Synthesis and properties of a new quadruple perovskite: A-site ordered PbMn3Mn4O12

Article abstract of DOI:10.1016/j.jssc.2012.02.036

PbMn₃Mn₄O₁₂ a quadruple perovskite was prepared by high pressure and high temperature synthesis. Powder X-ray diffraction (PXD) and differential scanning calorimetry reveal a structural phase transition at ～380 K. Rietveld refinement of the synchrotron room temperature data indicate rhombohedral symmetry (R-3) with a=6.43675(4) A and α=109.556(2)°. Similar 423 K PXD data refined in a body centered cubic cell (Im-3) with a=7.4283(9) A. The temperature variation of magnetization, shows a magnetic field dependent antiferromagnetic-like transition at 68 K, and dynamic fluctuations indicative of magnetic frustration. The semiconducting electrical behavior indicates a large decrease in the conductivity near 68 K. The temperature dependence of the real part of the dielectric constant, ε_real increases dramatically at ～68 K, and shows relaxor-type ferroelectric behavior as a function of frequency. The intimate coupling of magnetic, electrical and dielectric properties at 68 K in PbMn₃Mn₄O₁₂ suggests possible multiferroic behavior.

Full text of DOI:10.1016/j.jssc.2012.02.036

Journal of Solid State Chemistry 190 (2012) 277–284
Contents lists available at SciVerse ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Synthesis and properties of a new quadruple perovskite: A-site ordered
PbMn Mn O
3
4 12
a
a
a
b,n
a
T. Locherer , R. Dinnebier , R.K. Kremer , M. Greenblatt , M. Jansen
a
Max Planck Institute for Solid State Research, Heisenbergstr. 1, D-70569 Stuttgart, Germany
b
Department of Chemistry and Chemical Biology, Rutgers University, Piscataway, NJ 08854, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
3 4 12
PbMn Mn O a quadruple perovskite was prepared by high pressure and high temperature synthesis.
Received 23 October 2011
Received in revised form
Powder X-ray diffraction (PXD) and differential scanning calorimetry reveal a structural phase
transition at ꢀ380 K. Rietveld refinement of the synchrotron room temperature data indicate
5
February 2012
˚
rhombohedral symmetry (R-3) with a¼6.43675(4) A and
a
¼109.556(2)1. Similar 423 K PXD data
Accepted 12 February 2012
Available online 21 February 2012
˚
refined in a body centered cubic cell (Im-3) with a¼7.4283(9) A. The temperature variation of
magnetization, shows a magnetic field dependent antiferromagnetic-like transition at 68 K, and
dynamic fluctuations indicative of magnetic frustration. The semiconducting electrical behavior
indicates a large decrease in the conductivity near 68 K. The temperature dependence of the real part
Keywords:
3 4 12
PbMn Mn O
A-site ordered quadruple perovskite
Structure
of the dielectric constant,
behavior as a function of frequency. The intimate coupling of magnetic, electrical and dielectric
properties at 68 K in PbMn Mn 12 suggests possible multiferroic behavior.
ereal increases dramatically at ꢀ68 K, and shows relaxor-type ferroelectric
Magnetic and dielectric properties
Multiferroic
3
4
O
&
2012 Elsevier Inc. All rights reserved.
1
. Introduction
example, Ti in BaTiO
3
with respect to the oxygen octahedral
coordination [1]. One possible mechanism for the coexistence of
ferroelectricity and magnetism in perovskites is the presence in
the A site of an ion with a nonbonding pair of electrons in an outer
shell, a lone pair, whose stereochemical effect may be the origin
Materials that are both ferroelectric and magnetic, referred to
as multiferroics, are rare. This is because in most ferroelectrics,
such as BaTiO , the ferroelectricity is driven by a hybridization of
3
empty d orbitals with occupied p orbitals of the octahedrally
coordinating oxygen ions. This mechanism requires empty d
orbitals and thus can not lead to multiferroic behavior. There
are consequently very few ferroelectrics that exhibit long range
magnetic order, and rarer still are materials where these two
different order parameters are present together, and exhibit
substantial coupling. In multiferroics with significant coupling
of the magnetic and ferroelectric order parameters, it has been
established that it is possible to control the magnetic phase with
an applied electric field, and the electric polarization with an
applied magnetic field. Therefore, recently multiferroics have
been of particular interest both to understand the fundamental
aspects of the novel mechanism that gives rise to the magnetic–
ferroelectric coupling, and also for the intriguing possibility of
using these in novel device applications as actuators, switches,
magnetic field sensors, or electronic memory devices.
3 3
of ferroelectricity. Examples include BiFeO and PbVO [2,3].
Magnetic behavior is usually induced by partially filled d orbitals
on the B site.
0
Quadruple perovskites with the general formula AA
3
B
4
O
12
were first prepared by Marezio et al. where A is a monovalent
0
2þ
3þ
divalent or trivalent cation, A is Cu
(JT), and B a large variety of transition metal or main group
elements [4]. The structure with a cubic cell ofꢀ2a ꢁ 2a ꢁ 2a
(a is the aristotype perovskite lattice parameter) is a consider-
ably distorted variant of the perovskite structure; the B cations, as
in the ABO perovskite, occupy the center of significantly tilted,
or Mn , a Jahn–Teller ion
p
p
p
p
3
corner-sharing octahedra, which form a three-dimensional (3D)
0
network structure. The A and A cations, located in the vacancies
6
of the 3D structure of BO octahedra, order with a ratio 1:3. The A
cation site is 12-coordinated by oxygen atoms, similar to the
0
cubo-octahedral perovskite site. The A site is grossly distorted
0
In usual perovskite-based materials, the ferroelectric distor-
tion occurs due to the displacement of the B site cation for
due to the Jahn–Teller (JT) distortion in such a way, that the A ion
becomes square planar (4-fold) coordinated. The quadruple per-
ovskites are generally metastable and require high pressure and
temperature for their synthesis.
n
Recently, BiMn
3 4
Mn O12 a quadruple perovskite was prepared
Correxsponding author. Fax: þ732 445 5312.
E-mail address: martha@rutchem.rutgers.edu (M. Greenblatt).
at high pressure and shown to be multiferroic [5]. The space
0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2012.02.036

2
78
T. Locherer et al. / Journal of Solid State Chemistry 190 (2012) 277–284
group symmetry of BiMn
centrosymmetric, monoclinic Im, with the Bi
the cubooctahedra of oxygen atom environment, due to the 6s
lone-pair electrons [6]. The magnetic and dielectric anomalies
occur at ꢀ58 and 26 K and are presumed to be coupled.
Following the finding of multiferroic behavior in
3
Mn
4
O
12 was established to be non-
crystal before monitoring with a NaI scintillation detector. Data
were taken for 8.0 s at each 2 step of 0.0051 from 71 to 701. The
sample was spun during measurement for better particle statis-
tics. All data were normalized for storage ring current decay by an
ionization chamber monitor.
3
þ
ion off center in
y
2
7
PXD data of PbMn O12 were also collected at a temperature of
2
þ
BiMn
3
Mn
4
O
12, we prepared the Pb
analog, PbMn
3
Mn
4
O
12, also
3þ/4þ
423 K with a Bruker D8 diffractometer in Debye–Scherrer geo-
metry (primary beam Johannson-type Ge(2 2 0) monochromator
2
with 6s lone pair electrons on the A site, mixed valent Mn
on the B site, and find evidence for possible multiferroic behavior.
Results of this work are herein reported.
for MoK
strips of 500
sample contained in a sealed borosilicate glass capillary of
.3 mm diameter. For the measurement at elevated temperature,
a
1
radiation, one-dimensional LynxEye PSD with silicon
mm thickness and an opening angle of 3.51) with the
0
2
. Experimental
a calibrated water-cooled capillary heater stage (mri Physika-
lische Ger a¨ te GmbH) with a temperature stability better than 1 K
was used. PXD data were recorded for 24 h with a step width of
The title compound was synthesized from a stoichiometric
mixture of MnO, MnO
compound was synthesized from stoichiometric proportions of
the precursors via the reaction:
2
and PbO (Alfa Aesar, 99.9995%). The title
0.0091 in the 2
measurements for better particle statistics.
Y
range 6.0ꢃ60.01. The sample was spun during
For data evaluation of both PXD patterns, the program TOPAS
version 4.1. (Bruker-AXS) was used. Indexing of the diffraction
data of both phases was performed by iterative use of singular
value decomposition as implemented in the program TOPAS [8],
leading to a rhombohedral unit cell for the room temperature
phase and a body centered cubic unit cell for the high tempera-
ture phase (Table 1). The most probable space groups were
determined as R3, R-3, R32, R3m, R-3m for the room temperature
2 7 12
PbOþ3MnOþ4MnO -PbMn O
Reactive MnO was obtained by thermal decomposition of
Mn (99.995%, Sigma Aldrich) under a 90% Ar, 10% H flow
600 1C, 2 h, then 800 1C, 18 h). MnO was synthesized by thermal
decomposition of Mn(NO
2
O
3
2
(
2
3
)
2
ꢂ 4H O (97.5þ % Acros Organics) in
2
air (400 1C, 72 h). In order to introduce a slight excess of oxygen,
commercial PbO (Sigma Aldrich, 99.999%) was combined with
1
phase and I23, I2 3, Im-3, I432, I-43m, or Im-3m for the high
temperature phase from the observed extinction rules. From
volume increments, Z was determined to be 1 for the room
temperature and 2 for the high temperature phase. The peak
profile and precise lattice parameters were determined by a Le
Bail fit with the fundamental parameter (FP) approach of TOPAS
Pb
3
O
4
in the proportions 8.79:1 by weight, keeping the total Pb
was prepared
mass consistent with reaction stoichiometry Pb
3 4
O
by precipitation from a basic solution of K–Pb hydroxides [7]. The
precursors were ball milled for 1 h to ensure small grain size and
intimate mixing. The resulting powder was compacted into
pellets which were packed into gold capsules (diameter: 4 mm).
[
9,10]. As the geometry of the LynxEye PSD is not fully character-
ized by FP’s, fine tuning of the available parameters was per-
formed with refined values of the FP’s from precise
measurement of the NIST line profile standard SRM 660a (LaB
2 3
The gold capsules were loaded into MgO/Cr O octahedra (edge
a
length 25 mm). Subsequently, synthesis was done under high
pressure and high-temperature utilizing a 6–8-type multi anvil
press (maximum load 106 kg, Max Voggenreiter GmbH, Mainleus,
Germany) equipped with a Walker module and 32 mm WC-cubes
6
)
in a 0.1 mm capillary over the full two theta range of the
diffractometer. For the modeling of the background, Chebychev
polynomials of higher order were employed. In addition, for the
high temperature phase, several broad, but small reflections
originating from the capillary furnace were successfully modeled
by Lorentzian-shaped peaks.
(truncation edge length 15 mm). The sample pressure was deter-
mined by a pre-established pressure/load calibration curve. High
temperatures were provided by a stepped graphite resistance
heater. The temperature was monitored with a type C thermo-
couple. Typical experimental conditions were p¼7.5 GPa,
T¼1173–1273 K for t¼4–6 h. Experiments were terminated by
rapidly quenching the sample to ambient temperature followed
by a slow pressure release (15 h). The sample was recovered from
the capsule with tools fabricated from nonmagnetic Cu–Be-alloy.
Prior to X-ray powder diffraction experiments the black powder
obtained was thoroughly ground and annealed at 353–373 K in
air to release induced strain. Samples used for impedance spectro-
scopy and conductivity measurements were cylindrical bulk
samples, as recovered after high pressure synthesis, and were
not subjected to further grinding.
Table 1
Crystallographic and rietveld refinement data for the room (RT) and high
7 12
temperature (HT) phases of PbMn O .
RT-phase
PbMn
HT-phase
7 12
PbMn O
Sum formula
7
O
12
Formula weight (g/mol)
Space group
320.75
320.75
R3
Im3
Z
1
2
a ( A˚ )
6.43675(4)
7.4283(9)
a
(1)
3
109.556(2)
204.647(4)
–
V (A˚
Temperature (K)
)
409.9(1)
2.1. Powder X-ray diffraction
298
423
ꢃ3
r
(calc.)/ (g cm
)
6.360(1)
6.350(2)
0.7093
Wavelength ( A˚ )
0.7000(1)
7
Powder X-ray diffraction (PXD) data of PbMn O12 was col-
n
R-exp (%)
11.25
11.01
13.92
5.45
1.24
6.0
8.48
7.29
10.06
2.18
1.19
3.0
lected at room temperature in the Debye–Scherrer mode at the
X16C beamline at the National Synchrotron Light Source, Broo-
khaven National Laboratory. The sample was contained in a
sealed borosilicate glass capillary with 0.3 mm diameter (Hilgen-
n
R-p (%)
n
R-wp (%))
2
n
R-F (%)
GoF
˚
Starting angle (1 2y)
berg glass No. 50). X-ray radiation of wavelength ofꢀ0.7 A was
Final angle (1 2
Step width (1 2
Time/scan (h)
y
y
)
60.0
0.005
24
60.0
0.009
24
selected by a double Si(1 1 1) monochromator. The wavelength
and zero point error have been calibrated with eight precisely
measured peaks of the NBS1976 flat plate Alumina standard. The
diffracted beam was analyzed by reflection from a Ge(1 1 1)
)
n
2
R-exp, R-p, R-wp, and R-F as defined in TOPAS (Bruker AXS).

T. Locherer et al. / Journal of Solid State Chemistry 190 (2012) 277–284
279
2.2. Thermal analysis
or an Alpha High Resolution Dielectric Analyzer from 300 K to
73 K on a pellet of 0.8 mm thickness and 11 mm diameter with a
4
Analysis of the thermal behavior of PbMn
with a Netzsch DSC 404 calorimeter, which for c
was calibrated against a sapphire standard. Samples were placed
into platinum crucibles and closed with loosely fitting platinum
lids. A constant argon flow was applied during measurement to
prevent sample deterioration.
7
O
12 was performed
measurements
test signal level of 1 Vrms. The two capacitor electrodes were
applied to the pellet with a two-component silver epoxy resin.
Analysis of the measured complex impedances was performed in
terms of a parallel circuit of a capacitance and a resistor.
p
2.4. Electric and magnetic properties
2.3. Dielectric properties
Temperature and field dependent magnetic susceptibilities
were determined with a SQUID magnetometer (MPMS, Quantum
Design) in the temperature range of 1.7 to 300 K. The samples
were placed into a gelatin capsule. The measured magnetization
values were corrected for the sample holder contributions. Data
Temperature and frequency dependent impedance spectro-
scopy was performed on polished bulk samples either with a HP
284 A Precision LCR Meter in the temperature range 20 to 300 K
4
Fig. 1. Scattered X-ray intensities of PbMn
a function of diffraction angle 2 . The observed pattern (diamonds) measured in Debye–Scherrer geometry, the best Rietveld fit profiles (red line) and the difference curve
blue line) between the observed and the calculated profiles (below) are shown. The high angle part starting at 291 2 is enlarged for clarity. The inset shows the 2 range
around the cubic (682) reflection (bottom) and its splitting at the lower rhombohedral symmetry (top). The minor impurity phases in the sample are (small amounts of )
Mn
(bixbyite) and metallic Pb which have not been included in the refinement; a few unaccounted small reflections at T¼423 K (bottom) are contributions from the
furnace.
7
O12 at ambient conditions (synchrotron radiation, l l
¼0.7 A˚ ) (top) and at T¼423 K (Lab-diffraktometer, ¼0.7093 A˚ ) (bottom) as
y
(
y
y
2 3
O

2
80
T. Locherer et al. / Journal of Solid State Chemistry 190 (2012) 277–284
collection typically started at 2 K after zero-field cooling (ZFC) the
sample and subsequent to reaching ambient temperature, the
sample was cooled to 2 K again and magnetization was measured
again with heating (field cooling cycle, FC).
Table 3
Selected bond distances ( A˚ ) and angles (1) for RT-PbMn
7 12
O .
Pb-O
2.74(1) (6x)
.81(2) (6x)
1.90(2) (2x)
O–Pb–O
63.6(3) - 119.5(4),
180
2
Mn(1)-O
O-Mn(1)-O
86.8(7) (2x), 93.2(7) (2x)
180.0 (2x)
1
2
2
.94(1) (2x)
.63(2) (2x)
.70(1) (2x)
3
. Results and discussion
Mn(2)-O
Mn(3)-O
1.99(1) (6x)
O-Mn(2)-O
O-Mn(3)-O
85.3(4) (6x)
3
.1. Crystal structure
9
1
4.7(4) (6x)
80.0 (2x)
Starting values for the atomic coordinates for Rietveld refine-
1.98(1) (4x)
2.00(1) (2x)
82.9(8) (2x)
83.3(6) (2x)
85.7(5) (2x)
ment [11] of both phases of PbMn
similar compounds taken in the ICSD database (FIZ Karlsruhe):
PrMn 12 (R-3) [6] for the room temperature phase of PbMn
and BiMn 12 (Im-3) [12] for the high temperature phase of
PbMn 12. For the final Rietveld refinement, all profile and lattice
7
O12 were taken from CIF files of
9
9
9
1
4.3(5) (2x)
6.7(6) (2x)
7.1(8) (2x)
80.0 (2x)
7
O
7 12
O ,
7
O
7
O
parameters, isotropic temperature factors, atomic positions and a
cylindrical intensity correction were refined (Fig. 1). In general,
the positions of light atoms like oxygen in the neighborhood of
strong scatterers cannot be determined with high precision from
X-ray powder diffraction data since the shape of the least squares
minimum is quite shallow around the ’’true’’ position. Therefore,
two slack soft constraints (restraints) for Mn–O distances were
introduced and their weights iteratively reduced until the
obtained R-factors were practically identical to the ones from
the unconstraint refinement. The refinements converged quickly.
Crystallographic data including reliability factors for all reflec-
tions are given in Table 1. Atomic coordinates are listed in Table 2.
Mn(1)-O(1)-Mn(3)
Mn(1)-O(1)-Mn(2)
Mn(1)-O(2)-Mn(3)
Mn(2)-O(1)-Mn(3)
Mn(3)-O(2)-Mn(3)
111.1(5)
111.7(5)
110.4(6)
137.2(7)
139.2(8)
O-O (min)
2.64(2)
At room temperature, PbMn 12 was found to be isostructural
7
O
to CaMn 12 [13] and PrMn O12 [6] in space group R-3 (148).
7
O
7
Within such a rhombohedral ‘‘quadruple-perovskite’’ the Mn
0
atoms are located on Wyckoff position 1b, 3d and 3e (B1, B2, A
site, respectively). The B1 and the B2 site exhibit a trigonal
antiprismatic (slightly distorted octahedral) oxygen environment,
0
whereas the Mn atoms on the A site have an almost quadratic
planar oxygen coordination (see Table 3). Comparing the oxygen
coordinations of the manganese cations with those found for
Fig. 2. Comparison of the packing in the room temperature (T¼298 K; left; view
along [0 1 1]) and in the high temperature phase (T¼423 K; right; view along c
7
CaMn O12, all distortions are similar in the margins of the
7
axis) of PbMn O12. Whereas at ambient conditions two different octahedral
experimental errors (see Table 3, Fig. 2). The local environments
of these manganese atoms thus suggest an assignment of oxida-
tion states for the cations corresponding to
coordinations (orange and blue) for Mn are present, they become indistinguish-
able in the HT modification.
3
þ
3þ
4þ
3þ
Pb Mn(1)
3
Mn(2) Mn(3)
3
O12. However, the similarity of
the Mn–O distances for Mn(1) and Mn(3) would allow for a high
degree of charge disorder among these two Wyckoff positions.
At a temperature of ꢀ380 K a structural phase transition from
rhombohedral to cubic symmetry (SPGR R-3 No. 148-SPGR Im-3,
No. 204) occurs.
Table 4
Selected bond distances ( A˚ ) and angles (1) for HT-PbMn O .
7 12
Pb-O
2.77(1) (12x)
O–Pb–O
60.3(3)- 115.7(1),
80.0
1
Mn(1)-O
1.92(1) (4x)
2.68(1) (4x)
O-Mn(1)-O
87.3(4) (2x),
92.8(4) (2x)
7
In the high temperature modification of PbMn O12, Mn occupies
1
80.0 (2x)
only two different Wyckoff positions, (6b) and (8c). The (1b) and
Mn(2)-O
1.99(1) (6x)
O-Mn(2)-O
84.6(3) (6x)
9
1
5.4(2) (6x)
80.0 (2x)
Table 2
Atomic coordinates of the two polymorphs of PbMn
7
O
12: c-PbMn
7
O
12 (T¼423 K,
Mn(1)-O-Mn(2)
Mn(2)-O-Mn(2)
110.8(1)
138.2(1)
Im-3 (2 0 4)), and r-PbMn
7
O
12 (T¼298 K, R-3 (1 4 8)).
O-O (min)
2.65(1)
Wyckoff positions
X
y
z
c-PbMn
Pb
7 12
O
(
3d) Mn positions in the rhombohedral low-temperature modifica-
2a
0
0
0
tion merge into the (8c) site in the cubic phase. At the (6b) positions
Mn exhibits close to ideal quadratic planar oxygen coordination,
whereas on the (8c) positions Mn shows trigonal antiprismatic
coordination (see Table 4, Fig. 2), which does not affect the assign-
ment of oxidation states as proposed above (see Fig. 3).
Mn(1)
Mn(2)
O
6b
8c
0
½
½
¼
0
¼
¼
24g
0.322(1)
0.187(1)
7 12
r-PbMn O
Pb
1a
3e
1b
3d
6f
0
0
0
Mn(1)
Mn(2)
Mn(3)
O(1)
0
½
½
½
½
½
3
.2. Thermal analysis
½
0
0
0.678(2)
0.869(2)
0.816(2)
0.676(3)
0.497(2)
0.193(3)
O(2)
6f
The transformation to the cubic high temperature modification
is accompanied by an endothermic DSC signal of ꢀ4.2 J/g, or

T. Locherer et al. / Journal of Solid State Chemistry 190 (2012) 277–284
281
Fig. 3. Wyckoff positions’ splitting upon phase transition from cubic to rhombo-
hedral symmetry.
Fig. 4. Phase transition enthalpy maximum (a) and increased conductivity
(
b) coincide at a temperature of ꢀ370 K.
Fig. 5. (a) Resistance vs. temperature plot showing drastically increasing resis-
tances at temperatures below 68 K. (b) Formation of a frequency dependency of
the dielectric constant between 68 K and ambient temperature. (c) Sharp cusp in
the magnetic susceptibility observed at 68 K which is suppressed with increasing
magnetic field.
ꢀ
3.3 kJ/mole, respectively. The transformation temperature was
determined to be ꢀ380 K whereas the reverse transformation
upon cooling occurs at ꢀ370 K (see Fig. 4). Upon further heating,
PbMn
7
O
12 starts decomposing into stable binary decomposition
Curie constant converts to an effective magnetic moment (per one
products at 1010 K. Endothermic peaks at higher temperatures
can be attributed to melting of PbO and manganese oxides.
formula unit)
six Mn atoms in oxidation state 3þ (Mn , high spin, spin S¼2,
gE2) with an effective moment close to 4.8 and one Mn atom
in oxidation state 4þ (Mn , spin S¼3/2, gE2) with an effective
m
eff¼12.3(1)
m
B
, which is consistent with assuming
3
þ
m
B
4
þ
3.3. Magnetic properties
3
þ
2
4þ 2 1/
moment close to 3.9
m
B
(i.e.,
m
B
.
meff¼[6
m
B
(Mn ) þ
B
m (Mn ) ]
2
The magnetic susceptibilities of PbMn
7
O
12, corrected for dia-
¼12.38
m
B
magnetic contributions of the electrons in the closed core shells
14] can be described by a Curie–Weiss behavior at high tem-
peratures (TZ150 K; Fig. 5(c), inset) with a Curie constant of
8.9(2) emu K/mol (per one formula unit) and a negative Curie–
At low temperature a field-dependent sharp cusp arises in the
magnetic susceptibility at 68(2) K. The cusp is readily suppressed
by increasing the applied static external magnetic field (Fig. 5(c)).
This drastic field dependence disappears below and above the
cusp as can be seen from the magnetization experiments shown
in Fig. 6. The magnetization measured versus the magnetic field at
[
1
Weiss temperature of ꢃ45(2) K, indicating predominant antifer-
romagnetic spin exchange interactions between the Mn ions. The

2
82
T. Locherer et al. / Journal of Solid State Chemistry 190 (2012) 277–284
Fig. 7. Frequency dependence of the real part of the relative dielectric constant at
Fig. 6. Magnetization of PbMn
7
O
12 at 10 K and 100 K. The inset resolves a slight
the indicated temperature. The solid lines are fits with a Cole-type relaxation
function as described in detail in the text.
hysteresis between the pristine and the traces measured with increasing and
decreasing field as indicated.
1
00 K and 10 K above and below the cusp varies linearly with the
magnetic field for fields above ꢀ1000 Oe (Fig. 6). Below the cusp
maximum and in small magnetic fields (o1000 Oe) a slight
thermal hysteresis with a weak splitting of the FC and ZFC
susceptibility cycles is indicative that long-range magnetic order-
ing remains (see Fig. 6, inset). However, spontaneous magnetiza-
tion pointing to ferro- or ferrimagnetic ordering is not observed.
The magnetic susceptibility resembles that of an antiferromagnet.
The sharp field-dependent susceptibility cusp shortly above the
N e´ el temperature (T
N
) maybe ascribed to a regime of critical
fluctuations with easily magnetizable domains, which subse-
quently freeze out and condense in an antiferromagnetic arrange-
ment below the cusp. The magnetic cusp anomaly at 68 K is
paralleled by anomalies in the electric and dielectric properties as
can be seen in Fig. 5(a) and (b).
3.4. Electrical properties
Impedance spectroscopy measurements above room tempera-
Fig. 8. Temperature dependence of the characteristic relaxation time
t obtained
ture indicate PbMn
conductivity which can be well described by an exponential
behavior with an activation energy, E
7
O12 to be a semiconductor with an activated
from Cole-type relaxation fits of the frequency dependence of the real part of the
relative dielectric constant (displayed in Fig. 7).
a
ꢀ120 meV. A step in the
conductivity with an increase of the conductivity by about 50%
coincides with the structural phase transition atꢀ370 K from
rhombohedral to the cubic phase (cf. Fig. 4(b)). The activation
energies above and below the structural phase transition are
identical.
dependence of the relaxation with a single frequency Debye
relaxation gave a reasonable order of magnitude for the relaxation
time,t, but failed to give good quantitative agreement over the
whole frequency range. Better quantitative agreement was
achieved with a Cole–Cole-type relaxation behavior [16,17] with
3
.5. Dielectric properties
the frequency dependence of the complex dielectric constant
n
e
ð
o
Þgiven by
Fig. 5(b) displays the frequency dependence of the real part of
e ꢃe
1
S
n
e
ð
o
Þ ¼
e
1
þ
,
a
ð1Þ
the relative dielectric permittivity,
frequency dependence of
e
r’el , versus temperature. A
þðiotÞ1ꢃ
1
0
e
ð
o
Þ becomes apparent, which at
rel
ꢀ
70 K is followed by a step-like increase to higher temperatures
where
-0 (static dielectric constant) and for
0r r1 is the correction factor introduced by Cole and Cole.
Exploratory attempts to fit the frequency dependence of the real
e
S
and
e
1
are the dielectric constants for small frequencies
0
and, finally towards room temperature and above,
e
ð
o
Þ merge
o
o
-1, respectively, and
rel
at a value of ꢀ1500. In the intermediate temperature range,
a
0
e
ðo
Þ measured at higher frequency are significantly lower
rel
n
indicating a characteristic dynamics in the system with charac-
teristic frequencies of the order of 10 Hz. Fig. 7 shows the
frequency dependence of
indicating a typical relaxation behavior with a decrease of
towards higher frequencies reminiscent of the behavior of
relaxor-type ferroelectrics [15]. Trial attempts to fit the frequency
part of
subsequent fits
the frequency dependence of
peratures. Fig. 8 shows the temperature dependence of the general-
ized relaxation time increases exponentially
ðTÞ. Belowꢀ150 K
and grows by almost four orders of magnitude paralleling the
e
ðo
Þ indicated
was fixed to this value. Fig. 8 displays the fits of
’ ð Þ at some characteristic tem-
a to be of the order ofꢀ0.67. In the
3
a
0
e
ð
o
Þ at several constant temperatures
e
rel
o
rel
0
e
ð
o
Þ
rel
t
t

T. Locherer et al. / Journal of Solid State Chemistry 190 (2012) 277–284
283
4
configuration 3d and a high spin state as derived from the
magnetic susceptibility data with one e
g
electron occupying the
z z
d orbital. The d orbital is oriented towards the oxygen atoms at
2
2
largest distance from the central Mn atoms. The remaining Mn
ion is at 1b, with oxidation state 4þ, i.e., electronic configuration
3
4þ
3
d with half filling of the t2g orbitals. The Mn
ions located on
the 1b Wyckoff site center, in an almost perfect octahedral oxygen
environment with six identical Mn–O distances, but the octahe-
dra are slightly tilted and the three occupied t2g orbitals are
2
3þ
directed away from the d
z
orbitals of the neighboring Mn
ions.
4
þ
Each Mn
elongated Mn
direction pointing towards the central octahedra. The six Jahn–
is connected via corner-sharing to six Jahn–Teller
3
þ
O
6
octahedra with the elongated octahedral
3
þ
Teller distorted neighboring octahedra share corners. The Mn
–
4
þ
3þ
O–Mn
bond angle to the Mn
ions located on the Wyckoff
3
þ
4þ
position 3e is ꢀ1111, while the Mn –O–Mn
bond angle with
ion in the center of the
3
þ
4þ
the Mn
six-member ring is ꢀ1371.
According to the resistivity data, PbMn
tor. Unlike in the doped perovskite manganites a broader e
ion in the periphery of a Mn
7
O
12 is a semiconduc-
band
g
is not formed. However, according to the experimental results,
the electronic differentiation of the Mn lattice sites below the
cubic-to-rhombohedral phase transition still allows some elec-
tron hopping (phonon assisted variable range hopping) to occur.
7
Fig. 9. The resistance of PbMn O12 at temperatures above 68 K is in good
agreement with the linear dependency expected for the variable range hopping
mechanism.
3
þ
4þ
Transfer of an electron from a Mn
to a Mn
ion requires
temperature dependence of the electrical resistance. Belowꢀ68 K
the frequency dependence of the dielectric constant is negligible.
Assuming an Arrhenius-type law for the generalized relaxation type
according to,
substantial Jahn–Teller local reconstructions of the distorted
3
þ
4þ
Mn
and the highly symmetric Mn
oxygen environments,
respectively, which is consistent with the observed slow
9
0
dynamics of an attempt frequency of 1/t E10 Hz indicated by
the dielectric measurements. With decreasing temperature the
activated electronic hopping processes gradually freeze out and
finally disappear belowꢀ70 K, and at low temperatures a charge-
t
ðTÞ ¼
t
0
expð
D
E=k
B
TÞ
ð2Þ
we arrive at an activation energy of
D
E ¼ 0:22ð2Þ eV.
3
þ
4þ
Below room temperature the electrical resistivity increases, a
ordered electronic distribution of Mn /Mn
results, also
shoulder is seen near 150 K (Fig. 9). Belowꢀ70 K the resistivity
3 4 12
observed in other Mn-quadruple perovskites, e.g., NaMn Mn O
3
þ
saturates to high values of the order of ꢀ1 G
O
. The resistivity
[19]. Due to the elongation of the Mn O6 octahedra located in
4
þ
between room temperature andꢀ80 K can be well described by a
three-dimensional variable hopping mechanism with a tempera-
ture variation given by [18]
the periphery of a central Mn O , antiferromagnetic interactions
6
via superexchange, from the center to the Mn ions in the
periphery, may be expected to be small. This coupling scheme
leaves a complex geometrically frustrated exchange network,
which allows the formation of complex and sensitive magnetic
arrangements, and could explain the sensitivity of the suscept-
ibility to magnetic fields near 70 K. The low temperature suscept-
ibility, however, indicates a long-range ordered antiferromagnetic
phase well below the critical regime found near 70 K. Within a
small temperature interval around 70 K hopping slows down and
large domains may be formed. Statistically, due to the complex
geometrically frustrated exchange pathways, a full compensation
of the magnetic moments in a well ordered antiferromagnetic
arrangement, however, is not fully established and uncompen-
sated spins or domains will be left (see Fig. 6, relatively high
magnetization remains at 10 K) forming a kind of a cluster glass
involving very slow spin dynamics and also a large magnetoelas-
tic coupling with a possible tendency to multiferroic behavior.
Finally, well below 70 K a fully ordered antiferromagnetic phase
condenses. We ascribe the observed slowing down in the mag-
netic moment dynamics to the complex local distortion and
reconstruction of the octahedral oxygen environment of the Mn
ions involving optical phonons which are less and less available at
lower temperatures.
s
ðTÞ ¼
s
0
exp½ꢃðT
and T
0
=TÞ1=ðdþ1Þꢄ
ð3Þ
where
the given lattice.
s
0
0
are constants and d¼3 equals the dimension of
4
. Concluding remarks
There appears to be an intimate coupling of the magnetic
properties, the electrical conductivity and the dielectric relaxation
behavior of the new high-pressure synthesized quadruple-per-
7
ovskite-type compound PbMn O12. All three properties indicate a
critical temperature near 70 K below which the dynamics
becomes very slow and the electrical resistivity diverges. In a
small temperature regime around the critical temperature the
compound has a relatively high susceptibility, i.e., can be easily
magnetized. This ability is lost if the external field is increased.
We ascribe these anomalies to the diversification of the
oxygen octahedral environments of the Mn ions induced by the
structural transition occurring atꢀ380 K. The rhombohedral dis-
tortion leads to small, but noticable changes of the oxygen
environment of four out of the seven Mn ions. In view of the
magnetic susceptibility results, which are consistent with a
simple counting scheme for the oxidation states of the Mn ions,
we propose the following scenario: the Jahn–Teller elongation of
the oxygen octahedral environment of the six Mn ions occupying
the 3d and the 3e Wyckoff sites in the RT rhombohedral phase
suggests Mn to be in oxidation state 3þ, i.e., electronic
Acknowledgments
Financial support by the Fonds der Chemischen Industrie (FCI)
for TL, RKD, RD and MJ is gratefully acknowledged. This work was
partially supported by NSF-DMR-0966829 to MG. Special thanks

2
84
T. Locherer et al. / Journal of Solid State Chemistry 190 (2012) 277–284
go to Peter Stephens (SUNY at Stony Brook) for his help during
[8] A.A. Coelho, J. Appl. Cryst. 36 (2003) 86–95.
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7 12
data collection of the room temperature phase of PbMn O .
Science Ltd, 1988 447-452.
[10] R.W. Cheary, A.A. Coelho, J.P. Cline, J. Res. Nat. Inst. Stand. 109 (2004) 1–25.
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