The ordered double perovskite PrBaCo2O6: Synthesis, structure, andmagnetismMd. Motin Seikh, V. Pralong, O. I. Lebedev, V. Caignaert, and B. Raveau Citation: J. Appl. Phys. 114, 013902 (2013); doi: 10.1063/1.4812368 View online: http://dx.doi.org/10.1063/1.4812368 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v114/i1 Published by the AIP Publishing LLC. Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

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The ordered double perovskite PrBaCo2O6: Synthesis, structure,and magnetism

Md. Motin Seikh,a) V. Pralong, O. I. Lebedev, V. Caignaert, and B. Raveaub)

Laboratoire CRISMAT, UMR 6508 CNRS ENSICAEN, 6 bd Mar�echal Juin, 14050 CAEN, France

(Received 16 May 2013; accepted 10 June 2013; published online 1 July 2013)

The stoichiometric layered perovskite cobaltite PrBaCo2O6 has been synthesized using an oxidative

reaction of PrBaCo2O5.80 by sodium hypochlorite. The ferromagnetic properties of this oxide,

which exhibits the highest TC of 210 K among the “112” layered cobaltites, are interpreted by

double exchange mechanism. In contrast, the creation of oxygen vacancies in this framework leads

for the oxides PrBaCo2O5þd (0.80� d< 1) to a strong competition between ferromagnetism and

antiferromagnetism due to the appearance of superexchange Co3þ—O—Co3þ antiferromagnetic

interactions. VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4812368]

INTRODUCTION

Layered oxygen deficient “112” cobalt perovskites with

the generic formulation LnBaCo2O5þd have been the object

of considerable investigations due to their attractive physi-

cal properties such as ferromagnetism, metal-insulator

transition, and high magnetoresistance [for a review see

Ref. 1]. In this structural family, the oxygen content and the

ordering of the Ln3þ and Ba2þ cations and of the anion

vacancies play a crucial role in the magnetic properties, as

exemplified from their ability to exhibit metal-insulator

transition,2–4 giant magnetoresistance,5–9 charge order-

ing,6,10 and also spin-state transition.11,12 Such systems are

also quite remarkable by the great difficulty to synthesize

the ordered stoichiometric perovskite, corresponding to

d¼ 1. The ordered stoichiometric perovskite LnBaCo2O6,

which consists of the 1:1 ordered stacking of LnO and

BaO layers, could only be synthesized to date for Ln¼La

and Nd.13–17 In fact, the synthesis of the “112” layered

LaBaCo2O6 perovskite requires special conditions of syn-

thesis, starting from the reduced cobaltite LaBaCo2O5

and annealing it in oxygen at low temperature at around

�350 �C, in order to avoid the formation of the disordered

stoichiometric perovskite La0.5Ba0.5CoO3.13–16 The ordered

layered “112” perovskite NdBaCo2O6 cannot even be

obtained by direct solid state reaction method and requires

soft chemistry synthesis.17

The recent studies of the “112” layered cobaltites

PrBaCo2O5þd [Refs. 18–20] have shown a strong competi-

tion between antiferromagnetic (AFM) and ferromagnetic

(FM) interactions in this system. Interestingly, the authors

have demonstrated that the dependence of the magnetic

properties on oxygen content in this series can be linked to

the distortion of the cobalt oxygen polyhedra but could not

reach d values larger than 0.9, due to their experimental

conditions. In the present paper, we show the possibility

to synthesize the ordered perovskites PrBaCo2O5þd, with

0.8� d� 1, using an efficient oxidation reaction by sodium

hypochlorite. The evolution of the magnetic properties of

these oxides versus the oxygen content is studied, showing

that a pure ferromagnetic state is observed for PrBaCo2O6,

which TC of 210 K is the highest that has been reached to

date for the “112” layered cobaltites.

CHEMICAL SYNTHESIS

Bearing in mind that the aim of this study was to suc-

ceed to synthesize the layered “112” cobaltite PrBaCo2O6,

we have first prepared an oxygen rich member of the

PrBaCo2O5þd family, using the classical solid state reaction

from the oxides Pr6O11, Co3O4, and BaCoO3 mixed in the

stoichiometric Pr:1/Ba:1/Co:2 ratio, heated in air at 1100 �Cand slowly cooled down to room temperature. The mixture

was heated several times for 24 h, and ground between each

thermal treatment in order to get a good homogeneity. The

iodometric titration analysis of the final product, allowed the

oxygen content to be determined leading to the composition

PrBaCo2O5.80. The purity of this phase and its crystallo-

graphic nature were checked from its X-ray powder diffrac-

tion (XRPD) pattern, confirming that it belongs to the “112”

tetragonal ordered oxygen deficient perovskite family in

agreement with previous studies.18–20 Then in a second step

the oxide PrBaCo2O5.80 was oxidized at room temperature in

the presence of a sodium hypochlorite solution, according

to a method previously used to oxidize cobalt or nickel

oxides.21 Using NaClO as oxiding agent, the chemical oxida-

tion was performed from 0.5 g of PrBaCo2O5.80 added to a

solution of 200 ml NaClO (13% chlorine active) and stirred

for different times, the solution being renewed every week.

The samples were then washed with distilled water, in order

to eliminate NaClO, filtered, and dried in an oven at 60 �C.

In this way, different compounds could be isolated and were

shown to exhibit the “112” structure with the generic for-

mula PrBaCo2O5þd. The oxygen content corresponding to

d values of 0.93, 0.95, and 1, depending on the time of

exposure to the solution (Table I), were determined from

iodometric titration. Note that the synthesis of the limit stoi-

chiometric oxide PrBaCo2O6 (d¼ 1) required an exposure

a)On leave from Department of Chemistry, Visva-Bharati University,

Santiniketan 731235, India.b)Author to whom correspondence should be addressed. Electronic mail:

bernard.raveau@ensicaen.fr. Tel.: þ33 2 31 45 26 16. Fax: þ33 2 31 95

16 00.

0021-8979/2013/114(1)/013902/5/$30.00 VC 2013 AIP Publishing LLC114, 013902-1

JOURNAL OF APPLIED PHYSICS 114, 013902 (2013)

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time of one month to the NaClO solution, according to the

reaction:

PrBaCo2O5:80 þ 2NaClO! PrBaCo2O6 þ 2NaClþ 0:9O2:

STRUCTURAL CHARACTERIZATION

The XRPD patterns of these PrBaCo2O5þd oxides (Fig. 1)

registered with a Panalytical X’Pert Pro diffractometer under a

continuous scanning mode in the 2h range 5�–120� and step

size D2h¼ 0.017� with Cu Ka radiation, confirm that they are

single phase, with the tetragonal P4/mmm symmetry, character-

istic of the “112”-type structure, showing a doubling of

the c parameter with respect to the cubic perovskite, i.e., a ¼ b

� ap and c� 2ap. The lattice parameters deduced from Rietveld

refinements22 (Table I) vary only slightly in this small composi-

tion range, indicating a small expansion of the c parameter and

a small contraction of the a parameter for PrBaCo2O6 (d¼ 1)

compared to PrBaCo2O5.8 (d¼ 0.80). The structural quality of

synthesized compound was verified by transmission electron

microscopy (TEM) using a Tecnay G2 30 UT microscope oper-

ated at 300 kV and having 0.17 nm point resolution. The

electron diffraction (ED) patterns along the main zone axis

(Fig. 2) of PrBaCo2O6 show that these oxides are well crystal-

lized, attest double-perovskite type structure and could be

indexed with the reference to the P4/mmm symmetry with a ¼b � ap and c� 2ap. The ED patterns clearly confirm the pro-

posed crystal structure of PrBaCo2O6 and indicate absence of

any superstructure reflections due to possible oxygen or cation

vacancies ordering. One observes a pronounced twin micro-

structure of PrBaCo2O6 which is mostly represented along

[100] zone axis. In this case, the [100] ED pattern (Fig. 2)

shows the superposition of two [100] ED patterns produced by

different domain fragments rotated at 90� along [100] axis with

respect to each other and having the {012} twinning plane. It

should be noticed that because of the small size of twinned

domains and relatively big size of selective apertures, all [100]

ED patterns show a twinning character. The latter assumption

is supported by high resolution transmission electron micros-

copy (HRTEM) observation (Fig. 3(c)).

TABLE I. Chemical composition, exposure time to NaClO and cell parame-

ters of the cobaltites PrBaCo2O5þd synthesized from oxidation of

PrBaCo2O5.80 by NaClO.

Chemical formula Exposure time Cell parameters v2

PrBaCo2O5.80 As prepared a¼ 3.901(1) A 2.52

c¼ 7.648(1) A

V¼ 116.43 A3

PrBaCo2O5.93 2 days a¼ 3.898(1) A 1.87

c¼ 7.6637(1) A

V¼ 116.09 A3

PrBaCo2O5.95 2 weeks a¼ 3.892(1) A 6.05

c¼ 7.648(1) A

V¼ 115.85 A3

PrBaCo2O6.00 4 weeks a¼ 3.887(1) A 2.63

c¼ 7.655(1) A

V¼ 115.69 A3

FIG. 1. X-ray diffraction patterns (red circles) along with the fits (black

lines) for PrBaCo2O6. The residues are shown as blue lines at the bottom

and the Bragg peaks are shown as green bars. Inset shows the ordered double

perovskite structure projected along the b-axis.

FIG. 2. Electron diffraction patterns of PrBaCo2O6 along main zone axis.

Note the presence of twinning structure in the [100] ED pattern.

013902-2 Seikh et al. J. Appl. Phys. 114, 013902 (2013)

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The atomic coordinates of the stoichiometric phase

PrBaCo2O6 (Table II) deduced from the Rietveld refinements

were used for the simulation of the HRTEM images taken

along main zone axis. The combination of these two techni-

ques allows the perfectly ordered character of the structure

(inset Fig. 1) to be evidenced. One indeed observes from

HRTEM measurements the perfect crystallinity of the sam-

ple and a structure free of any defect. Fig. 3 shows HRTEM

images of PrBaCo2O6, taken along most informative [001]

(Fig. 3(a)) and [100] (Fig. 3(b)) zone axis. The calculated

images are given as inset. A comparison of experimental and

calculated HRTEM images shows that under the present

imaging conditions the darkest dots correspond to Co-O col-

umns, Pr-Ba columns being brighter than Co-O and bright

dots correspond to oxygen columns. One can observe good

correspondence between the calculated and the experimental

images. The interatomic distances (Table III) obtained from

XRPD refinements, clearly confirm that the Pr3þ and Ba2þ

cations occupy two kinds of sites with Pr–O distances rang-

ing from 2.580 to 2.749 A and Ba–O distances ranging from

2.749 to 2.888 A, respectively. The Co—O bonds are rather

close to those observed previously for PrBaCo2O5.8,18 with

two apical distances of 1.884–1.948 A and four equatorial

distances of 1.953 A.

MAGNETIC PROPERTIES

The magnetization measurements were performed using a

superconducting quantum interference device (SQUID) mag-

netometer with a variable temperature cryostat (Quantum

Design, San Diego, USA). The temperature dependence of the

magnetization M(T) measured under 0.3 T (Fig. 4) shows that,

whatever d comprised between 0.80 and 1, the samples exhibit

a PM to FM transition in agreement with the results previously

observed for PrBaCo2O5.80 [Ref. 18] and PrBaCo2O5.90

[Ref. 19]. Importantly, the TC increases significantly with the

oxygen content from 165 K for PrBaCo2O5.80 to 210 K for the

stoichiometric oxide PrBaCo2O6. It is worth pointing out that

this is the highest TC value that has been reached in the

LnBaCo2O5þd series (to be compared to 175–179 K for

LaBaCo2O6 [Refs. 13–16] and 200 K for NdBaCo2O6

[Ref. 17]). The second remarkable feature concerns the diver-

gence between the zero field cooled (ZFC) and field cooled

(FC) curves at low temperature, observed for the oxygen defi-

cient phases. This feature was already observed for

PrBaCo2O5.80 and ascribed either to magnetic anisotropy or to

a competing interaction between FM and AFM states.18

However, in our oxide PrBaCo2O5.80 (Fig. 4(a)), the amplitude

of the divergence is significantly smaller than observed by pre-

vious authors, suggesting that either the oxygen content may

not be exactly the same in both oxides, i.e., slightly larger in

our case, or the ordering of the anionic vacancies may be

slightly different. This difference is also confirmed from the

TC value, which is slightly higher in our case (TC� 165 K)

compared to that previously observed by Ganorkar et al.18

(TC� 148 K). The great sensitivity of this phenomenon to oxy-

gen stoichiometry is in fact confirmed for higher d values. One

observes that the divergence between the ZFC and FC curves

decreases abruptly as d increases as shown for d¼ 0.93 (Fig.

4(b)) and d¼ 0.95 (Fig. 4(c)) and almost disappears for d¼ 1.0

(Fig. 4(d)). Thus, these results show that TC increases dramati-

cally as the oxygen content increases from TC¼ 165 K for

PrBaCo2O5.80, leading to a pure ferromagnetic state for the

stoichiometric layered “112” perovskite LaBaCo2O6 with a

high TC of 210 K, the M(T) curve (Fig. 4(d)), showing a very

abrupt transition.

The isothermal magnetization curves M(H) registered at

5 K (Fig. 5) clearly support the effect of oxygen deficiency

upon the competition between ferromagnetism and antiferro-

magnetism in this series for d ranging from 0.80 to 1. For

d¼ 0.80, one observes that the hysteresis loop indicating the

presence of ferromagnetism (Fig. 5(a)) does not reach the

saturation, due to the competing effect of antiferromagnetism

originating from oxygen vacancies, whereas for larger oxy-

gen contents, d¼ 0.93 (Fig. 5(b)) and d¼ 0.95 (Fig. 5(c)),

the hysteresis loops are much closer to the saturation under

FIG. 3. (a) and (b) HRTEM images of PrBaCo2O6 along the [001] and [100]

zone axis. (c) (100) HRTEM image of twinning area and corresponding struc-

tural model. The calculated images for [001] (defocus value Df¼�45 nm,

t¼ 6 nm) and [100] (defocus value Df¼�55 nm, t¼ 7.5 nm) are given as

inset.

TABLE II. Atomic coordinates of PrBaCo2O6.00.

Atom Wyck. x/a y/b z/c

Co 2h 1/2 1/2 0.2458(2)

Pr 1a 0 0 0

Ba 1b 0 0 1/2

O1 1c 1/2 1/2 0

O2 1d 1/2 1/2 1/2

O3 4i 1/2 0 0.2213(2)

TABLE III. Interatomic distances of PrBaCo2O6.00.

Co-O

distances d (A)

Pr-O

distances d (A)

Ba-O

distances d (A)

Co O1 1.884 Pr O3 8� 2.580 Ba O2 8� 2.888

O2 1.9482 O1 4� 2.749

O3 4� 1.9530

013902-3 Seikh et al. J. Appl. Phys. 114, 013902 (2013)

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an applied magnetic field of 4 T, the magnetic moment

increasing significantly with respect to d¼ 0.80. Finally, the

saturation is reached for the stoichiometric phase PrBaCo2O6

(Fig. 5(d)), with a magnetic moment of 3.3 lB per f.u. under

5 T. Quite remarkably, the coercive field of the oxides

PrBaCo2O5þd decreases as the oxygen content increases

from HC¼ 0.4 T for d¼ 0.8 to HC¼ 0.05 T for d¼ 1.0. The

larger coercive field observed for the oxygen deficient com-

positions may originate from the presence of anion vacancies

which play the role of pinning centers.

The behavior of these cobaltites can be interpreted either

through a superexchange or through a double exchange (DE)

mechanism. However, for the stoichiometric perovskite

PrBaCo2O6, if the ferromagnetism can be explained by the

presence of FM superexchange Co3þ—O—Co4þ interactions

according to Goodenough-Kanamori rule,23 a competition

with AFM Co3þ—O—Co3þ super exchange interactions

should also be observed, which is not the case. Thus, it is most

probable that the DE mechanism24 previously proposed for the

La0.5Ba0.5CoO3 perovskite13 and LaBaCo2O6 [Ref. 14] is the

right one. Indeed, the great tendency of Co-3 d and O-2 p orbi-

tals to hybridize, as mixed d6/d7L for Co3þ and d5/d6L for

Co4þ, allows the hole on the apical oxygen of CoO6 octohedra

to be coupled to eg electrons, leading to ferromagnetism and

metallic conductivity. In the present case, the conductivity of

the samples cannot be measured, due to their powdery nature,

but this feature was observed for isostructural layered

perovskite LaBaCo2O6 [Refs. 14–16]. Then, as vacancies are

FIG. 4. MZFC(T) (open symbols) and

MFC(T) (closed symbols) curves of

PrBaCo2O5þd: (a) d¼ 0.80, (b)

d¼ 0.93, (c) d¼ 0.95, and (d) d¼ 1.00,

recorded under a magnetic field of

0.3 T.

FIG. 5. M vs H curves of

PrBaCo2O5þd: (a) d¼ 0.80, (b)

d¼ 0.93, (c) d¼ 0.95, and (d) d¼ 1.00,

recorded at 5 K.

013902-4 Seikh et al. J. Appl. Phys. 114, 013902 (2013)

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created on the apical oxygen sites of PrBaCo2O6, the coupling

between eg electrons and holes is dramatically decreased.

Consequently, the number of Co3þ—O—Co3þ AFM superex-

change interactions increase abruptly as shown for the oxide

PrBaCo1.43þCo0.6

4þO5.80, suggesting the appearance of elec-

tronic phase separation, where AFM and FM states are

competing.

In conclusion, we have successfully synthesized the

112-layered perovskite PrBaCo2O6 using soft-chemistry

technique. It is remarkable that this ferromagnetic oxide

exhibits a TC of 210 K larger than those of LaBaCo2O6

(TC� 175–179 K) and of NdBaCo2O6 (TC� 200 K), suggest-

ing that the size of the lanthanide may influence slightly the

double exchange mechanism, i.e., the Co—O—Co bond

angles in these oxides. It would be of great interest to investi-

gate the spins state of cobalt in this compound, using spec-

troscopy in order to better understand the mechanism which

governs its magnetic behaviour.

ACKNOWLEDGMENTS

We gratefully acknowledge the CNRS and the Minister

of Education and Research for financial support through their

Research, Strategic, and Scholarship programs.

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