0012-5008/05/0012- © 2005 Pleiades Publishing, Inc.0247

Doklady Chemistry, Vol. 405, Part 2, 2005, pp. 247–250. Translated from Doklady Akademii Nauk, Vol. 405, No. 4, 2005, pp. 489–492.Original Russian Text Copyright © 2005 by Gizhevskii, Zhuravlev, Zakharov, Zinigrad, Kozlov, Leont’ev, Naumov, Petrova, Pilyugin, Fishman, Chebotaev.

Production of bulk (massive) materials with submi-crocrystalline and nanocrystalline structures is one ofthe main challenges of modern materials science [1, 2].These materials have advanced mechanical, physical,and chemical properties for construction and functionalapplications.

Quasi-static severe plastic deformation methodshave been widely used for producing bulk porelessnanomaterials [1]. However, these methods (equalchannel angular pressing, high-pressure torsion, etc.),which are well developed for metals and alloys, are vir-tually unused for preparing nanomaterials based onoxides and other brittle compounds. This is associatedwith the necessity of using high and superhigh pres-sures, as well as with the instability (reduction) of manyoxides under the action of shear deformations [3]. Asimilar situation is observed for dynamic (pulsed)methods of producing nanomaterials.

In this work, we obtained bulk nanostructured oxidematerials based on lanthanum manganite LaMnO

3 +

δ

(LMO) by high-pressure torsion and shock-wave load-ing and studied the effect of severe plastic deformationson the microstructure and crystal lattice of this com-pound. We showed that the density of the compactedmaterial can be close to the density of correspondingsingle-crystal samples. Lanthanum manganites aregood candidates for spintronics applications due totheir colossal magnetoresistance, as well as to theirlarge tunnel magnetoresistance [4]. In addition, thesematerials are of interest for use in solid fuel cells andcatalysis [5, 6].

EXPERIMENTAL

An LMO powder for the high-pressure torsionexperiments was obtained by the ceramic method from

La

2

O

3

(LaO-Lyum grade) and

Mn

3

O

4

(pure grade). Pre-liminary annealings were carried out at

1300°ë

withintermittent grinding. The final annealing was per-

formed for 10 h at

1000°ë

in a vacuum of

10

–2

mmHg.This procedure yielded a single-phase powder oforthorhombic lanthanum manganite. For the experi-ments on dynamic deformation by shock-wave loading,a larger amount of LMO powder (about 500 g) wasrequired. The solid-phase synthesis of LMO in this casewas carried out in four steps with intermittent grinding.The final annealing was performed for 6 h at

600°ë

. AnX-ray diffraction study of the initial powder fordynamic deformation showed that it is a 3 : 1 mixture

of the rhombohedral ( ) and cubic polymorphs oflanthanum manganate.

The LMO powder was subjected to severe plasticdeformation by high-pressure torsion using a 100-tpress and Bridgman anvils [7]. The anvils, made of theVK6 alloy, were 5 mm in diameter. The powder wasplaced between the anvils and pressed at a pressure ofas high as 9 GPa. Shear deformation was achieved byrotating one of the anvils with respect to the other oneat a rate of 0.3 rpm. The experiments were carried outat room temperature in air.

To produce nanostructured ceramics by the shock-wave loading method, the initial LMO powder shapedinto a sphere by repeated static compaction and anneal-ing was placed in a sealed spherical steel case with aninner diameter of 49 mm. Loading by a convergingspherical shock wave was performed by detonation ofan explosive layer at the surface of the sealed case [8,9]. The integrity of the sealed case persisted duringloading. The pressure at the sphere surface was about20 GPa. This pressure sharply increased as the spheri-cal shock wave front propagated toward the center ofthe sphere and was several hundreds of gigapascals at adistance less than 1 mm from the sphere center. Uponloading, the initial sphere was compressed and sub-jected to severe plastic deformation.

X-ray diffraction studies of the initial and strainedsamples were carried out on a DRON-UM1 diffracto-meter in Bragg–Brentano geometry with radia-tion (a pyrographite monochromator in the diffractedbeam). X-ray powder diffraction patterns were taken ina continuous mode and in a scan mode with the step

∆

(2

θ

) = 0.02°

or

0.04°

depending on the reflection half-

R3c

CuKβ

Synthesis of Bulk Nanostructured Manganites LaMnO

3 +

d

by Quasi-Static and Dynamic Deformation Methods

B. A. Gizhevskii, V. D. Zhuravlev, R. G. Zakharov, M. I. Zinigrad, E. A. Kozlov,

Academician

L. I. Leont’ev, S. V. Naumov, S. A. Petrova, V. P. Pilyugin, A. Ya. Fishman, and N. M. Chebotaev

Received August 15, 2005

Institute of Metallurgy, Ural Division, Russian Academy of Sciences, ul. Amundsena 101, Yekaterinburg, 620016 Russia

CHEMISTRY

248

DOKLADY CHEMISTRY

Vol. 405

Part 2

2005

GIZHEVSKII

et al

.

width. For the scan mode, the exposure time was from50 to 200 s per point depending on the reflection inten-sity. Silicon, with

a

= 0.54309(1) nm, was used as anexternal standard. Profile analysis of experimentalX-ray diffraction patterns was carried out using apseudo-Voigt function. The coherent scattering domain(crystallite) size and lattice strain were calculated fromX-ray diffraction data using the Hall–Williamsonmethod [10].

RESULTS AND DISCUSSION

Being subjected to high-pressure torsion deforma-tion, the LMO powder was compacted to give pellets4

−

5 mm in diameter and 50–100

µ

m thick in the centralpart. Some samples were split into separate layers. Theexperimental conditions and major characteristics ofthe samples subjected to quasi-static deformation byhigh-pressure torsion are summarized in Table 1. Allthe samples under consideration were single-phase andturned out to be orthorhombic lanthanum manganite(space group

Pnma

). In some samples, X-ray photo-electron spectroscopy revealed the presence of tung-sten, which is presumably due to contamination withthe material of the anvils.

After shock-wave loading, a nearly spherical sample46–48 mm in diameter was obtained. The sphere has acavity in the center extended along one of the axes. Thestrength of the resulting material is not uniform

throughout the volume. Samples were taken for analy-sis from different parts of the sphere: from near the cav-ity (the central part), at a distance of 11–14 mm fromthe center, and at a distance of 20–23 mm from the cen-ter (from near the surface of the compressed sphere).The major parameters of the LMO samples obtainedunder dynamic deformation conditions are given inTable 2. The content of the cubic phase increases withdistance from the surface so that only cubic LMO existsin the central part of the sphere. The lattice parametersof both phases vary slightly with the distance from thecenter of the sphere. Thus, when subjected to high pres-sures and strains, lanthanum manganite in the centralpart of the sphere undergoes the structural phase transi-tion from the rhombohedral to the cubic phase.

The magnitude and character of the line broadeningin the X-ray powder diffraction patterns of all strainedsamples, as well as the “express” comparison of theratio between the true physical broadenings of two lines

β

2

/

β

1

with the

and

sec

θ

2

/sec

θ

1

ratios,indicated that this broadening is caused by both crystallattice microstrain and a decrease in coherent scatteringdomain size. For all samples subjected to deformation,the crystallite sizes and microstrains calculated for dif-ferent crystallographic directions are considerably dif-ferent, which points to the anisotropic character ofdeformation processes.

θ2/ θ1tantan

Table 1.

Treatment conditions and characteristics of the LaMnO

3 +

δ

samples after quasi-static deformation by high-pressuretorsion (

ϕ

is the anvil rotation angle)

SampleLattice parameters, nm Volume,

nm

3

Crystallite size

⟨

D

⟩

, nmLattice strain

ε

, %

a b c

ϕ

= 0 0.5655

1

0.7722

1

0.5528

1

0.2414

1

>2000 –

ϕ

=

π

/3 0.5648

2

0.7723

2

0.5526

2

0.2410

2

22 0.46

ϕ

=

π

0.5643

3

0.7722

3

0.5528

3

0.2409

3

21 0.96

ϕ

= 2

π

0.5640

3

0.7725

3

0.5529

3

0.2409

3

23 1.20

ϕ

= 6

π

0.564

1

0.777

2

0.553

1

0.242

2

23 1.40

Table 2.

Treatment conditions and characteristics of the LaMnO

3 +

δ

samples after dynamic deformation by shock wave load-ing (

r

is the distance from the center of the sphere)

SampleXRD Lattice parameters, nm Volume, nm

3

Crystallite size

⟨

D

⟩

, nmLattice strain

ε

, %

R c

Pm m c

R

a

c

V

R

V

c

R c

Pm m R c

Pm m

Initial 75% 25% 0.5522

1

1.3324

2

0.3887

1

0.3518

2

0.0587

1

>2000 >2000 – –

r

= 0 mm – 100% – – 0.3901

1

– 0.0594

1

– 38 – 0.21

r

= 11–14 mm 40% 60% 0.5526

5

1.3370

7

0.3889

5

0.3536

5

0.0588

5

31 48 0.20 0.38

r

= 20–23 mm 60% 40% 0.5525

5

1.3401

7

0.3886

5

0.3542

5

0.0587

5

33 62 0.42 0.68

* The indices

R

and

c

refer to the rhombohedral and cubic modifications, respectively.

3 3 aR* 3 3 3 3

DOKLADY CHEMISTRY Vol. 405 Part 2 2005

SYNTHESIS OF BULK NANOSTRUCTURED MANGANITES 249

Upon dynamic deformation, the lowest degree ofanisotropy was observed for the cubic phase. Tables 1and 2 summarize the crystallite sizes and microstrainsaveraged over all reflections.

Under high-pressure torsion conditions, the size ofcrystallites in the LMO powder decreases to 20–30 nmeven at an anvil rotation angle of 60°. This causes con-siderably lower deformations than those required forcreating a nanostructure in metals [1]. A furtherincrease in the anvil rotation angle does not lead tochanges in crystallite size but considerably enhancesmicrostrains, up to 1.4% (Fig. 1).

After dynamic deformation of LMO under loading byconverging spherical shock waves, the crystallite sizeand microstrains in both the rhombohedral and the cubicphases decrease as the center of the sphere isapproached, i.e., with an increase in the pressure in thefront of the converging shock wave and with an increasein the impact on the material. Lower microstrains in thecentral part of the sphere can be due to relaxation pro-cesses at high residual temperatures (Fig. 2).

Both quasi-static deformation and dynamic defor-mation of LMO lead to the formation of a rather finemicrostructure with an average crystallite size of20−30 nm, which allows us to assign the resulting bulkmaterials to nanostructures.

Changes in the lattice parameters are associatedwith defects generated by plastic deformation. As canbe seen from Fig. 3, the lattice parameters of the ortho-rhombic phase change monotonically with the anvilrotation angle: the a parameter decreases and the bparameter increases, the c parameter being constant.This behavior of the structural parameters of LMO as afunction of the degree of deformation differs from thechange in the orthorhombic lattice parameters underhydrostatic compression, when all three lattice parame-ters decrease, although at a different rate [12].

40

0 400 800 1200

35

30

25

20

1.5

0400 800 1200

1.0

0.5

ε, %⟨D⟩, nm

15

ϕ, degϕ, deg

Fig. 1. Crystallite size ⟨D⟩ and lattice strain ε vs. the anvilrotation angle ϕ under high-pressure torsion.

0 10 20

45

25

20

r, mm

30

35

40

50

55

65

60

155 25

ε, %⟨D⟩, nm

0 10 20

0.5

0.2

r, mm

0.3

0.4

0.6

0.7

155 25

⟨ε⟩R⟨ε⟩c

⟨D⟩R

⟨D⟩c

Fig. 2. Crystallite size ⟨D⟩ and lattice strain ε vs. the dis-tance from the center of the sphere r after shock wave load-ing. The indices R and c refer to the rhombohedral and cubicmodifications, respectively.

5.48

5.52

5.54

5.56

5.50

7.72

7.76

7.78

7.74

5.64

5.66

5.65

200 400 600 800 1000 12000

c, Å

b, Å

a, Å

ϕ, deg

Fig. 3. Lattice parameters vs. the anvil rotation angle ϕ afterquasi-static deformation by high-pressure torsion.

250

DOKLADY CHEMISTRY Vol. 405 Part 2 2005

GIZHEVSKII et al.

The trends in the lattice parameters observed instrained samples are associated with specific distortionsof the orthorhombic lattice of LMO under shear defor-mation. Two deformation stages can be discerned:deformations at angles below ϕ = 2π, at which the unitcell volume remains virtually constant, and more severedeformations, which are accompanied by an increase inthe unit cell volume (Table 1). The same tendency isobserved for the cubic phase located in deeper-lyinglayers of the compacted sphere (Table 2).

The increase in the unit cell volume can beexplained by partial loss of oxygen [11], which isaccompanied by a decrease in the concentration ofMn4+ ions and an increase in the occupancy of cationicsites.

Thus, the possibility of producing bulk high-densitynanostructured oxide materials from coarse-grainedpowders by quasi-static deformation (high-pressuretorsion) and dynamic deformation (loading by converg-ing spherical shock waves) was exemplified by lantha-num manganites. High-pressure torsion results in morefine-grained nanomaterials. The dynamic deformationmethod with the use of spherical explosive systems hasthe advantage that it is suitable for producing ratherlarge amounts of nanomaterials within one processingcycle, as well as that it ensures a lack of external contam-ination. The density of the nanoceramics thus obtainedcan be as high as 99% of the theoretical density.

ACKNOWLEDGMENTSThis work was supported by the Russian Foundation

for Basic Research (project no. 04–03–34971) and the

Israeli Center for Academic Relationships (nanomateri-als, grant no. 039.7524).

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