Formation of a cubic Sr2MnWO6 phase at elevated temperature;

a neutron powder diffraction study

A.K. Azada,b,*, S.-G. Erikssona,b

aStudsvik Neutron Research Laboratory, Uppsala University, SE-611 82 Nykoping, SwedenbDepartment of Inorganic Chemistry, University of Gothenburg, SE-412 96 Goteborg, Sweden

Received 17 February 2003; accepted 20 March 2003 by C.N.R. Rao

Abstract

In a temperature dependent neutron powder diffraction (NPD) study we observed the high temperature cubic phase at 973 K

in the polycrystalline double perovskite Sr2MnWO6. Rietveld analysis of the NPD data shows that the room temperature

tetragonal phase exists up to 573 K (space group P42=n; a ¼ 8.0119 (4) A, c ¼ 8.0141(8) A). At 773 K, the primitive tetragonal

symmetry change to body-centred tetragonal (space group I4=m; a ¼ 5.6935(5) A, c ¼ 8.077(1) A) and finally at 973 K it

becomes face-centred cubic (space group Fm-3m; a ¼ 8.0864(8) A). The changes in the structural symmetry are connected to

the small distortion of the B-site octahedra, which are insensitive to the Differential Thermal Analysis (DTA) signal.

q 2003 Elsevier Science Ltd. All rights reserved.

PACS: 61.12. 2 q; 61.12.Ld

Keywords: C. Crystal structure and symmetry; D. Phase transitions; E. Neutron diffraction

1. Introduction

Some of the oxides of the double perovskites family

A2B0B00O6 (A is an alkaline earth; B0,B00 are heterovalent

transition metals) have recently been described to exhibit

half-metallic ferromagnetism with a high spin polarization

at the Fermi level, making them promising candidates for

future spin electronics [1–3]. The modification of structural

and magnetic properties of B-site ordered double perovskite

oxides, caused by a change of B-site cations, is of great

interest, and of use when trying to understand the

mechanism of e.g. colossal magnetoresistance. Some

compounds with ordered perovskite structure show a

considerable distortion with c=a . 1 which depends on the

radius of the A-site cations and/or a Jahn–Teller effect.

These structural distortions are of interest not only from a

crystallographer’s point of view, but also because they can

have important effects on the physical properties of

perovskite compounds, particularly its electric and magnetic

properties. Recently, motivated by the theoretical and

experimental studies, we have studied the room temperature

to low temperature structural and magnetic properties of the

series A2MnWO6 (A ¼ Ca, Sr, Ba) [4–7].

In crystal structure analysis the principal interest in

lattice vibrations is their influence on the intensities of the

Bragg reflections. Due to the increase of the lattice vibration

with temperature some of the reflections of the powder

pattern with close d-spacing can be combined together and

the decrease/increase of the Bragg intensities convert the

crystal structure to the higher symmetry. Considering these

effects, we have done the high-temperature neutron diffrac-

tion study of the Sr2MnWO6 double perovskite to look at the

changes of the crystal symmetry and the individual

temperature factors of the atoms.

0038-1098/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S0038-1098(03)00280-1

Solid State Communications 126 (2003) 503–508

www.elsevier.com/locate/ssc

* Corresponding author. Address: Studsvik Neutron Research

Laboratory, Uppsala University, SE-611 82 Nykoping, Sweden.

Tel.: þ46-155-221871; fax: þ46-155-263001.

E-mail address: azad@studsvik.uu.se (A.K. Azad).

2. Experimental

The preparation of Sr2MnWO6 is reported in more

details elsewhere [5]. In short, high purity powders of

SrCO3, MnO and WO3, were mixed and calcined at 1223 K

for 15 h. The sample was pressed into a pellet and reacted at

1373 K for 48 h, 1473 K for 72 h, 1523 K for 48 h and

1613 K for 48 h in an N2 environment. The sample was

reground before each new sintering step, and grinding and

pelleting cycles were carried out to ensure the homogeneity

of the sample.

X-ray diffraction patterns were obtained from Guinier

film data ðCu Ka1 ¼ 1:540598 �AÞ: These data were used to

index the pattern. Indexing and refinement of the lattice

parameters were made with the programs TREOR90 [8] and

Chekcell [9]. Neutron powder diffraction data were

collected at the 50MW R2 Research Reactor at Studsvik,

Sweden. The double monochromator system consisting of

two parallel copper crystals in (220) orientation was aligned

to give a wavelength of 1.470(1) A. A vanadium can was

used as the sample holder. The step scan covered a 2u-range

48–139.928 with a step size of 0.088. The sample was placed

inside a controlled high temperature vanadium furnace. The

collected NPD data sets were refined by the Rietveld method

[10] using the FullProf software [11]. Diffraction peak

shapes were considered as pseudo-Voigt. Background

intensities were described by a Chebyshev polynomial

with six coefficients. Peak asymmetry corrections were

made during refinements.

3. Results and discussions

The neutron diffraction data were collected at four

different temperatures, 295, 573, 773 and 973 K. Fig. 1

shows the changes in the neutron diffraction pattern

corresponding to a change in lattice symmetry as a function

of temperature. At room temperature (295 K) the structure

was refined using the tetragonal space group P42=n (no. 86,

setting 2). The refinement procedure used here has been

derived in Ref. [5] in detail. This tetragonally distorted

ða2a2a2Þ [12] tilt system can illustrate the combined effect

of cation ordering and octahedral tilting. The unit cell

parameters are related to that of ideal cubic perovskite as

a < 2ap; b < 2ap; c < 2ap ðap < 3:89 �AÞ where c=a <1:00027: Mn and W are found to occupy alternate B-sites.

The refinement resulted in fairly good R-factors

(Rp ¼ 3:89%; Rwp ¼ 5:15%; RBragg ¼ 2:97%; x2 ¼ 1:81).

Structure parameters, temperature factors, occupancies and

R-factors obtained from the analysis at 295, 573, 773 and

973 K are summarised in Table 1. The MnO6 octahedra

(volume ¼ 12.9820(5)) are significantly larger than WO6

octahedra (volume ¼ 9.3995(3)). The MnO6 and WO6

octahedra are ordered and alternate along the three

directions in the crystal structure in such a way that each

MnO6 octahedra is linked to six WO6 octahedra and vice-

versa for all structures. Fig. 2 shows the 3D view of

tetragonal and cubic double perovskites at 773 and 973 K,

respectively. Bond valence calculation [13] from our

observed data shows that the charge distribution of Mn

and W cations is Mn2þ and W6þ, respectively. At 573 K, the

structure remains in the same symmetry and space group.

Only small changes in the refinement parameters are

observed. Thermal vibration of the atoms was increased

due to the increased temperature.

In crystal structure analysis the temperature induced

lattice vibrations are very interesting to study, especially

their influence on the intensities of the Bragg reflections. It

is well known that, the effect of thermal motion in the

calculation of the structure factor is known as the

temperature factor of the atom. Ignoring thermal motion,

the temperature factor may be described as the quantity

representing the reduction in the effective scattering factor

due to lattice vibrations. The corresponding quantity

representing the reduction in the intensity of the Bragg

reflection is the Debye–Waller factor. It is even possible for

the Bragg intensity to increase rather than decrease with

rising temperature, either because individual atoms have

widely different temperature factors or because of anhar-

monicity. From a thorough examination of Fig. 1, it is clear

that the intensity of the Bragg reflections is decreasing with

temperature due to the increase of the atomic vibrations.

However, the intensity of some of the reflections, like (200)

and (220), in the cubic phase, remains the same in the whole

temperature range. Many reflections disappear in the

background at high temperature. Some of the peaks, like

(531) and (622), have satellite reflections at room tempera-

ture, which disappears with the increase of temperature. The

intensity of the more overlapping reflections are more

affected by the atomic vibration due to the smaller

difference in the d-spacing. Exact and/or close-overlapping

reflection permits us to consider higher symmetry in the

refinement.

According to the observed diffraction pattern at 773 and

973 K, the difference in the peak intensity distribution is

very small (Fig. 1). If we refine the 773 K data in cubic

Fm-3m space group, the thermal parameters of the

individual atoms goes negative, which is indicative to

change lattice symmetry in this case to tetragonal symmetry.

At 773 K, the crystal structure changes from primitive

tetragonal symmetry (space group P42=n) to body centered

tetragonal symmetry (space group I4=m; no. 87). The

reflection condition hkl; h þ k þ l ¼ 2n; hk0; h þ k ¼ 2n;

0kl; k þ l ¼ 2n; hhl; l ¼ 2n; 00l; l ¼ 2n and 0k0; k ¼ 2n

allows us to select the space group. This space group is a

subgroup of the space group, which describes the simple

cubic B-site ordered perovskite, Fm-3m: The unit cell

parameters are related to that of ideal cubic perovskite as

a <ffiffi

2p

ap; b <ffiffi

2p

ap; a < 2ap ðap < 3:89 �AÞ: The Wyckoff

positions were, Mn on 2b; W on 2a; Ba on 4d and O on 4e

and 8h: This gives us an ordered perovskite structure of

A.K. Azad, S.-G. Eriksson / Solid State Communications 126 (2003) 503–508504

so-called elpasolite type. Mn and W are found to occupy

alternate B-sites. The distances between Mn and O in the ab

plane and parallel to the c-axis were 2.157(5) A and

2.15(1) A. The distances between W and O in the ab

plane and parallel to the c-axis were about 1.896(5) A and

1.89(1) A, respectively. The volume of the MnO6 and WO6

octahedra was calculated to be 13.3509(6) A3 and

9.0428(5) A3. The Glazer notation [12] of the distortion is

a8a8c2 (one tilt system), which can illustrate the combined

effect of cation ordering and octahedral tilting.

Fig. 1. Observed neutron diffraction patterns at 295, 573, 773 and 973 K.

Fig. 2. The crystal structure of the double perovskite Sr2MnWO6 at (a) 773 K (tetragonal, s.g. I4=m) and (b) 973 K (cubic, s.g. Fm-3m).

A.K. Azad, S.-G. Eriksson / Solid State Communications 126 (2003) 503–508 505

At 773 K, the crystal structure changes from body

centered tetragonal symmetry to face centered cubic

symmetry (space group Fm-3m; no. 225). The observed

and calculated patterns, differences and the peak positions

at 295, 773 and 973 K are shown in Fig. 3. The lattice

parameter, a; at 295 K is found to be 8.0864(8) A. Mn

and W form together with oxygen a NaCl-type lattice

where both the cations show perfect octahedral anion

coordination. The MnO6-octahedra are larger than the

WO6-octahedra, an observation in accordance with the

larger ionic size of Mn2þ ðrMn2þ ¼ 0:97 �AÞ compared to

W6þ ðrW6þ ¼ 0:74 �AÞ: Each strontium ion is coordinated

Fig. 3. Observed (circles) and calculated (continuous line) NPD intensity profiles for Sr2MnWO6 at 295, 773 and 973 K. The short vertical lines

indicate the angular position of the allowed Bragg reflections. At the bottom in each figure the difference plot, Iobs 2 Icalc; is shown.

A.K. Azad, S.-G. Eriksson / Solid State Communications 126 (2003) 503–508506

to twelve oxygen ions, and being part of the ccp layers.

The average Sr–O bond lengths at 295 K compare well

with the expected values calculated as the sum of the

ionic radii [14]. The observed Mn–O distance is

2.1759(1) A and the calculated distance is 2.23 A. The

observed and calculated W-O distance are 1.9234(1) A

and 2.00 A, respectively.

It is worth to notice that we have done DTA

measurement in order to determine the exact transition

temperature, which was not successful. The reason may be

that the DTA signal of the instrument was not sensitive

enough to the change of the small polyhedral distortion,

taking place.

4. Conclusions

In summery, we have investigated the high temperature

crystal structures of the polycrystalline double perovskite

Sr2MnWO6. We found a tetragonal unit cell with rock-salt

type order of Mn and W ions for the whole temperature

range (295–973 K) investigated. Due to the increase in

lattice vibrations with temperature, the symmetry changes

from primitive tetragonal (space group P42=n; at 295 K) to

body centered tetragonal (space group I4=m; at 773 K) to

face centered cubic (space group Fm-3m; at 973 K). From

BVS calculation the charge distribution between Mn and W

ions in Sr2MnWO6 was calculated, indicating the presence

of Mn2þ and W6þ.

Acknowledgements

We are grateful to P. Berastegui, J. Eriksen and

H. Rundlof for their help and cooperation during the work.

This work was supported by the Royal Swedish Academy of

Sciences, the Swedish Natural Science Research Council

(NFR) and the Swedish foundation of Strategic Research

(SSF). The authors are also grateful to Bangladesh Atomic

Energy Commission for cooperation.

Table 1

Cell parameters, variable fractional atomic coordinates and Rietveld refinement reliability factors of Sr2MnWO6 at 295, 573, 773 and 973 K

Parameters Sr2MnWO6

295 K 573 K 773 K 973 K

Space group P42=n P42=n I4=m Fm-3m

a (A) 8.0119(4) 8.0385(7) 5.6935(5) 8.0864(8)

b (A) 8.0119(4) 8.0385(7) 5.6935(5) 8.0864(8)

c (A) 8.0141(8) 8.042(1) 8.077(1) 8.0864(8)

V (A3) 514.44(6) 519.6(1) 261.83(5) 528.77(9)

B (A2) of Sr(1) 0.7(3) 2.1(5) 1.64(9) 2.8(1)

B (A2) of Sr(2) 0.2(2) 1.4(7)

z of Sr(3) 0.2671(7) 0.251(2)

B (A2) of Sr(3) 0.2(1) 0.9(3)

B (A2) of Mn 0.03(9) 0.9(2) 1.6(2) 0.9(1)

B (A2) of W 0.08(7) 0.6(1) 1.1(1) 0.6(1)

x of O(1) 0.2624(9) 0.266(2) – 0.2646(3)

y of O(1) 20.0399(7) 20.034(1) – –

z of O(1) 20.0123(7) 20.014(5) 0.234(2) –

B (A2) of O(1) 0.12(6) 0.1(1) 3.6(1) 3.4(7)

x of O(2) 0.2341(8) 0.231(2) 0.2638(9)

y of O(2) 0.0375(8) 0.037(1) 0.2945(9)

z of O(2) 0.5234(7) 0.516(1) –

B (A2) of O(2) 0.24(6) 1.2(2) 1.65(9)

x of O(3) 0.0148(9) 0.007(1)

y of O(3) 0.0321(7) 0.018(1)

z of O(3) 0.2698(8) 0.260(2)

B (A2) of O(3) 2.7(1) 2.8(1)

Volume of MO6 octahedra 12.9820 13.3509 13.7355

Volume of WO6 octahedra 9.3995 9.0428 9.4869

R-factors, Rp (%) 3.89 3.62 4.33 4.45

Rwp (%) 5.15 4.67 5.53 5.99

RBragg (%) 2.97 3.47 6.41 8.05

x2 1.81 1.61 2.21 2.56

A.K. Azad, S.-G. Eriksson / Solid State Communications 126 (2003) 503–508 507

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A.K. Azad, S.-G. Eriksson / Solid State Communications 126 (2003) 503–508508