbitals for O were treated using a down-
folding technique. The spin-polarized case was consid-
ered; all Mn sites were treated as equivalent. The ELF
distribution was calculated using the intrinsic procedure
of the TB-LMTO-ASA program complex.
3.2. Crystal structure and ELF distribution analysis
The crystal structure of Pb5Sb2MnO11 was solved ab
initio from X-ray powder diffraction data. The integral
intensities of the reflections over the angular range 10–
80ꢀ 2y were determined by profile fitting and then used
for the direct structure solution. The Cmcm space group
was assumed for the initial structure model. The analysis
of the resulting electron density map revealed the
positions of the Pb atoms. The atomic coordinates of
the Sb, Mn and oxygen atoms were found by a sequence
of difference Fourier syntheses. Prior to the Rietveld
refinement of the obtained structure model the impacts
of six impurity peaks of Mn3O4 and the unknown phase
were subtracted from the raw data using their fitted
profiles since they did not overlap with the peaks from
the main phase. The refinement was carried out in an
isotropic approximation for the atomic displacement
parameters. Preferred orientation along the [110] direc-
tion was taken into account using the March–Dollase
function. After sequential iterations, a good agree-
ment between the experimental and calculated profiles
was achieved: RI ¼ 0:045; RP ¼ 0:059; RwP ¼ 0:052: An
3. Results and discussion
3.1. Sample preparation
The target compound was prepared starting from the
4PbO2+PbO+MnO2+Sb2O3 mixture. After annealing
at 650ꢀC for 50 h, the Pb5Sb2MnO11 phase with a large
amount of the Pb2Sb2O7 pyrochlore, PbO and Mn3O4
was obtained. The subsequent annealing for 50 h after
regrinding the sample significantly improved the sample
quality leaving only a small amount of the pyrochlore
admixture (the intensity of the strongest peak is
Irel ¼ 4:0%) and traces of metallic Pb. Pb5Sb2MnO11
has a yellow-greenish color. The X-ray powder pattern

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attempt to refine the crystal structure with the Cmc21
Table 3
Selected interatomic distances for Pb5Sb2MnO11 (A)
space group did not improve reliability factors and
resulted in strong correlations between the atomic
coordinates of Pb atoms, which are symmetrically
related in the Cmcm space group. The crystallographic
parameters, atomic coordinates and the most relevant
interatomic distances for Pb5Sb2MnO11 are listed in
Tables 1–3. The observed, calculated, and difference
X-ray diffraction patterns are shown in Fig. 1.
Pb1–O1
Pb1–O4
Pb2–O1
Pb2–O3
Pb2–O4
Pb3–O1
Pb3–O3
Pb3–O4
2.317(13) ꢁ 1
2.401(6) ꢁ 4
2.514(1) ꢁ 1
2.467(6) ꢁ 2
2.396(5) ꢁ 2
2.494(1) ꢁ 1
2.403(6) ꢁ 2
2.559(5) ꢁ 2
Sb–O2
Sb–O3
Sb–O4
Sb–Sb
2.041(5) ꢁ 2
1.999(5) ꢁ 2
1.961(6) ꢁ 2
3.159(1) ꢁ 1
2.454(13) ꢁ 1
2.379(8) ꢁ 2
2.261(6) ꢁ 4
Mn–O1
Mn–O2
Mn–O3
To our knowledge, the Pb5Sb2MnO11 structure has no
analogues, and therefore it represents a new structure
type. The crystal structure of Pb5Sb2MnO11 can be
considered as quasi-one-dimensional, built up of chains
of manganese-oxygen and antimony-oxygen polyhedra
running along the c-axis and joined together by Pb
atoms located between the chains (Fig. 2). Each chain
consists of Sb2O10 units built up of two SbO6 octahedra
with the O2, O3 and O4 atoms as vertices linked by the
common O2–O2 edge. The Sb2O10 units are located
above each other along the chain direction so the two
neighboring units are mirror-related. Two O2 and four
O3 atoms form a trigonal prismatic cavity between these
two units, which is filled by Mn cations. The coordina-
tion environment of the Mn atoms is completed up to
Table 1
Selected parameters from Rietveld refinement of X-ray powder data
for Pb5Sb2MnO11
Fig. 1. The experimental, calculated and difference X-ray diffraction
patterns for Pb5Sb2MnO11
.
Formula
Space group
Pb5Sb2MnO11
Cmcm
CN=7 by an O1 atom which forms a cap above one
rectangular face of the trigonal prism. The tendency of
the SbO6 octahedra to share common edges forming
Sb2O10 pairs (K3Sb5O14 [6], MSbO3, (M=Li, K, Ag)
[7–9], MSb2O6 (M=Co, Cu) [10,11]), chains (Na3SbO4
[12]), layers (CdSb2O6 [13], MSb2O6 (M=Mn, Ca)
[14,15], TlSbO3 [16]) or frameworks (K2Sb4O11 [6]) is a
characteristic feature of Sb+5 cations. The average
interatomic distances /dðSb2OÞS ¼ 2:000 A and
/dðMn2OÞS ¼ 2:322 A are in agreement with the ionic
radii of the Sb+5 and Mn+2 cations (r(Sb+5)=0.75 A
a (A)
b (A)
c (A)
9.06282(5)
11.48250(6)
10.94024(6)
4
Z
Cell volume (A3)
Calculated density (g/cm3)
2y range, step (deg)
Number of reflections
Refinable parameters
Preferred orientation parameter and axis
Reliability factors:
RI; RP; RwP
1139.08(3)
5.721
10p2yp110, 0.01
431
27
1.428(7), [110]
0.045, 0.059, 0.052
for
CN=6,
r(Mn+2
,
CN=7)E1/2(r(Mn+2
,
CN=6)+r(Mn+2, CN=8))=1.02 A, r(OÀ2)=1.26 A).
The coordination environments of the Pb1, Pb2 and Pb3
atoms are virtually identical. They can be represented as
a distorted tetragonal pyramid where the apical vertex is
formed by O1 atoms, and the square base consists of
either four O4 atoms (for Pb1) or two O3 and two O4
ones (for Pb2 and Pb3) (Fig. 3). The lead atoms are not
located inside the pyramids, but are shifted outside,
away from the O1 atom. One may assume that such a
distortion can be understood if the influence of the lone
electron pair of the Pb+2 cations is taken into account.
The lone pair completes the coordination environment
of the Pb atoms to a PbO5E (E=lone electron pair)
octahedron and is directed along the apical axis of the
Table 2
Positional and atomic displacement parameters of atoms for
Pb5Sb2MnO11
Atom Position x/a
y/b
z/c
Biso (A2)
Pb1
Pb2
Pb3
Sb
4c
8g
0
0.27674(7) 0.21715(6) 3/4
0.99957(8) 1/4
1.45(2)
1.57(2)
0.21923(7) 0.52277(5) 1.32(2)
8f
8e
0
0.3257(1)
1/2
0
1/2
1/4
1.04(3)
1.31(8)
0.4(3)
0.7(2)
1.1(2)
0.8(2)
Mn
O1
4c
4c
0.0841(3)
0.202(1)
0.0635(7)
0.1401(5)
0.0729(4)
0
1/2
3/4
O2
O3
8f
0.5974(7)
0.3918(5)
0.6116(5)
16h
16h
0.3330(7)
0.1839(6)
O4

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Fig. 3. Coordination environment of the Pb1, Pb2 and Pb3 cations in
the Pb5Sb2MnO11 crystal structure. Orientation of the lone electron
pairs of the Pb cations is shown. The right bottom figure represents the
Pb5O9 cluster consisting of PbO5E octahedra.
electron localization, or, in other words, the lone
electron pair (Fig. 5, Z ¼ 0:85 isosurface), exists near
the open corner of the PbO5E octahedra. The absence of
an electron localization near Sb and between two Sb
atoms of the Sb2O10 units allows to suggest that Sb in
the Sb2O10 units has the formal oxidation number +5
and there is no Sb–Sb bonding even in spite of the
relatively short Sb–Sb distance of 3.159(1)A. After
analysis of DOS, projected DOS (Fig. 6) and moments
on the atoms one can conclude that Mn2+ is strongly
spin-polarized, or, in other words, there is high-spin d5
configuration of Mn2+ ion in this compound. Fermi
level in this case lies just above the occupied non-
Fig. 2. [100] and [001] projections of the Pb5Sb2MnO11 crystal
structure. Sb atoms are located inside the octahedra, Mn atoms are
inside the capped trigonal prisms. White, gray and black spheres
represent Pb1, Pb2 and Pb3 atoms, respectively.
octahedron away from the O1 atom. The location of the
O3 and O4 atoms then follows the valence shell electron
pair repulsion model assuming the requirement of a
larger space for the lone pair in the vicinity of the Pb
atom than for a bonding pair. The PbO5E octahedra
around one Pb1, two Pb2 and two Pb3 atoms are joined
together by common faces forming a Pb5O9 cluster with
the O1 atom at the center and the lone pairs directed
away from the cluster center (Fig. 3). The location of
these clusters in the unit cell is shown in Fig. 4. The
orientation of lone pair clouds with respect to the crystal
axes can be revealed from this figure: the lone pair
clouds are aligned along [010] for Pb1, [100] for Pb2 and
[001] for Pb3. The assumption about the lone electron
pairs, localized on the Pb atoms, was further proved by
the analysis of the ELF distribution, which confirms the
nature of the coordination peculiarities in the
Pb5Sb2MnO11 structure. Thus, an area of strong
bonding eg orbitals of octahedrally coordinated Mn2+
.
All other atoms are practically spin-unpolarized. The
substance in this case is a semiconductor with the
estimated band gap of about 0.88 eV.
The experimentally determined coordination environ-
ments, the Pb5Sb2MnO11 composition and the ELF
distribution analysis suggest the formal valences of Pb,
Sb and Mn cations to be equal to +2, +5 and +2,
respectively. The coexistence of Mn+2 and Sb+5 in the
same structure was also found before in MnSb2O6 [14]
and Mn2Sb2O7 [17] oxides. The valences were also
verified by BVS calculations which give the values of
+2.40 for Pb1, +2.03 for Pb2, +2.01 for Pb3, +5.14
for Sb and +1.70 for Mn. For the Pb2, Pb3 and Sb
atoms the calculated BVS values agree well with the
proposed valence distribution, but the bond valence sum
is overestimated for Pb1 and underestimated for Mn.
The most probable explanation is that the X-ray powder

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Fig. 5. ELF distribution near Pb in PbO5E octahedra (Z ¼ 0:85
isosurface, Pb2 is taken as an example, the Pb1 and Pb3 atoms exhibit
similar features).
Fig. 4. Location of the Pb5O9 clusters in the Pb5Sb2MnO11 crystal
structure.
Fig. 6. Total DOS and Mn-d-spin up/spin down projected DOS curves
near Fermi level.
refinement does not allow to determine precisely the y/b
coordinate for the O1 atom. Indeed, shifting the O1
atom from the 0, 0.202, 0.75 position towards 0, 0.235,
0.75 allows to obtain the BVS values of +2.04 and
+1.99 for the Pb1 and Mn cations, respectively and
almost does not change the BVS values for the Pb2 and
Pb3 cations. However, the atomic coordinate for the O1
atom always converges to y/b=0.202. One might expect
that the refinement of the positions of the oxygen atoms
in the presence of such heavy scatterers as Pb cannot be
very precise in the case of conventional X-ray powder
diffraction, and the refinement from neutron diffraction
data will be helpful in this case.
with the unit cell parameters determined using X-ray
powder diffraction. The presence of the h k l: h+k=2n
and h 0 l: h,l=2n reflection conditions is clearly visible
on the [010]ꢂ, [001]ꢂ and [101]ꢂ ED patterns. The
forbidden 0 0 l: la2n reflections on the [100]ꢂ and
[110]ꢂ patterns are caused by multiple diffraction as
was confirmed by tilting the crystal around the cꢂ-axis.
No other extinction conditions or extra reflections were
observed, which confirms the chosen unit cell and the
Cmcm space symmetry.
The HREM images taken along the [100] and [110]
zone axes are shown in Fig. 8. Oxygen columns and
cation columns are projected separately viewed along
the [100] direction. At this thickness and defocus value
the nearly square pattern of the bright dots represents
the columns of oxygen atoms. The inserted theoretical
image, calculated at Df=À200 A and t ¼ 110 A using
3.3. Electron microscopy analysis
ED patterns for Pb5Sb2MnO11 taken along the most
prominent zone axes are shown in Fig. 7. All patterns
can be indexed completely on an orthorhombic lattice

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Fig. 8. The [100] and [110] HREM images of Pb5Sb2MnO11. Insets
show the theoretical images calculated at Df=À200 A and t ¼ 110 A
(for the [100] zone axis) and at Df=300 A and t ¼ 40 A (for the [110]
zone axis).
Fig. 7. [100]ꢂ, [010]ꢂ, [001]ꢂ, [101]ꢂ and [110]ꢂ ED patterns for the
Pb5Sb2MnO11 phase.
the atomic coordinates as determined from X-ray
powder diffraction, reproduces well the square pattern
of bright dots and the intensity variation in the dot rows
along the c-axis. The [110] HREM image shows the
projection of the cation columns as a centered rectan-
gular bright dot pattern. The theoretical image calcu-
lated with Df=300 A and t ¼ 40 A is in good agreement
with the experimental one. The HREM images exhibit a
faultless structure and no extended defects are found.
Fig. 9. Temperature dependence of the dc magnetic susceptibility of
Pb5Sb2MnO11. Inset shows inverse susceptibility vs. T plot.
3.4. Magnetic properties
are well separated by non-magnetic entities (Sb2O10
octahedral clusters).
The temperature dependence of dc susceptibility
revealed paramagnetic behavior (Fig. 9). A fit of the
inverse susceptibility to the Curie–Weiss law yielded the
effective moment of 5.81 mB, in agreement with a
theoretical spin-only value for the Mn+2 (d5, high spin)
of 5.92 mB. The Weiss constant (y) is zero within the
error, indicating the absence of significant magnetic
interactions between Mn+2 spins. This behavior is not
unexpected because magnetic ions (Mn+2) in the chains
Acknowledgments
The authors are grateful to M. Lobanov and M.
Greenblatt for the magnetic measurements. The work
was supported in part by the program IAPV-1 of the
Belgium government. Acknowledgment is made to the

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A.M. Abakumov et al. / Journal of Solid State Chemistry 177 (2004) 2855–2861
2861
donors of the American Chemical Society Petroleum
Research Fund for partial support of this research
(project 38459-AC5). A.M.A. is grateful to INTAS for
the Fellowship grant for Young Scientists YSF 2002-48.
E.V.A. is grateful to Russian Science Support Founda-
tion for the financial support.
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