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A. Faik et al. / Journal of Molecular Structure 933 (2009) 53–62
The Sr2MnWO6 was originally reported as cubic, with a ¼ 8:01Å
tiated, a serial preparation of powder samples for pure and solid
solution double perovskite oxides following four different meth-
ods. In this work, we report the two most efficient methods (effi-
ciency in the sense of time and effort, in the one hand; and, in
the other, in the sense of obtaining high quality samples) used
for obtaining the samples of Sr2Mn2þW6þO6 we have analyzed.
The first one is the DTPA method, which we have used to pre-
pare the sample 1 (Sr2Mn2þWO6). As far as we know, this is the
first time that the double perovskite tungstate oxide powder was
prepared by the Diethylene Triamine Pentaacetic Acid (DTPA)
method (sample 1). We prepared it as follows: stoichiometric
amounts of SrCO3; MnCO3 and WO3 were dissolved in dilute nitric
solution. Then, suitable amounts of ammoniac and C12H20N3O8
(DTPA) (the number of moles of DTPA equals the sum of the num-
bers of the moles of the reagents) as coordinate agent, were added
in the reactor, and a completely homogeneous light yellow trans-
parent solution was achieved, after heating at 100 °C for 30 min,
with a control of the pH between 8 and 10. This solution was sub-
jected to evaporation, until a highly viscous residual was formed
and a gel was developed during heating at 170 °C. The gel was ther-
mally treated at 300 °C, for 5 h, and at 450 °C, for 10 h, to decom-
pose the organic precursor. The resulting powder was heated,
under nitrogen environment, progressively at different tempera-
tures 900 °C for 10 h, 950 °C for 10 h and 1000 °C for 10 h, with
intermediate regrinding. We have obtained a small amount of
SrWO4 as impurity (3.35%). The sample was obtained after four
days, and the maximal temperature in the heat treatment was
1000 °C.
[12]. More recently, the structure of the double perovskite
Sr2MnWO6, at room temperature, was determined, by neutron
powder diffraction methods, by two groups. The first study, by
Azad et al. [13], described this compound as tetragonal, with the
P42=n space group (No. 86), which has never been reported for
double perovskite tungstate oxides (and which is not a common
tetragonal space group for double perovskite oxides), with
a ¼ b ¼ 8:0119ð4ÞÅ and c ¼ 8:0141ð8ÞÅ. The second work, by
Muñoz et al. [14], reported the Sr2MnWO6 compound as
monoclinic, with the P21=n space group: a ¼ 5:6803ð2ÞÅ;
b ¼ 5:6723ð2ÞÅ; c ¼ 8:0990ð2ÞÅ and b ¼ 89:936ð3Þꢀ. As it is stated
in [14], in the P42=n space group tetragonal description, it is neces-
sary to consider three different positions for the Sr2þ cations and
the volume of the tetragonal cell is nearly twice the monoclinic
volume. Muñoz et al. [14] carried out a fitting of the crystallo-
graphic structure with the P42=n space group: the discrepancy fac-
tors were slightly worse than those obtained for P21=n. Their
conclusion was that as the structure can be described in P21=n,
with a smaller unit cell and with considerably fewer atoms per unit
cell and variable parameters, they preferred the monoclinic
description for Sr2MnWO6. Moreover, they found that in the
Sr2MnWO6 unit cell the monoclinic distortion is very small, an ef-
fect that has been widely observed in many 1:1 ordered perovsk-
ites with a strong pseudo-cubic character. Azad et al. have
carried out a neutron diffraction study of Sr2MnWO6 at high tem-
peratures [15], collecting data at four different temperatures, 295,
573, 773 and 973 K. In that work, they report that at 573 K, the
structure remains in the same symmetry and space group, pointing
out that only small changes in the refinement parameters were ob-
served. However, at 773 K, the crystal structure changes from the
primitive tetragonal symmetry (space group P42=n) to a body cen-
tered tetragonal symmetry (space group I4=m). And, finally, at
973 K, the crystal structure changes from body centered tetragonal
As second method of synthesis we used the co-precipitation for
the preparation of sample 2 (Sr2Mn2þWO6). Stoichiometric quanti-
ties of SrðNO3Þ2ðIÞ; ðOOCH3Þ2Mn ꢁ 4H2O (II) and ðNH4Þ10W12O41 (III)
were dissolved separately in distilled water. A slow addition,
under magnetic agitation, of (III) in (I) and (II) mixture at room
temperature induces the formation of a precipitation. After slow
evaporation at about 60 °C, the resulting powder was heated pro-
gressively, at different temperatures, with intermediate regrind-
ing: at 300 °C, 400 °C, 500 °C (for 8 h every temperature) and
900 °C for 24 h. The whole heat treatment was conducted under
ꢀ
symmetry to face centered cubic symmetry (space group Fm3m).
The aim of the present study is twofold. First, we want to pres-
ent a new route for synthesizing double perovskite oxides, which
has been proven to be very efficient: low amounts of impurity
are obtained, it needs lower reaction temperatures (although the
grade of crystallization is good) and the samples are obtained in
only two days. The second aim is to report the structural re-deter-
mination of Sr2Mn2þWO6, and the structural determination of a
a
nitrogen environment. The obtained final powder of
Sr2Mn2þWO6 is brown in color. In this case, we have obtained, an
even lower quantity of impurity (< 1%) of MnO in this sample, that
was included as secondary phase in the final refinement. The sam-
ple was obtained after two days, and the maximal temperature in
the heat treatment was 900 °C. Consequently, the method of co-
precipitation presents the shortest time and the lowest tempera-
ture of preparation.
new compound: Sr2Mn3þWO6þd
, by X-ray powder diffraction
methods. We also present the phase transitions observed in
Sr2Mn2þWO6 and in Sr2Mn3þWO6þd with a survey of the evolution
of the crystalline parameters according to the temperature.
2.2. X-ray powder diffraction
2. Experimental
The powder diffraction data were obtained using a Philips
X’Pert MPD System with Cu Ka (Ni-filter) radiation, equipped with
a proportional detector. The specimen for diffraction at room tem-
perature was prepared by depositing sample powder on a Si plate.
The intensity data were collected by continuous scanning with 2h
steps of 0.01° and counting times of 10 s at each step. The 2h range
covered was 15°–100°. X-ray diffractograms at high temperatures,
Sr2Mn2þWO6 and Sr2Mn3þWO6þd both in air, were obtained using
an Anton Paar HTK16 temperature chamber, with a temperature
stability of 0.5 K, mounted on the same diffractometer. In the case
of Sr2Mn2þWO6, another experiment was conducted under a vac-
uum environment, high temperature X-ray powder diffraction data
2.1. Sample preparation
The literature describes a large number of methods for synthe-
sizing double perovskite oxide materials. Without any pretention
of exhausting we can mention few different methods used in the
last five years: ceramic method [16], sol-gel method [17], freeze-
drying method [18] and exchange ionic reaction [19]. Each one
has its own advantages and disadvantages. In our group, we have
been using, quite successfully, the ceramic method which is well
known for the easy implementation, although it can take very long
synthesis time. This fact, due to the (in principle) unknown ad hoc
thermal treatments for each compound, is usually stated as the
counterpart of the easiness of the method. It has another obvious
counterpart: usually, they are needed (very) high temperatures
in the thermal treatment. With the aim of stablishing a good route
for obtaining powder samples of double perovskite oxides, we ini-
were recorded on
a Bruker D8 Advance h=h diffractometer,
equipped with a Vantec high speed one-dimensional detector
using CuKa radiation. An Anton Paar HTK2000 high-temperature
chamber with direct sample heating (Pt filament) and a tempera-
ture stability of 0.5 K was used. The data were collected using con-