The ordered double perovskite PrBaCo2O6 ...

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

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JOURNAL OF APPLIED PHYSICS 114, 013902 (2013)

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

Layered oxygen deficient “112” cobalt perovskites with the generic formulation LnBaCo2O5þd have been the object of considerable investigations due to their attractive physical 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 ordering,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 d ¼ 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 Ln ¼ La and Nd.13–17 In fact, the synthesis of the “112” layered LaBaCo2O6 perovskite requires special conditions of synthesis, 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 competition 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 a)

On leave from Department of Chemistry, Visva-Bharati University, Santiniketan 731235, India. Author to whom correspondence should be addressed. Electronic mail: [email protected]. Tel.: þ33 2 31 45 26 16. Fax: þ33 2 31 95 16 00.

b)

0021-8979/2013/114(1)/013902/5/$30.00

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 succeed 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 C and 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 crystallographic nature were checked from its X-ray powder diffraction (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 oxidation 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 formula 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 stoichiometric oxide PrBaCo2O6 (d ¼ 1) required an exposure

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TABLE I. Chemical composition, exposure time to NaClO and cell parameters of the cobaltites PrBaCo2O5þd synthesized from oxidation of PrBaCo2O5.80 by NaClO. Chemical formula

Exposure time

Cell parameters

v2

PrBaCo2O5.80

As prepared

˚ a ¼ 3.901(1) A ˚ c ¼ 7.648(1) A ˚3 V ¼ 116.43 A

2.52

PrBaCo2O5.93

2 days

˚ a ¼ 3.898(1) A ˚ c ¼ 7.6637(1) A 3 ˚ V ¼ 116.09 A

1.87

PrBaCo2O5.95

2 weeks

˚ a ¼ 3.892(1) A ˚ c ¼ 7.648(1) A ˚3 V ¼ 115.85 A

6.05

PrBaCo2O6.00

4 weeks

˚ a ¼ 3.887(1) A ˚ c ¼ 7.655(1) A ˚3 V ¼ 115.69 A

2.63

electron diffraction (ED) patterns along the main zone axis (Fig. 2) of PrBaCo2O6 show that these oxides are well crystallized, attest double-perovskite type structure and could be indexed with the reference to the P4/mmm symmetry with a ¼ b ap and c 2ap. The ED patterns clearly confirm the proposed crystal structure of PrBaCo2O6 and indicate absence of any superstructure reflections due to possible oxygen or cation vacancies ordering. One observes a pronounced twin microstructure 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 microscopy (HRTEM) observation (Fig. 3(c)).

time of one month to the NaClO solution, according to the reaction: PrBaCo2 O5:80 þ 2NaClO ! PrBaCo2 O6 þ 2NaCl þ 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 D2h ¼ 0.017 with Cu Ka radiation, confirm that they are single phase, with the tetragonal P4/mmm symmetry, characteristic 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 composition range, indicating a small expansion of the c parameter and a small contraction of the a parameter for PrBaCo2O6 (d ¼ 1) compared to PrBaCo2O5.8 (d ¼ 0.80). The structural quality of synthesized compound was verified by transmission electron microscopy (TEM) using a Tecnay G2 30 UT microscope operated at 300 kV and having 0.17 nm point resolution. The

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.


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J. Appl. Phys. 114, 013902 (2013) TABLE III. Interatomic distances of PrBaCo2O6.00. Co-O distances Co

O1 O2 O3

˚) d (A 1.884 1.9482 4 1.9530

Pr-O distances Pr

O3 O1

˚) d (A 8 2.580 4 2.749

Ba-O distances Ba

O2

˚) d (A 8 2.888

MAGNETIC PROPERTIES

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 structural model. The calculated images for [001] (defocus value Df ¼ 45 nm, t ¼ 6 nm) and [100] (defocus value Df ¼ 55 nm, t ¼ 7.5 nm) are given as inset.

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 techniques 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 sample 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 columns, 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˚ and Ba–O distances ranging from ing from 2.580 to 2.749 A ˚ 2.749 to 2.888 A, respectively. The Co—O bonds are rather close to those observed previously for PrBaCo2O5.8,18 with ˚ and four equatorial two apical distances of 1.884–1.948 A ˚ distances of 1.953 A. TABLE II. Atomic coordinates of PrBaCo2O6.00. Atom Co Pr Ba O1 O2 O3

Wyck.

x/a

y/b

z/c

2h 1a 1b 1c 1d 4i

1/2 0 0 1/2 1/2 1/2

1/2 0 0 1/2 1/2 0

0.2458(2) 0 1/2 0 1/2 0.2213(2)

The magnetization measurements were performed using a superconducting quantum interference device (SQUID) magnetometer 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 divergence between the zero field cooled (ZFC) and field cooled (FC) curves at low temperature, observed for the oxygen deficient 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 previous 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 oxygen 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 d ¼ 0.93 (Fig. 4(b)) and d ¼ 0.95 (Fig. 4(c)) and almost disappears for d ¼ 1.0 (Fig. 4(d)). Thus, these results show that TC increases dramatically as the oxygen content increases from TC ¼ 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 antiferromagnetism in this series for d ranging from 0.80 to 1. For d ¼ 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 oxygen contents, d ¼ 0.93 (Fig. 5(b)) and d ¼ 0.95 (Fig. 5(c)), the hysteresis loops are much closer to the saturation under


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FIG. 4. MZFC(T) (open symbols) and MFC(T) (closed symbols) curves of PrBaCo2O5þd: (a) d ¼ 0.80, (b) d ¼ 0.93, (c) d ¼ 0.95, and (d) d ¼ 1.00, recorded under a magnetic field of 0.3 T.

an applied magnetic field of 4 T, the magnetic moment increasing significantly with respect to d ¼ 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 HC ¼ 0.4 T for d ¼ 0.8 to HC ¼ 0.05 T for d ¼ 1.0. The larger coercive field observed for the oxygen deficient compositions 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 orbitals 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. 5. M vs H curves of PrBaCo2O5þd: (a) d ¼ 0.80, (b) d ¼ 0.93, (c) d ¼ 0.95, and (d) d ¼ 1.00, recorded at 5 K.


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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 superexchange interactions increase abruptly as shown for the oxide PrBaCo1.43þCo0.64þO5.80, suggesting the appearance of electronic 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), suggesting 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 investigate the spins state of cobalt in this compound, using spectroscopy 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. 1

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