Synthesis, structural and magnetic characterisation of the double perovskite A2MnMoO6 (A = Ba, Sr)

Article abstract of DOI:10.1016/S0925-8388(03)00611-X

A study of the crystallographic structure and magnetic properties of the double perovskites Ba₂MnMoO₆ and Sr₂MnMoO ₆ in polycrystalline form has been carried out by means of neutron powder diffraction (NPD) and magnetization measurements. The Rietveld analysis of room temperature data shows that the Mn²⁺ and Mo⁶⁺ ions are B-site ordered, i.e. the structure is a NaCl-type ordered double perovskite. Ba₂MnMoO₆ crystallizes in the cubic space group Fm3m (a = 8.1680(1)) and Sr₂MnMoO₆ crystallizes in the space group P4₂/n (a = 7.9575(5), c = 7.9583(9)). Bond valence sum (BVS) calculation revealed that these compounds have the valency pair of {Mn²⁺(3d⁵;t³_2ge² _g), Mo⁶⁺(4d⁰)}. The magnetic measurements suggest that these compounds transform to an antiferromagnetic state below 10 K.

Full text of DOI:10.1016/S0925-8388(03)00611-X

Journal of Alloys and Compounds 364 (2004) 77–82
Synthesis, structural and magnetic characterisation of the double
perovskite A2MnMoO6 (A = Ba, Sr)
A.K. Azada,b,∗, S.-G. Erikssona,b, S.A. Ivanovc, R. Mathieud,
P. Svedlindhd, J. Eriksena, H. Rundlöfa
a
The Studsvik Neutron Research Laboratory, Uppsala University, SE-611 82 Nyköping, Sweden
b
Department of Inorganic Chemistry, University of Gothenburg, SE-412 96 Göteborg, Sweden
c
Karpov Institute of Physical Chemistry, RU-103064 K-64, Moscow, Russia
d
Department of Materials Science, Uppsala University, SE 751 21 Uppsala, Sweden
Received 28 April 2003; received in revised form 26 May 2003; accepted 26 May 2003
Abstract
A study of the crystallographic structure and magnetic properties of the double perovskites Ba2MnMoO6 and Sr2MnMoO6 in polycrystalline
form has been carried out by means of neutron powder diffraction (NPD) and magnetization measurements. The Rietveld analysis of room tem-
perature data shows that the Mn2+ and Mo6+ ions are B-site ordered, i.e. the structure is a NaCl-type ordered double perovskite. Ba2MnMoO6
crystallizes in the cubic space group Fm3m (a = 8.1680(1)) and Sr2MnMoO6 crystallizes in the space group P42/n (a = 7.9575(5),
c = 7.9583(9)). Bond valence sum (BVS) calculation revealed that these compounds have the valency pair of {Mn2+(3d5;t32ge2 ), Mo6+(4d0)}.
g
The magnetic measurements suggest that these compounds transform to an antiferromagnetic state below 10 K.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Transition metal compounds; Oxide materials; Crystal structure; Magnetic measurements
1. Introduction
at the Fermi level, making them promising candidates for
future spin electronics. These findings have stimulated re-
search on other double perovskites. Antiferromagnetism at
low temperature is also very common in these types of com-
pounds depending on the cation composition [7,8]. The ideal
double perovskite structures can be viewed as a regular ar-
rangement of corner-sharing BO6 and BꢀO6 octahedra al-
ternating along the three directions of the crystal, with the
large A cations occupying the voids in between the octahe-
dra. Depending on the relative size of the B and Bꢀ cations
with respect to the A cations, the crystal structure can be
defined as cubic (Fm3m), tetragonal (I4/m) or monoclinic
(P21/n) [8,9].
In the present work we have given our attention to
the Mn analogues of Ba2FeMoO6 and Sr2FeMoO6. The
double perovskite Sr2MnMoO6 was first studied in the
1960s and described as cubic with a = 7.98 Å [10]. Re-
cently, the compound was reported as cubic, a = 7.9973(1)
Å and antiferromagnetic, TN = 12 K [11]; and mono-
clinic, a = 5.6671(1), b = 5.6537(1), c = 7.9969(2) Å,
β = 89.92(2)◦ and antiferromagnetic, TN = 15 K [12]. A
solid solution of Mn and Mo at the B-site gives a ma-
The double perovskites A2BBꢀO6 host two different B-site
cations, which tend to order in a NaCl-type superlattice
structure, doubling the elementary perovskite cell [1]. Com-
pounds where A is an alkaline earth cation and B = Cr,
Fe; Bꢀ = Mo, W, Re, are ferromagnetic [2–6]. Interest in
this type of compound was revived in a recent report by
Kobayashi et al. that Sr2FeMoO6 is a colossal magnetoresis-
tance (CMR)-type half-metallic ferromagnet [2] with a Curie
temperature of ∼415 K, significantly higher than for any
mixed-valence manganite. This interest has also been driven
by the possible technological applications of these materi-
als in magnetoelectronic devices, stimulating the search for
new CMR compounds. Recently, similar CMR properties
were described for other double perovskites which exhibit
half-metallic ferromagnetism with a high spin polarization
∗
Corresponding author. Present address: The Studsvik Neutron Re-
search Laboratory, Uppsala University, SE-611 82 Nyköping, Sweden.
Tel.: +46-155-221-871; fax: +46-155-263-001.
E-mail address: azad@studsvik.uu.se (A.K. Azad).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0925-8388(03)00611-X

78
A.K. Azad et al. / Journal of Alloys and Compounds 364 (2004) 77–82
terial of the type BaMn0.5Mo0.5O3 showing a hexagonal
structure with a = 11.238 and c = 15.758 Å, and a diffuse
ferroelectric behavior [13]. To our knowledge no struc-
tural characterization has previously been done for ordered
Ba2MnMoO6.
In the present study we have described the synthesis of
these materials, prepared by solid state reaction procedures,
and the results of high-resolution neutron powder diffrac-
tion (NPD) studies on well-crystallized samples. We have
reported complete structural data for these materials and
described the magnetic properties from the magnetization
measurements.
2.3. Magnetisation measurement
Magnetization measurements were carried out using a
Quantum Design Superconducting Quantum Interference
Device (SQUID) magnetometer. Magnetization versus tem-
perature curves were measured between 0 to 300 K in
field-cooled (FC) and zero-field-cooled (ZFC) modes with
applied fields (␮0H) of 20 Oe.
3. Results and discussions
3.1. Crystal structure
2. Experimental details
The crystal structure determination was performed on
the neutron powder diffraction data collected at room
temperature with a wavelength λ = 1.47 Å. In case of
Ba2MnMoO6, the structure was cubic with the unit cell
parameter a = 8.1680(1) Å at 295 K. The space group
was Fm3m and the Rietveld refinement resulted in good
R-factors (Rp = 3.31%, Rwp = 4.24%, RBragg = 2.71% and
χ2=1.85). The x coordinate of oxygen was varied during
the refinement and determined to be 0.2643(1). The possible
presence of oxygen non-stoichiometry was not confirmed
during the final Rietveld refinements. The cation occupan-
cies were also found to be very close to their nominal values.
From the Rietveld analysis we obtain an elemental ratio
Ba:Mn:Mo:O = 0.332(3):0.167(1):0.165(1):1.000, which is
in excellent agreement with the nominal composition of the
sample, Ba2MnMoO6. The Mn and Mo ions were ordered
at the B-sites and form together with oxygen a NaCl-type
lattice where both the cations show perfect octahedral an-
ion coordination. The MnO6 octahedra are larger than the
MoO6 octahedra, an observation in accordance with the
larger ionic size of Mn2+ (rMn2+ = 0.83 Å) compared to
Mo6+ (rMo6+ = 0.59 Å). The volumes of the octahedra
are calculated to be 13.41(1) Å3 for MnO6 and 9.51(1) Å3
for MoO6. Each barium ion is coordinated to 12 oxygen
ions, being part of the ccp layers. The average Ba–O bond
lengths at 295 K compare well with the expected values cal-
culated as the sum of the ionic radii. The average observed
Mn–O and Mo–O bond distances are 2.158(1) and 1.926(1)
Å, respectively. Bond valence sum (BVS) [17] calculation
from the average observed bond lengths (d ≤ 3.5 Å) be-
tween Mn or Mo and O shows that the charges of Mn and
Mo cations are +2.214 and +5.706, respectively. The tol-
erance factor was calculated to be 1.05 using the Shannon
ionic radii [18]. If we calculate the parameters considering
completely ordered and pure oxidation state for Mn2+ and
Mo6+ using SpuDS software [19], we get the lattice param-
eter a = 8.207 Å, bond lengths between Mn–O = 2.1965
Å and Mo–O = 1.907 Å, and a tolerance factor, t = 1.016,
which are in good agreement with the experimental values.
BVS calculation using Mn3+ and Mo5+ gives the value
2.046 and 5.718 Å, respectively, in which very different
from the expected value. The small disagreements may be
2.1. Sample preparation
The Ba2MnMoO6 and Sr2MnMoO6 compounds were pre-
pared as polycrystalline powders by the solid state reac-
tion technique. Stoichiometric amounts of analytical grade
BaCO3 or SrCO3, MnO and MoO3 were mixed together by
ethanol. The finely mixed powders were pre-sintered in a
furnace for 15 h at 950 ◦C. The powders were mixed in an
agate mortar again and pressed into small discs and heated
up to 1200 ◦C for 48 h. The sample was again reground and
heated at 1350 ◦C for 48 h and finally at 1400 ◦C for 48 h. All
heat treatments were conducted under nitrogen environment.
2.2. X-ray diffraction, neutron diffraction and
Rietveld analysis
X-ray powder diffraction measurements of the samples
were carried out to check the quality and phase distribu-
tion of the samples using a Guinier Camera (Cu K␣1 radi-
ation) at the KEMLAB of Studsvik Neutron Research Lab-
oratory, Sweden. Silicon (NBS 640b) was used as an inter-
nal standard and a computerized line scanner for evaluation
of the film. Indexing and refinement of the lattice param-
eters were made using TREOR90 [14] and Checkcell [15]
software, respectively. Neutron diffraction data were col-
lected at the NPD instrument at the R2 reactor, Sweden. The
double monochromator system consists of two copper crys-
tals aligned in (220) mode giving a wavelength of 1.470 Å.
The neutron flux at the sample position was ∼106 neutron
cm−2 −1. The step scan covered the 2θ range 4◦–139.92 ◦
s
with a step size 0.08◦. NPD data sets were refined by the
Rietveld method using FullProf software [16] including the
coherent scattering length supplied by the software. Diffrac-
tion peak shapes were quantified by a pseudo-Voigt func-
tion, with a peak asymmetry correction applied at angles
below 45◦ in 2θ. Background intensities were described by
a Chebyshev polynomial with six coefficients. Each struc-
tural model was refined to convergence, with the best result
selected on the basis of agreement factors, chemical sense
and stability of the refinement.

A.K. Azad et al. / Journal of Alloys and Compounds 364 (2004) 77–82
79
Fig. 1. Observed (circles) and calculated (continuous line) NPD intensity profiles for (a) Ba2MnMoO6 and (b) Sr2MnMoO6 at room temperature (295
K). The short vertical lines indicate the angular position of the allowed Bragg reflections (second row for the second phase). At the bottom in each figure
the difference plot, Iobs–Icalc, is shown.

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A.K. Azad et al. / Journal of Alloys and Compounds 364 (2004) 77–82
Table 1
Refined structural parameters of Sr2MnMoO6 at RT (295 K)
Atom Positions
x
y
z
B (Å)2
(a) Main phase: space group P42/n, a = b = 7.9575(5) Å, c = 7.9583(9) Å; Rp = 3.15%, Rwp = 4.08%, RBragg = 2.97%, χ2 = 2.12
Sr(1)
Sr(2)
Sr(3)
Mn
2a
2b
4e
4c
4d
8g
8g
8g
0.25
0.25
0.25
0.25
0.29(7)
0.66(8)
0.28(6)
0.11(8)
0.57(6)
0.34(9)
0.24(9)
0.65(8)
0.25
0.75
0.00
0.75
0.2638(7)
0.00
0.25
0.00
Mo
0.00
0.00
0.50
O(1)
O(2)
O(3)
0.2659(9)
0.2386(6)
0.0104(1)
−0.0361(2)
0.0296(2)
0.0237(5)
−0.0111(2)
0.5153(7)
0.2592(4)
(b) Impurity phase: space group I41/a, a = b = 5.3897(5) Å, c = 12.0209(8) Å
Sr
4b
4a
16f
0.00
0.25
0.625
0.125
0.0433(4)
0.067(6)
0.11(4)
0.62(7)
Mo
O
0.00
0.2373(4)
0.25
0.1146(2)
caused by the oxidation of a small amount of Mn2+ ions to
Mn3+ to form a Mn3+Mo5+ pair due to a strong tendency
of oxidation of divalent Mn+2 ions. An impurity phase
in the form of BaMoO4 was present. The weight fraction
of the impurity phase was determined to be 2.7(3)% from
the Rietveld refinement and was refined as a second phase
using the tetragonal symmetry (space group I41/a, No. 88,
choice 2). The Wyckoff positions of Ba, Mo and O were
4b, 4a and 16f, respectively. The unit cell parameters were
a = b = 5.569(2) and c = 12.854(8) Å. Fig. 1(a) shows the
observed, calculated, difference curve and Bragg reflection
positions for both phases.
In Sr2MnMoO6 the Bragg reflections were indexed with
a tetragonal unit cell and the structure was refined in the
tetragonal space group P42/n. The good agreement between
the observed and calculated patterns after the refinement is
shown in Fig. 1(b). The lattice parameters and final atomic
coordinates for original and impurity phases are given in
Table 1.
In a recent study of the crystallographic structure of
Sr2MnMoO6 [12], the monoclinic space group P21/n has
been proposed. In the monoclinic description, the Sr po-
sition can vary in all three x, y, z directions. Although in
the tetragonal space group P42/n, we need to use three
different positions for Sr, almost all positions are special,
and only one position can vary in the z-direction. The num-
ber of variable atomic co-ordinates is less in this model
than in the monoclinic model. Based on our experimen-
tal data we carried out a fitting of the crystallographic
structure in space group P21/n. The resulting R-factors ob-
tained were; Rp = 3.17%, Rwp = 4.00%, RBragg = 3.34%
and χ2 = 2.16. The cell parameters were a = 5.6392(9) Å,
b = 5.6223(6) Å, c = 7.946(1) Å and β = 89.96(2)◦. When
refining a model in space group P42/n (No. 86, choice 2)
the agreement was even better (Rp = 3.15%, Rwp = 4.08%,
RBragg = 2.97% and χ2 = 2.12). The cell parameters were
refined to a = b = 7.9575(5) Å, c = 7.9583(9) Å. Since it
is a well established convention to use symmetry as high
as possible, we preferred to use the tetragonal description
for the compound. The number of variable parameters
are also less and the unit cell parameters are very close
to the cubic description of the structure in the tetragonal
model.
The unit cell parameters are related to that of ideal cubic
perovskite as a ≈ 2ap, b ≈ 2ap, c ≈ 2ap (ap≈3.89 Å). To
illustrate the combined effect of cation ordering and octa-
hedral tilting, consider the tetragonally distorted (a+a+c−)
[20] tilt system. The Wyckoff positions were Mn on 4c, W
on 4d, Sr on 2a, 2b, 4e and O on 8g (three positions), which
gives an ordered perovskite structure of so-called elpasolite
type. Mn and Mo are found to occupy alternate B-sites. A
schematic representation of the MnO6 and MoO6 octahedral
distortion is shown in Fig. 2. The distances between Mn and
O in the a–b plane and parallel to c-axis are about 2.14(1),
2.07(1) and 2.08(4) Å, respectively. The distances between
Mo and O in the a–b plane and parallel to c-axis are about
1.88(1), 1.94(2) and 1.94(4) Å, respectively. BVS calcula-
tion from the average observed bond lengths (d ≤ 3.5 Å)
between Mn2+ or Mo6+ and O2− shows that the charge
distributions of Mn and Mo cations are +2.59 and +5.93,
respectively. Again, using Mn3+ and Mo5+ we get the
BVS values +2.394 and +5.94, which is a larger discrep-
ancy than expected indicating that a mixed valence state is
probable, and the difference from ideal charge distribution
Mn+2 and Mo+6 may be caused by the oxidation of a small
amount of Mn2+ ions to Mn3+ to form a Mn3+Mo5+ pair
due to a strong tendency of oxidation of divalent Mn+2 ions.
The MnO6 octahedra (volume = 12.20(2) Å3) are signif-
icantly larger than the MoO6 octahedra (volume = 9.40(3)
Å3). The MnO6 and MoO6 octahedra are ordered and al-
ternate along the three directions in the crystal structure in
such a way that each MnO6 octahedra is linked to six MoO6
octahedra and vice versa. Based on the ionic radii of Sr2+
(r = 1.44 Å), Mn2+ (r = 0.67 Å), Mo6+ (r = 0.59 Å) and
O2− (r = 1.40 Å) [17] in appropriate coordination number,
the tolerance factor was calculated to be t = 0.989.

A.K. Azad et al. / Journal of Alloys and Compounds 364 (2004) 77–82
81
3.2. Magnetic properties
The temperature dependence of the mass magnetic sus-
ceptibilities for Ba2MnMoO6 and Sr2MnMoO6 are shown
in Fig. 3. Both the Mn2+ and Mo6+ ions contribute to
the paramagnetic behavior and the effective magnetic mo-
ment of the compounds (µeff) are given by the follow-
ing equation: µeff = µeff(Mn2 + )2 + µeff(Mo6+)2. By fit-
2
ting the Curie–Weiss equation to the experimental data in
the temperature range 0–300 K, the effective magnetic mo-
ment (µeff) and the Weiss constant (θ) are determined to
be 5.80 µB and −94 K for Ba2MnMoO6 and 4.5 µB and
−108 K for Sr2MnMoO6. This negative Weiss constant
is indicative of the presence of predominant antiferromag-
netic interaction. No divergence between the ZFC and FC
magnetic susceptibilities or no magnetic hysteresis in the
magnetization-magnetic field curve indicates that at low tem-
perature (below 10 K) these compounds transform to an anti-
ferromagnetic state without weak ferromagnetic properties.
Fig. 2. A schematic representation of the MnO6 and MoO6 octahedral
distortion, volume and alternate arrangement. The O atoms are not shown
for simplicity, but are located at the corners of each octahedra.
4. Conclusions
Rietveld analysis of NPD data shows that the double-
perovskites Ba2MnMoO6 and Sr2MnMoO6 are B-site or-
dered. Ba2MnMoO6 crystallizes in the space group Fm3m
with the doubling of unit cell axes of the ideal perovskite.
Sr2MnMoO6 crystallises in the space group P42/n which
is related to the ideal cubic perovskite as a ≈ 2ap, b ≈ 2ap,
a ≈ 2ap (ap≈3.89 Å). Valency pair of these materials was
considered to be (Mn2+, Mo6+) from the bond valence sum
calculation and compared with the calculated values. The
magnetic measurements suggest that these compounds trans-
form to an antiferromagnetic state below 10 K. The study
of magnetic structure below 10 K is in progress.
A second phase, SrMoO4, was observed as an impu-
rity and refined in the tetragonal space group I41/a using
multi-pattern Rietveld analysis of FullProf software. The
amount of the impurity phase was calculated to be 3.4(2)%.
The refinement of the impurity phase gave the follow-
ing reliability factors: RBragg = 4.73% and RF = 2.83%.
The unit cell parameters were a = b = 5.3897(5) and
c = 12.0209(6) Å. The fractional atomic coordinates, Wyck-
off positions and isotropic thermal parameters are given in
Table 1(b).
Acknowledgements
The authors are grateful to the Royal Swedish Academy
of Sciences, to the Swedish Research Council (V.R.), and
to the Swedish Foundation of Strategic Research (SSF),
for financial support. One of the authors, A.K. Azad, is
grateful for the support of the Bangladesh Atomic Energy
Commission.
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