Crystal structure and phase transitions of Sr2 CdWO6

Article abstract of DOI:10.1016/j.jssc.2007.05.022

The crystal structure of Sr₂ CdWO₆, prepared by solid state reaction, was determined by high-resolution X-ray diffraction at different temperatures. At room temperature, this compound has a monoclinic structure (space group P 2₁ / n) with a = 5.7463 (1), b = 5.8189 (1), c = 8.1465 (1), β = 90.071 (1). At 1105 K the structure is converted to tetragonal (space group I 4 / m). Diffraction data also suggest that a cubic phase exists above 1220 K. Comparing the phase transition temperatures of Sr₂ CdWO₆ with those of other compounds of the Sr₂ MWO₆ family reported previously, it was observed that the transition temperatures are higher in compounds with low-tolerance factors. At the same time, the temperature range in which the intermediate tetragonal phase exists is reduced.

Full text of DOI:10.1016/j.jssc.2007.05.022

ARTICLE IN PRESS
Journal of Solid State Chemistry 180 (2007) 2248–2255
www.elsevier.com/locate/jssc
Crystal structure and phase transitions of Sr CdWO6
2
a
M. Gateshki , J.M. Igartua , A. Faik
b,
Ã
b
a
Australian Nuclear Science and Technology Organization, Private Mail Bag 1, Menai, NSW 2234, Australia
b
Fisika Aplikatua II Saila, Zientzia eta Teknologia Fakultatea, Euskal Herriko Unibertsitatea, P.O. Box 644, Bilbao 48080, Spain
Received 19 March 2007; received in revised form 24 May 2007; accepted 27 May 2007
Available online 3 June 2007
Abstract
The crystal structure of Sr2CdWO6, prepared by solid state reaction, was determined by high-resolution X-ray diffraction at different
temperatures. At room temperature, this compound has a monoclinic structure (space group P21=n) with a ¼ 5:7463ð1Þ, b ¼ 5:8189ð1Þ,
c ¼ 8:1465ð1Þ, b ¼ 90:071ð1Þ. At 1105 K the structure is converted to tetragonal (space group I4=m). Diffraction data also suggest that a
cubic phase exists above 1220 K. Comparing the phase transition temperatures of Sr2CdWO6 with those of other compounds of the
Sr2MWO6 family reported previously, it was observed that the transition temperatures are higher in compounds with low-tolerance
factors. At the same time, the temperature range in which the intermediate tetragonal phase exists is reduced.
r 2007 Elsevier Inc. All rights reserved.
PACS: 61.10.Nz; 61.50.Ks; 61.66.Fu; 64.70.Kb
Keywords: Double perovskite; X-ray diffraction; Crystal structure; Phase transitions
1
. Introduction
general formula Sr BWO and B ¼ Ni; Co; Zn; Ca; Mg. All
2
6
of these materials undergo temperature-induced phase
transitions with the following symmetry changes:
I4=m ðtetragonal; no: 87Þ ! Fm3m, for B ¼ Ni; Mg;
In recent years, the structural properties of double
0
perovskite compounds with general formula A2BB O6 have
been extensively studied [1–3]. The double perovskite
structure can be represented as a three-dimensional net-
and P21=n ðmonoclinic; no: 14; non
ꢀ
standard settingÞ !
I4=m ! Fm3m, for B ¼ Co; Zn; Ca. According to the
0
work of alternating BO6 and B O6 octahedra, with A-
Glazer classification [9] these distorted structures have the
þ ꢀ
following tilt systems: P2 =n, a b b ; I4=m, a a c and
1
ꢀ
ꢀ
0
0
atoms occupying the 12-coordinated interstitial spaces
between the octahedra. The aristotype structure of this
family is cubic [4] with the space group Fm3m (cubic, no.
0
0 0
a a a [4]. It was found that in materials with relatively
small B-site cation (Ni,Mg,Zn,Co) the cubic aristotype
structure is achieved at lower temperature than in the case
of Sr2CaWO6 with larger B cation. In order to extend this
study and establish a relation between the structural
parameters of the materials and the temperature at which
such transitions occur, in this article we report the crystal
structures observed in Sr CdWO at different tempera-
2
25) [5]; however, due to a mismatch between the size of the
A-site cation and the cuboctahedral space between the
octahedra, many such materials undergo one [6], two [7] or
even more [8] structural phase transitions at different
temperatures. These structural transformations can be
conveniently described as changes in the way the BO
0
6
2
6
and B O are rotated with respect to the crystallographic
6
tures, by means of synchrotron radiation powder diffrac-
tion study.
axes of the material to accommodate the size of the A-site
cations [9,10]. In previous articles [11,12], we studied the
structural phase transitions occurring in the materials with
The first preparation of Sr2CdWO6 was reported in [13].
The room-temperature structure of Sr2CdWO6 was found
to be of the ordered perovskite type, with the orthorhombic
Pmm2 (no. 25) space group. This space group was chosen
because of the piezoelectric properties observed in
Ã
Corresponding author. Fax:+34 94 601 3500.
E-mail address: josu.igartua@ehu.es (J.M. Igartua).
0022-4596/$ - see front matter r 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2007.05.022

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M. Gateshki et al. / Journal of Solid State Chemistry 180 (2007) 2248–2255
2249
Sr2CdWO6 by the author; although, no experimental
results evidencing such properties were presented. The
Pmm2 space group can accommodate the B cation ordering
and the presence of a polar moment in the structure. The
structure of Sr CdWO , presented in [13], was refined using
SrWO4 has symmetry I41=a. Known structural parameters
from the literature [16] were used; and these were not
refined.
2.2. X-ray powder diffraction measurements
2
6
conventional X-ray powder diffraction data. In this
structure, the CdO6 and WO6 octahedra are deformed,
Two different sets of diffraction experiments were
performed with the studied sample. First, in order to
establish the presence of phase transitions and their
temperatures, X-ray data were collected in the 2y interval
from 71.5 to 73.5, with a temperature step of 15 K. This
2þ
but without any tilt or rotation. Also, the cations Cd and
þ
6
W
were found to be displaced from the centers of their
respective octahedra. Due to this structural determination,
Sr2CdWO6 was included in the list of non-centrosymmetric
oxides [14]. In another work [15], the room-temperature
0
was done using a Philips X Pert MPD System with CuKa
crystal structure of Sr CdWO6 was referenced as tetra-
2
(Ni filter) radiation, equipped with a proportional detector
and an Anton Paar HTK16 temperature chamber, with a
temperature stability of 0.5 K. Intensity data were collected
using Bragg–Brentano parafocusing geometry and 12 s
counting time at each step.
Second, for structural determination and refinement of
Sr2CdWO6, high-resolution X-ray diffraction experiments
at selected temperatures (300, 770, 1120 K) were performed
using the X7A beam-line at the National Synchrotron
Light Source (Brookhaven National Laboratory) [17]. The
gonal; however, no structural details were reported.
The study of Sr2CdWO6 at high temperatures, reported
in [13], consists of DTA measurements and one structural
determination at 1133 K. The DTA measurements showed
a reversible thermal anomaly at about 1077 K; and the
¯
structure at 1133 K was refined with the Fm3m space
group.
2
. Experimental
˚
radiation wavelength was 0.8005 A. It was obtained using a
Si(1 1 1) monochromator; and was calibrated with a CeO2
standard sample. Diffracted radiation was collected by a
linear position sensitive detector mounted on the 2y arm of
the diffractometer, at a distance of 1 m from the sample. In
this case, the sample was placed in a quartz capillary and
rotated during the experiment to reduce preferred-orienta-
tion effects.
2
.1. Sample preparation
For the preparation of the sample, the standard method
of solid-state chemical reaction was used. Stoichiometric
amounts of the reacting compounds were mixed according
to the following chemical reaction:
2
SrCO þ CdO þ WO ! Sr CdWO þ 2CO " .
3
3
2
6
2
The Rietveld refinement of the structures was performed
with the FullProf program [18].
The reacting compounds (all delivered by Sigma-
Aldrich) had the following purities: SrCO3(99.995%),
WO3(99.995%) and CdO(99.99%). All compounds were
used as received. The starting materials were mixed and
ground in an agate mortar; and subsequently heated in air
in alumina crucibles. The following heat treatment was
used: 24 h at 1120 K; 24 h at 1170 K; and 48 h at 1220 K.
After each heating, the sample was cooled down slowly
3. Results and discussion
At all temperatures covered, the 2y region chosen
(71.5–73.5) for the phase transition search contains a
group of diffraction peaks that has been identified as
especially sensitive to the structural changes occurring in
double perovskite materials [12]. This experiment showed
that the only changes that can be interpreted as indications
of phase transitions are the appearance of a shoulder on
the low-angle side of the peak, at 1105 K (Fig. 1); and the
subsequent unification of this shoulder with the main peak,
occurring at about 1220 K. This behavior is very similar to
that observed in Sr CaWO [12], where these changes were
(
3 K/min); and re-ground (re-mixed) to improve homo-
geneity. In order to control the quality of the obtained
material, X-ray diffraction measurements were performed
after each heating. Higher heating temperatures were also
attempted, but we observed volatilization of Cd. This was
deduced from the color of the material that changes form
light gray to yellow and the progressive increase of the
Sr WO impurity in the sample as observed by X-ray
2
6
interpreted as evidences of the material undergoing two
phase transitions; first, from monoclinic structure to
tetragonal one, and (at higher temperature) from tetra-
gonal to cubic. To clarify the structural changes occurring
in Sr2CdWO6, high-resolution diffraction measurements
using synchrotron radiation were performed at three
different temperatures: 300, 770 and 1120 K. It was not
possible to reach higher temperatures due to instrument
limitations.
2
5
diffraction. The X-ray studies also showed that the
diffraction lines of Sr2CdWO6 are broader compared to
those of the other members of the Sr2MWO6 family studied
previously [11,12]. This is probably due to the fact that the
synthesis temperature was lower, and such lower tempera-
ture can give as a result crystalline grains with smaller sizes.
Small amounts (ꢁ 1:7% weight fraction) of the impurity
SrWO was found in the final material: this impurity was
4
included as a known additional phase in the Rietveld
refinements of the structure of the studied compound.
The diffraction pattern collected at 300 K is shown in
Fig. 2. The attempt to refine the structure of Sr2CdWO6

ARTICLE IN PRESS
M. Gateshki et al. / Journal of Solid State Chemistry 180 (2007) 2248–2255
2250
1
220 K
1
1
1
1
1
200
150
100
050
000
1225 K
4
2
0
0
0
0
1
105 K
6
1105 K
30
0
6
3
0
0
0
970 K
9
9
50
00
71.8
72.0
72.2
72.4
72.6
72.8
73.0
73.2
71.5
72.0
72.5
73.0
73.5
Angle 2θ, deg
Angle 2θ, deg
Fig. 1. Temperature evolution of the (620) cubic reflection of Sr2CdWO6. In (a), scattered intensity is represented with shades of gray—black corresponds
to high intensity and white to low intensity. The tetragonal splitting existing in the temperature range from 1105 to about 1220 K can be observed. The
shoulder appearing at 1105 K is marked with an arrow in (b).
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
10
15
20
25
30
35
40
45
50
55
60
65
Brag Angle 2θ (deg)
Fig. 2. Experimental (symbols) and calculated (line) powder diffraction profiles for the Rietveld refinement of Sr2CdWO6 at room temperature using a
structural model with P21=n space group. The bars in the lower part of the graphics represent the Bragg peak positions of the main phase and the SrWO4
impurity (lower set of bars).
using a structural model with the Pmm2 space group, as
suggested previously [13], gave a reasonably good fit of the
diffraction profile: no unindexed lines or large differences
between the experimental and calculated intensities were
observed. However, many fine details in the pattern were
not well accounted for by the model. These problems
were successfully solved by performing a refinement based
on the monoclinic structure with space group P21=n
observed in many double perovskite compounds [19–22],
including several compounds with the general formula
Sr2MWO6 [11,12,23]. One example of a diffraction reflec-
tion where the monoclinic model gives improvement, is
shown in Fig. 3. The peak located at higher 2y angle is
considerably broader than the peak at lower angle, also
shown in the figure. This difference in the widths of the two
peaks can be easily explained as a result of a monoclinic
distortion of the unit cell, while the orthorhombic
structural model is not able to reproduce this experimental
observation. The better visual fit of the experimental data
with the P21=n model is accompanied, also, with significant

ARTICLE IN PRESS
M. Gateshki et al. / Journal of Solid State Chemistry 180 (2007) 2248–2255
2251
Fig. 3. Experimental diffraction data for Sr2CdWO6 at 300 K (symbols) compared with the diffraction patterns calculated after refinement (lines) using
structural models with symmetries Pmm2 (a) and P21=n (b). Only the monoclinic model reproduces correctly the diffraction intensities observed in the
experiment.
2
1
0
5
3
2
1
0
0
0
20
15
1
120 K
1
120 K
1120 K
1
2
0
0
1
2
0
0
(044)
3
2
1
0
0
0
7
70 K
15
770 K
770 K
1
5
(423)
(044)
(-404)
(404)
(
-243)
10
(
-111) (111)
(-423)
1
0
0
(243)
2
3
2
1
0
0
0
20
15
300 K
300 K
300 K
1
1
5
0
(044)
(-404)
(243)
(-243)
10
(
-111) (111)
(404)
12.0 12.2 12.4 12.6 12.8 13.0
35.6
35.8
36.0
36.2
36.4
39.2 39.4 39.6 39.8 40.0 40.2
Bragg Angle 2θ (deg)
Fig. 4. Temperature evolution of three groups of diffraction peaks located at different 2y positions. The calculated and difference diffraction profiles
lines) are obtained after Rietveld refinement of the structure at each temperature. The structural models used in the refinement have the following
(
symmetries: P21=n for 300 and 770 K, and I4=m for 1120 K. For clarity, only the Bragg positions of the main phase are shown as bars in the lower part of
the graphics.
improvement in the agreement factors of the refinement. In
the diffraction data collected at 770 K, the presence of the
monoclinic distortion is even clearer. There is a visible
splitting (middle panel of Fig. 4c) of the peak, caused by
the difference in 2y positions of the ðꢀ404Þ and (404)
reflections. The structural details of Sr CdWO at room
cannot present such properties. If the crystal structure had
a symmetry described by some of the non-centrosymmetric
subgroups of P21=n: P21, Pn or P1, additional diffraction
peaks should be observed, since these space groups have
different extinction conditions. Such additional peaks
could not be observed in our diffraction data, and it was
not possible, on the base of the collected X-ray powder
2
6
temperature and 770 K, refined by assuming monoclinic
lattice and the P2 =n space group, are shown in Table 1.
diffraction data, to confirm that Sr CdWO6 is non-
2
1
With respect to the piezoelectric properties observed for
Sr2CdWO6 in [13], it should be pointed out that a structure
with the P21=n space group, which is centrosymmetric,
centrosymmetric.
Before analyzing the structure of Sr2CdWO6 at 1120 K,
it should be noted that the diffraction experiment at this

ARTICLE IN PRESS
M. Gateshki et al. / Journal of Solid State Chemistry 180 (2007) 2248–2255
2252
Table 1
Crystal structure data and refinement results for Sr2CdWO6 at 300, 770 and 1120 K
Temperature space group
Agreement factors
Lattice parameters
˚
(A)
Atom
Wyck. pos.
x
y
z
˚
2
BisoðA Þ
3
00 K
a ¼ 5:7463ð1Þ
b ¼ 5:8189ð1Þ
c ¼ 8:1465ð1Þ
W
2a
2b
4e
4e
4e
4e
0
1/2
0
1/2
0
0.5(1)
0.7(1)
0.9(1)
2.7(1)
2.7(1)
2.7(1)
P21=n
Cd
Sr
0
Rwp ¼ 7:18%
RB ¼ 4:29%
0.0081(7)
0.089(1)
0.254(2)
0.182(2)
0.5375(1)
0.021(2)
ꢀ0.194(2)
0.250(2)
0.2508(3)
ꢀ0.23(1)
0.032(2)
0.058(2)
ꢂ
b ¼ 90:071 ð1Þ
O1
O2
O3
2
w ¼ 11:6
7
73 K
a ¼ 5:7905ð1Þ
b ¼ 5:8122ð1Þ
c ¼ 8:1974ð1Þ
W
2a
2b
4e
4e
4e
4e
0
1/2
0
1/2
0
0.7(1)
1.2(1)
1.8(1)
3.9(1)
3.9(1)
3.9(1)
P21=n
Cd
Sr
0
Rwp ¼ 7:19%
RB ¼ 4:06%
0.0050(1)
0.073(3)
0.282(2)
0.179(3)
0.5261(3)
0.024(2)
ꢀ0.203(2)
0.264(3)
0.2512(5)
ꢀ0.216(2)
0.009(3)
0.046(2)
ꢂ
b ¼ 90:091 ð1Þ
O1
O2
O3
2
w ¼ 9:86
1
120 K
a ¼ 5:8273ð1Þ
c ¼ 8:2564ð1Þ
W
Cd
Sr
2a
2b
4d
4e
8h
0
0
0
1/2
1.5(1)
2.2(1)
3.4(1)
6.1(1)
6.1(1)
I4=m
0
0
1/2
Rwp ¼ 8:02%
RB ¼ 4:5%
0
1/4
O1
O2
0
0.265(2)
0
0.181(3)
0.230(2)
0
2
w ¼ 14:1
The atomic positions (in fractional coordinates) and isotropic atomic displacement parameters were refined in the space groups P21=n (300 K and 770 K)
and I=4m (1120 K).
1
1
20
00
8
6
4
2
0
0
0
0
0
10
15
20
25
30
35
40
45
50
55
60
65
Bragg Angle 2θ (deg)
Fig. 5. Experimental (symbols) and calculated (line) powder diffraction profiles for the Rietveld refinement of Sr2CdWO6 at 1120 K using a structural
model with I4=m space group. The bars in the lower part of the graphic represent the Bragg peak positions of the main phase, the SrWO4 impurity (middle
set of bars) and SiO2 (lower set of bars). The arrow points to the most intense diffraction peak of SiO2.
temperature (Fig. 5) shows that the weight fraction of
SrWO increases to about 4%, indicating partial decom-
shows that there is a change from a primitive unit cell to a
body-centered one. Also, while some of the peaks remain
split (Fig. 4b) at this temperature, others are well
represented by a single diffraction reflection (Fig. 4c).
Analyzing the peak splitting, it becomes clear that at
1120 K the unit cell is tetragonal.
4
position of the sample. Also, at this temperature, several
low-intensity peaks appear that are incompatible with the
perovskite structure. Through search–match procedure,
these peaks were identified as originating from quartz, and
are probably due to crystallization process in the material
of the capillary tube.
If this result is compared with the list of the possible
double perovskite structures suggested by group-theoreti-
cal analysis [10,11], it can be assumed that at 1120 K
Regarding the Sr CdWO material under study, there
2
6
0
0
ꢀ
Sr CdWO has the symmetry I4=m (tilt a a c ). The
2 6
are several important observations that can help identify its
structural type at 1120 K. For example, all diffraction
peaks with indices of the type h þ k þ l ¼ 2n þ 1 present at
lower temperatures are missing at 1120 K (see Fig. 4a). This
pattern calculated using this symmetry accounts for all
important features of the experiment (Fig. 5) and provides
reasonably good agreement factors. Structural details for

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M. Gateshki et al. / Journal of Solid State Chemistry 180 (2007) 2248–2255
2253
Sr2CdWO6 at 1120 K are given in Table 1. It can be seen
that the isotropic atomic displacements of the oxygen
atoms are rather large even considering that the experiment
is performed at high temperature. Refinement of the
structure was attempted also with the space group I4
which allows not only octahedral rotations but also atomic
displacements along the c-axis. This did not lead to any
improvement of the agreement factors despite the increased
number of refinable atomic coordinates. One likely cause
for the unusually large isotropic displacements of the
oxygen atoms is the modified chemical composition of the
material due to the ongoing decomposition process.
The tetragonal distortion of the unit cell at 1105 K is also
responsible for the peak splitting observed in Fig. 1. As
already mentioned, this splitting gradually disappears at
higher temperature (upper panel of Fig. 1b), and suggests
that at a temperature of about 1220 K the unit cell is
converted to cubic. This cubic phase most probably has the
symmetry of the aristotype double perovskite structure
with space group Fm3m. It was not possible to confirm
this hypothesis through structural refinement, due to the
instrumental limitations, and to the fact that at this
temperature the decomposition process accelerates (note
the diminished intensity of the peak shown in the upper
panel of Fig. 1b, compared to the other two temperatures).
Further studies are required to confirm the presence of the
cubic phase at high temperatures and to establish its
symmetry.
Rb2KBF6 ðB ¼ Sc; In; Lu; Er; HoÞ elpasolites [29], and
confirm that the most important factor governing the
appearance of octahedral tilts in the double perovskite
structure and the temperature range in which they exist is
the mismatch between the size of the A cation and
0
interstitial space between the BO6 and B O octahedra.
6
There is one exception, however, and this is the case of
Sr2MgWO . This compound has a tolerance factor that is
6
very close to that of Sr2CoWO6 but considerably lower
temperature of the cubic-to-tetragonal transition. Also, the
low-temperature studies of this compound [12] showed that
the tetragonal structure is preserved at least down to 26 K,
while in Sr CoWO the monoclinic phase is observed
2
6
below 250 K [11,23]. This suggests that other factors can
also influence the phase transition temperatures. Such
factors could be the presence of oxygen or cation vacancies,
and the electronic properties of the particular cations. If
the lattice parameters of Sr2MgWO [12] are compared
6
with those obtained in other recent studies, [30–32] using
also different precursor materials and synthesis tempera-
tures, it can be seen that they are within 0.1% of each
other. This makes unlikely the presence of excessive
amount of vacancies in our sample and also any other
synthesis related effect. The temperature of the tetragonal-
cubic phase transition reported in [30] is identical (570 K)
to the one observed in our previous study. Similarly, low-
temperature studies of Sr2MgWO6 [32] show that the
tetragonal phase is retained down to 15 K. Further studies
The structural analysis based on the presented tempera-
ture-dependent high-resolution diffraction experiments
suggests that the phase transition sequence existing in
Sr2CdWO6 is P21=n ! I4=m ! Fm3m; being the first
transition discontinuous, and the second one continuous.
The same sequence is known to exist in many other double
perovskites [24], including Sr2BWO6 ðB ¼ Co; Zn; CaÞ
are required to establish the reason for Sr MgWO not
2
6
following the general trend observed in Fig. 6.
1
1
1
400
200
000
800
Ca
Cd
[
11,12], suggesting that the temperature evolution of their
Mn
structures is governed by the same principles. It is usually
assumed [24] that the driving force of the structural
changes in this kind of materials is the mismatch between
Zn
Co
2
þ
the size of the A cation (Sr
in this case) and the
0
Ni
interstitial space between the BO6 and B O6 octahedra.
This mismatch is measured by means of the so-called
tolerance factor [25]:
6
4
00
00
Mg
d
AꢀO
þ d
t ¼
.
1
2
200
0
pffiffi ðd
0
Þ
BꢀO
B ꢀO
The mean d
, dBꢀO and d
0
bond lengths can be
AꢀO
B ꢀO
0
.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99
obtained from tables of ionic radii [26] or by calculating the
distances that give the nominal oxidation states of the
cations in the bond–valence method [27,28]. Fig. 6 shows
the phase transition temperatures as a function of the
tolerance factor t, calculated using the second method. As
the figure shows, the phase transition temperatures increase
at lower values of t. Also, the temperature range in
which the tetragonal phase exists is narrower at low t.
These results are very similar to those observed in
Tolerance Factor
Fig. 6. Phase transition temperatures of Sr2MWO6 compounds, as a
function of their tolerance factors. The temperature values used were
taken from previously reported studies: [12], for M ¼ Ca; Mg; and [11], for
M ¼ Zn; Co; Ni. The phase transition temperatures for M ¼ Mn are based
on results obtained from high-temperature XRD that will be published
elsewhere. Open diamonds represent the tetragonal-cubic transition, and
solid squares represent the monoclinic-tetragonal transition (if existing) in
each compound. Lines are guides to the eye.

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M. Gateshki et al. / Journal of Solid State Chemistry 180 (2007) 2248–2255
2254
the phase transition temperatures of Sr2CdWO6 are
2
2
2
2
2
2
2
90
80
70
60
50
40
30
compared with those of other compounds of the
Sr2MWO6 family reported previously, it becomes clear
that the transition temperatures increase at low-tolerance
factors, and, that, at the same time, the temperature range
in which the intermediate tetragonal phase exists is
reduced. This comparison also showed that the tempera-
Ca
Cd
Mn
Co
Cu
ture-dependent structural evolution of the Sr2MgWO6
Zn
compound considerably deviates from the general trend
observed in the rest of the Sr2MWO6 materials.
Ni
Mg
Acknowledgments
Part of this work was carried out at the National
Synchrotron Light Source (X7A beam-line), Brookhaven
National Laboratory (BNL), which is supported by the
U.S. Department of Energy, Division of Material Sciences
and Division of Chemical Sciences. The authors thank to
Dr. T. Vogt and Dr. B. Noheda from BNL for their kind
and precise assistance during the experiments. This work
was done in part under project numbers: UPV 0063.310-
0
.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99
Tolerence Factor
Fig. 7. Variation of the unit-cell volumes at ambient temperature of
Sr2MWO6 compounds, as a function of the tolerance factor. Both the
experimental values (solid squares) and those calculated with the program
SPuDS (open circles) show linear dependence with the same slope. The
unit-cell volume of Sr2MgWO , obtained from the structural data given in
6
the work by Khalyavin et al. [31], is also included as a reference (open
diamond).
1
3564/2001-2006 and FIS2005-07090. The authors thank
the technician of SGIker, Dr. J.P. Chapman, financed by
the Programa Nacional de Potenciacion de Recursos
Humanos del Plan Nacional de Investigacion Cientıfica,
Desarrollo e Innovacion-Ministerio de Ciencia y Tecnolo-
gıa y Fondo Social Europeo (FSE), for the X-ray
´
´
´
If the unit cell volumes at ambient temperature of the
studied double perovskite oxides is plotted as a function of
the tolerance factor (Fig. 7), a linear dependence can be
observed. This, again, reinforces the result that structural
properties of double perovskites are predetermined by the
cationic radii. On the same figure the unit cell volumes
calculated for the studied compounds using the program
SPuDS (Version 2—Creation date December 2005) [19,33]
are given. For each compound the tilt pattern observed in
the experiment was used in the calculation. The calculated
values have a linear dependence on the tolerance factor
with the same slope observed in the experiment. The only
difference is that the calculated values slightly overestimate
the unit cell volumes for all compounds.
´
´
diffraction experiments.
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