Synthesis, crystal structure, and magnetic characterization of the double perovskite Ba2MnWO6

Synthesis, crystal structure, and magnetic characterizationof the double perovskite Ba2MnWO6

A.K. Azada,c,*, S.A. Ivanovb, S.-G. Erikssona,c, J. Eriksena, H. Rundlofd,R. Mathieue, P. Svedlindhe

aStudsvik Neutron Research Laboratory, Uppsala University, SE-611 82 Nyko¨ping, SwedenbKarpov Institute of Physical Chemistry, RU-103064 K-64, Moscow, Russia

cDepartment of Inorganic Chemistry, University of Gothenburg, SE-412 96 Go¨teborg, SwedendDepartment of Materials Chemistry, The Ångstro¨m laboratory, Box 538, SE-751 21 Uppsala, Sweden

eDepartment of Materials Science, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden

(Refereed)Received 23 March 2001; accepted 21 May 2001

Abstract

The double perovskite Ba2MnWO6 has been prepared as a pure powder by a conventionalsolid-state reaction process and studied by X-ray, neutron powder diffraction (NPD), magnetization,and AC susceptibility measurements. NPD, magnetization, and AC susceptibility measurements havebeen carried out at different temperatures to correlate structural and magnetic properties. Rietveldanalysis of NPD patterns show that the sample is a B-site ordered perovskite with elpasolite-typestructure. At T5 295 K the crystal structure is cubic witha 5 8.1985(3) Å, (space group Fm-3m).Temperature-dependent magnetization and AC susceptibility measurements show a transition at;45K to a weak ferromagnetic state, with a spontaneous moment at 15 K corresponding tom 5 2.2 1023

mB/Mn21. A second transition, to an antiferromagnetic state, still having a weak spontaneous magneticmoment, is detected at;9 K. This transition is also confirmed by NPD measurements performed at4.2 K, showing a number of new distinct magnetic Bragg peaks corresponding to a G-type magneticstructure. © 2001 Elsevier Science Ltd. All rights reserved.

Keywords:A. Oxides; A. Magnetic materials; C. Neutron scattering; D. Crystal structure; D. Magnetic properties

* Corresponding author.E-mail address:[email protected] (A.K. Azad).

Pergamon Materials Research Bulletin 36 (2001) 2215–2228

0025-5408/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.PII: S0025-5408(01)00707-3

1. Introduction

The discovery of high-temperature superconductivity in cuprates [1] and of colossalmagnetoresistance (CMR) in manganites [2,3] have during the last years raised a stronginterest in complex transition metal oxides containing mixed valence ions. Perovskite oxideshave the general formula ABO3, where the A-cation can be an alkaline metal, an alkalineearth metal, or a lanthanide. The B-site is often occupied by transition metals, and canaccommodate two or more different metal ions. Complex metal oxides with the generalformula A2B9B0O6, where B9 and B0 sites are occupied alternately by different cations,depending on their valences and relative ionic radii, are known as double perovskites orelpasolites [4]. A continued interest in their synthesis and physical properties, especially theirmagnetic behavior, can be seen [5,6]. These oxides can be classified into two main types,namely B-site ordered and B-site disordered perovskites [7–9].

B-site ordered double perovskites, where B9 is magnetic while B0 is nonmagnetic, wasinitially studied by Blasse [10]. Some compounds of this type with an ordered perovskitestructure and a Jahn-Teller active ion (Mn31, Ni31 (low spin) or Cu21), show a considerabletetragonal distortion withc/a . 1. These compounds are also characterised by a two-dimensional antiferromagnetic behavior, because of theab-plane superexchange interactionbetween the magnetic ions via an array of nonmagnetic ions. When double perovskitesinclude transition metal ions within the B sublattice, the magnetic properties are stronglyinfluenced by the ordering of the cations within this sublattice.

Barium perovskites have revealed a variety of magnetic structures, such as antiferromag-netic in Ba2CoWO6 and Ba2NiWO6 [10,11], ferrimagnetic in Ba2MnReO6 [12,13],Ba2FeMoO6 [14], and Ba2FeReO6 [15], and a spin spiral arrangement has been found inBa2CoReO6 [16]. Moreover, the double perovskites Ba2CoNbO6 [7], Sr2FeRuO6, Ba-LaNiBiO6, and Ba2LnNbO6 [17–19] have been suggested to exhibit spin-glass like behaviorat low temperature. The structures are face-centered cubic, and the perovskite cell is doubledalong all three axes. Khattak et al. [20] showed, from Neutron Powder Diffraction (NPD)data, that the ordered perovskite Ba2MnWO6 (BMW) is cubic with the unit cell parametera 5 8.19 Å. Moreover, magnetic measurements indicated an antiferromagnetic state at lowtemperature with a Ne´el temperature of 7.5 K. However, the investigated sample was notpure, and an impurity content of about 5% each of BaWO4 and BaMnO3 was estimated. Tothe best of our knowledge, no single-phase material of BMW has been made and charac-terized so far. In our current research on Ba2MnWO6, we have succeeded in the synthesis ofsingle-phase material and characterized its structural and magnetic properties.

2. Experimental

Polycrystalline Ba2MnWO6 was prepared by a traditional solid-state reaction from stoi-chiometric amounts of BaCO3, MnO, and WO3, to give the desired composition, i.e., the2:1:1 cation ratio. All the starting materials were mixed in an agate mortar and as a grindingaid ethanol was added. The ground powder was placed in an Al2O3 crucible and calcined at1223 K for 15 h. The sample was reground and pressed into a pellet, thereafter fired at 1373

2216 A.K. Azad et al. / Materials Research Bulletin 36 (2001) 2215–2228

K for 48 h, 1473 K for 72 h, and 1523 K for 48 h. Between each sintering step the pelletswere ground to a fine powder and new pellets formed before the consecutive heat treatment.All the heat treatments were performed under a controlled N2 atmosphere, and heating aswell as cooling rates were 4 K/min.

In between each sintering step phase purity was checked by X-ray diffraction. When noimpurity phases could be detected the reaction was considered to be complete. X-raydiffraction patterns were obtained from Guinier film data (CuKa1 5 1.540598 Å). Silicon(NBS 640b) was used as an internal standard, and a computerized line-scanner system [21]was used for evaluation of the films. Indexing and refinement of the lattice parameters weremade with the programs TREOR90 [22] and PCPIRUM [23].

NPD data were collected at the 50MW R2 research reactor at the Neutron ResearchLaboratory at Studsvik, Sweden, by means of a Huber two-circle diffractometer with an arrayof 35 3He detectors. A double monochromator system consisting of two copper crystals inthe (220) mode was aligned to give a wavelength of 1.470(1) Å. The neutron flux at thesample position was approximately 106 neutron cm22s21. A vanadium can was used as asample holder. The step scan covered the 2u range 4°–139.92°, with a step size of 0.08°. Datawere collected at eight different temperatures, 295, 60, 50, 40, 30, 20, 10, and 4 K. The 4 Kdata was collected at the POLARIS neutron powder diffractometer at ISIS, RutherfordAppleton Laboratory, UK.

DC magnetization measurements have been performed in a QuantumDesign supercon-ducting quantum interference device (SQUID) magnetometer. Magnetisation versus temper-ature curves were measured between 5 and 200 K in field-cooled (FC) and zero-field-cooled(ZFC) modes. Fields up tom0H 5 3 T was applied. Magnetisation versus field isothermcurves were measured at 5, 15, and 40 K. AC susceptibility versus temperature measure-ments were performed in a LakeShore AC susceptometer. These measurements were per-formed in the temperature range 5 to 120 K, using an AC field ofh 5 800 A/m.

SEM micrographs and EDX analysis were taken in a Philips XL30 EnvironmentalElectron Scanning Microscope at an acceleration voltage of 20 kV. Second harmonicgeneration (SHG) was measured using a powder technique with a pulsed Nd:YAG laser (l 51.064mm). The measured signal was normalized toa-quarts.

3. Results and discussions

3.1. Crystal structure

The NPD data sets collected at Studsvik were refined by the Rietveld method [24] usingthe FullProf software [25]. Peakshapes were considered as pseudo-Voigt, and backgroundintensities were described by a Chebyshev polynomial with six coefficients. Peak asymmetrycorrections were made during refinements, and a small influence of preferred orientation inthe [100] direction was observed, and also corrected for.

Structural refinements were carried out for the data sets at seven different temperatures,namely at 295 K, and between 60 and 10 K in steps of 10 K (see Fig. 1a–c for representativeresults). The results of the Rietveld refinements and SHG measurements (at 295K) show that

2217A.K. Azad et al. / Materials Research Bulletin 36 (2001) 2215–2228

the BMW sample has a centrosymmetric cubic lattice, crystallizing in space group Fm-3m(No. 225). As expected, the lattice parameter becomes compressed as the temperaturedecreases. However, the behavior close to 40 K is anomalous; first an abrupt contraction ofthe lattice takes place, which is followed by an expansion when the temperature is furtherdecreased (see Fig. 2). Moreover, some of the peak intensities show a discontinuous decreaseat 40 K, which is followed by an increase in intensity on further lowering the temperature.Fig. 3 shows the integrated intensity versus temperature for different reflections. Thevariation in normalized intensity as a function of temperature of these reflections cannot beexplained solely on the basis of the atomic structure. These findings, together with magne-tization data (see section 3.2) indicate that a magnetic phase transition takes place at atemperature close to 45 K. Below this temperature a weak ferromagnetic state is stable. Thiswill be discussed in more detail below.

During Rietveld refinements of neutron data at 50, 40, 30, 20, and 10 K, some of theisotropic temperature factors became negative. If the data up to 2u 5 70° is excluded,however, all temperature factors are positive. The low temperature magnetic ordering is notincluded in the Rietveld model, and as the magnetic contribution to the scattering mainlyoccurs at low angles, this may explain why some of the temperature factors become negativeif data below 70° is used in the refinement. A summary of structure parameters, interatomicdistances, isotropic temperature factors, occupancies and R-factors, obtained at differenttemperatures are presented in Table 1.

The lattice parameter,a, at 295 K, is found to be 8.1985(2) Å, which is in agreement withthe previously reported value of 8.19(3) [20]. Thex coordinate of oxygen is varied during therefinement and determined to be 0.2654(1), which is very close to the value 0.265(1) foundin ref. [20]. The possible presence of oxygen nonstoichiometry was not confirmed during thefinal Rietveld refinements. The cation occupancies were also found to be very close to theirnominal values. From the Rietveld analysis we obtain an elemental ratio Ba:Mn:W:O50.332(3):0.167(1):0.165(1):1.000, which is in excellent agreement with the nominal compo-sition of the sample, Ba2MnWO6. Mn and W form together with oxygen a NaCl-type latticewhere both the cations show perfect octahedral anion coordination. The MnO6-octahedra arelarger than the WO6-octahedra, an observation in accordance with the larger ionic size ofMn21 (rMn21 5 0.97 Å) compared to W61 (rW61 5 0.74Å). Each barium ion is coordi-nated to 12 oxygen ions, and being part of theccp layers. The average Ba–O bond lengthsat 295 K compare well with the expected values calculated as the sum of the ionic radii [26].The observed Mn–O distance is 2.1759(1) Å, and the calculated distance is 2.23 Å. Theobserved and calculated W–O distances are 1.9234(1) Å and 2.00 Å, respectively.

Fig. 1. Observed (circles) and calculated (continuous line) NPD intensity profiles for Ba2MnWO6 at (a) roomtemperature (295 K), and (b) 10 K. The short vertical lines indicate the angular position of the allowed Braggreflections. At the bottom in each figure the difference plot, Iobs2 Icalc, is shown. (c) Plot of intensity versus 2uobtained at different temperatures.


Fig. 1. Continued


Fig. 2. The temperature dependence of the lattice parameter,a, for Ba2MnWO6.

Fig. 1 Continued


3.2. Magnetic properties

Materials adopting the cubic elpasolite type structure, like BMW, are ideal systems to study180° superexchange interactions between different cations. The magnetic interactions to beconsidered are of Mn–O–W superexchange type where the Mn–O–W angle is exactly 180°.There are no Mn–O–Mn or W–O–W interactions, and there are no Mn–O–W interactions otherthan 180°. The antiferromagnetic properties arise from the superexchange interaction betweenMn ions via an array of nonmagnetic ions, O–W–O. Bond valence sum calculations [27] basedon the results of our neutron data analysis unambiguously show that the valence states are Mn21

and W61. These results are consistent with Blasse’s prediction [10].The temperature dependence of the magnetization was measured for the sample under

both ZFC and FC conditions. Fig. 4 shows the FC (MFC/H) and ZFC (MZFC/H) curves forBMW. They are measured with the applied fieldsm0H5 0.002, 0.05, 0.2, and 3 T, and thecurve corresponding to the smallest field shows that a magnetic transition occurs at Tc ;45K. The magnetic interactions are predominantly antiferromagnetic (AF), but below 45 K asmall spontaneous moment develops, like in a canted-AF state [28], resulting in a magneticstate that can be described as a weak ferromagnetic state.

The presence of a spontaneous moment is further emphasized by the magnetic irrevers-ibility exhibited by the FC and ZFC curves; below the transition temperature, the ZFC andFC curves show large deviations. With increasing field, the transition is masked by the largerand linear in field AF-like response of the material. This behavior is analogous to thatobserved for the double perovskite Ba2CoNbO6 [7], where a peak like susceptibility anomalyappears around 43–45 K for low fields (below 1000 Oe) and disappears with a field of 5000Oe. Magnetic hysteresis curves are shown in Fig. 5. Again, it can be seen that a weakferromagnetic response exists but that the predominant character of the response is linear in

Fig. 3. The temperature dependence of the integrated intensity for different reflections for Ba2MnWO6.


Tab

le1

Str

uctu

ralp

aram

eter

sex

trac

ted

from

refin

emen

tof

NP

Dda

tafo

rth

eco

mpo

und

Ba

2M

nWO

6at

diffe

rent

tem

pera

ture

s

Ba 2

MnW

O6

10K

20K

30K

40K

50K

60K

295

K

Cry

stal

syst

emC

ubic

Cub

icC

ubic

Cub

icC

ubic

Cub

icC

ubic

Spa

cegr

oup

Fm

-3m

Fm

-3m

Fm

-3m

Fm

-3m

Fm

-3m

Fm

-3m

Fm

-3m

a(A

)8.

1844

(3)

8.18

56(3

)8.

1858

(3)

8.18

39(3

)8.

1859

(3)

8.18

61(3

)8.

1985

(2)

Vol

ume,

V(A

3)

548.

26(2

)54

8.47

(3)

548.

49(3

)54

8.12

(3)

548.

53(3

)54

8.57

(3)

551.

06(3

)O

(x)

0.26

49(2

)0.

2648

(2)

0.26

49(2

)0.

2652

(2)

0.26

47(2

)0.

2650

(2)

0.26

54(1

)B

ond

leng

ths,

Mn–

O2.

1681

(1)

2.16

81(1

)2.

1682

(1)

2.16

96(1

)2.

1671

(1)

2.16

99(1

)2.

1759

(1)

W–O

1.92

41(1

)1.

9247

(1)

1.92

46(1

)1.

9223

(1)

1.92

58(1

)1.

9232

(1)

1.92

34(1

)B

a–O

2.89

62(1

)2.

8966

(1)

2.89

66(1

)2.

8961

(1)

2.89

67(1

)2.

8969

(1)

2.90

14(1

)Is

otro

pic

tem

p.fa

ctor

s,B

a2

0.04

2(83

)0.

017(

82)

0.03

1(87

)0.

007(

87)

20.

070(

79)

0.04

1(79

)0.

458(

79)

Mn

0.02

9(59

)0.

071(

55)

0.16

0(58

)0.

124(

58)

20.

033(

56)

0.10

0(55

)0.

213(

50)

W2

0.12

4(50

)2

0.06

3(48

)2

0.09

9(50

)2

0.03

3(50

)2

0.08

2(48

)0.

019(

48)

0.04

0(43

)O

0.30

6(32

)0.

321(

31)

0.31

9(30

)0.

328(

33)

0.33

9(32

)0.

351(

31)

0.63

4(30

)O

ccup

anci

es,

Ba

0.33

3(4)

0.33

3(3)

0.33

0(4)

0.33

3(1)

0.33

9(4)

0.33

8(3)

0.34

1(4)

Mn

0.16

6(1)

0.16

6(1)

0.16

6(1)

0.16

6(1)

0.16

6(1)

0.16

6(1)

0.16

7(1)

W0.

167(

1)0.

167(

1)0.

165(

1)0.

168(

1)0.

167(

1)0.

167(

1)0.

165(

1)O

1.00

1.00

1.00

1.00

1.00

1.00

1.00

R-f

acto

rs,

Rp

(%)

4.36

4.03

4.20

3.78

4.15

3.93

3.40

Rw

p(%

)5.

745.

315.

544.

975.

405.

194.

53R

Bra

gg

(%)

1.82

1.51

1.41

1.80

1.35

1.30

1.74

x2

1.90

1.61

1.74

1.72

1.66

1.54

2.12

The

atom

icpo

sitio

nsfo

rth

esp

ace

grou

pF

m-3

m(N

o22

5)w

ere:

Ba:

8c

(1/4

,1/

4,1/

4),

Mn:

4a(0

,0,

0),

W:

4b(1

⁄2,1⁄2,

1⁄2)

and

O:

24e

(x,

0,0)

.


Fig. 4. MFC/H and MZFC/H versus temperature for Ba2MnWO6 measured with the applied fieldsm0H (a) 0.002T and 0.05 T, and (b) 0.2 T and 3T.


Fig. 5. Magnetization versus applied field measured at (a) 40 K, and (b) 15 K and 5 K. The inset in (b) showsthe spontaneous magnetisation versus applied field at T5 15 K, obtained by subtracting the linear high fieldresponse from the measured hysteresis curve.


Fig. 6. In-phase (x9) and out-of-phase (x0) components of the AC susceptibility versus temperature. The differentcurves correspond to different frequencies;f 5 30 Hz, 125 Hz, and 1 kHz. The results forx0 have been multipliedby a factor of 50.

Fig. 7. A plot showing the diffuse peak at an angle 2u ; 8° at different temperatures for Ba2MnWO6.


the field. The linear high field response was studied in fields up tom0H 5 4 T. By subtractingthe linear high-field dependence from the hysteresis curve measured at 15 K (inset Fig. 5b),the magnitude of the spontaneous moment at this temperature can be estimated. Themeasured magnetic moment is mainly determined by the Mn21 ions. The spontaneous(uncompensated) manganese moment so obtained becomesm 5 2.2 1023 mB/Mn21.

To further characterize the magnetic state below the transition temperature Tc, ACsusceptibility measurements were performed. Fig. 6 shows the temperature dependence ofthe in-phase (x9) and out-of-phase (x0) components of the AC susceptibility obtained forthree different frequencies. Both components exhibit frequency-dependent features at thetransition temperature. In case ofx9, the frequency dependence is;0.5% comparing theresults obtained atf 5 30 Hz andf 5 1 kHz; this difference is too small to be resolved inFig. 6. The frequency dependence close to the transition temperature do not show thecharacteristics expected for a spin-glass or a cluster-glass transition, and the existence of asuch a low-temperature disordered magnetic phase can be ruled out for the BMW sample. Itis our strong belief that, in the region from 10 to 45 K, we have a system showing weakferromagnetic order, which is supported by the magnetization and AC susceptibility data.

NPD patterns in the temperature range 10 to 60 K show a broad diffuse peak at an angle 2u;8° that grows in magnitude as the temperature is decreased below 50 K (see Fig. 7). The diffusefeature exhibits similar temperature dependence as does the low-field FC magnetization. Weattribute the broad feature, centered at 2u ; 8°, to be magnetic in origin. The most plausibleinterpretation of the neutron data is that there are small magnetic domains, showing antiferro-

Fig. 8. Plot of neutron powder diffraction data collected at the POLARIS instrument, ISIS, RAL at 4.2 K. Theextra diffraction peaks correspond to the evolution of a G-type antiferromagnetic structure. Magnetic reflectionsare indicated by stars (*).


magnetic order, but being of a size too small to give rise to distinct magnetic Bragg peaks. Sucha scenario should give rise to diffuse scattering of the type observed indicating a disorderedmagnetic state.

At lower temperature, this disordered state is replaced by a commensurate antiferromag-netic state, as evidenced by a number of low-angle magnetic Bragg reflections appearing inour T 5 4.2 K data (see Fig. 8). The extra magnetic peaks could be indexed on the basis ofa doubling of the chemical unit cell in all three directions, which is in agreement with earlierwork [20]. It is worth noting that the first Bragg peak at 4.2 K corresponds to the broaddiffuse peak seen in the diffraction data at 2u ; 8° in the temperature range 10–60 K (seeFig. 7). The antiferromagnetic structure is most likely of G-type.

The antiferromagnetic structure will be presented in a subsequent work [29], and thenature of the disorderd magnetic state, in the temperature region 10 to 60 K, will be analyzedby means of a reverse Monte Carlo modeling technique developed at Studsvik [30,31]. Themethod can handle ordered as well as disordered magnetic structures.

This low temperature transition is also indicated by the cusp in the magnetizaton and ACsusceptibility (x9) versus temperature results at a temperature of TN ; 9 K. However, themagnetic state below this temperature is not that of an ideal antiferromagnet, becausemagnetic irreversibility remains down to the lowest temperature studied (5 K), as evidencedby the results shown in Fig. 5. Thus, a small canting angle exists, giving rise to a netspontaneous magnetic moment.

Above the transition temperature Tc the sample shows typical Curie-Weiss behavior. Thedata were fitted to a Curie-Weiss law,x 5 C/(T 2 u), where C is the Curie constant anduis the Weiss temperature, yielding C5 0.658 andu 5 264.4 K. The effective number ofBohr magnetons (p) is from these results calculated to be 5.85, which can be compared to theexpected p5 5.92 for the Mn21 ion obtained from p5 2=S (S1 1) with S5 5/2. In thework of Khattak et al. [20],u 5 236 K and p5 5.4 was obtained. The small discrepancybetween our results and the results of Khattak et al. may be due to the presence of impuritiesin their sample. The value of p for BMW is nearly the same as the calculated value,indicating that thed-electrons of the Mn21 ions are to a large extent localized. The Weisstemperature of the sample is negative in sign, confirming that the magnetic interactionbetween Mn ions is AF to its nature.

4. Conclusions

The Ba2MnWO6 was obtained as a yellowish-green, well-crystallized powder. Its single-phase nature was first confirmed by X-ray diffraction analysis, and was supported by EDXanalysis. Bond valence calculations based on the results of the NPD study indicate that thevalence states of Mn and W are Mn21 and W61, respectively. The crystal structure is, for alltemperatures measured, cubic (Fm-3m) witha, ; 8.19 Å. From room temperature down to;45 K the compound is in a paramagnetic state. At lower temperatures, down to 10 K, aweak ferromagnetic state is stabilized with a spontaneous magnetic moment of 2.2z1023

mB/Mn21. The nature of this magnetic state is disorderd in the sense that it most likely is builtup of small antiferromagnetic domains. Below 10 K this disordered magnetic state undergoes


a phase transition, and an antiferromagnetic structure of G-type is formed, while still a smallnet spontaneous magnetic moment remains.

Acknowledgments

The authors are grateful to the Royal Swedish Academy of Sciences and to the SwedishNatural Science Research Council (NFR) for financial support. One of the authors, A.K.Azad, gratefully acknowledge the financial support from the “Research, development andtraining project of Bangladesh Atomic Energy Commission”. Moreover, we are grateful toMarkus Valkeapa¨a (University of Goteborg), for his help and cooperation during ESEM andEDX measurements and to Dr. S. Stefanovich for his SHG study of the sample.

References

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Synthesis, crystal structure, and magnetic characterization of the double perovskite Ba2MnWO6

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Transcript of Synthesis, crystal structure, and magnetic characterization of the double perovskite Ba2MnWO6