ISSN 0020�1685, Inorganic Materials, 2011, Vol. 47, No. 12, pp. 1361–1366. © Pleiades Publishing, Ltd., 2011.Original Russian Text © T.I. Chupakhina, G.V. Bazuev, 2011, published in Neorganicheskie Materialy, 2011, Vol. 47, No. 12, pp. 1491–1496.

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INTRODUCTION

Over the past few decades, materials based onmixed oxides of alkaline�earth elements and transitionmetals have been the subject of extensive studies. Ofparticular interest are oxides containing manganeseand cobalt. The interest in Ln1 – xAxMnO3 (A = alka�line�earth element) manganites has been sparked bythe discovery of the colossal magnetoresistance effect.The unusual properties of the manganites are due to anoptimal balance between spin, charge, and orbitalordering [1, 2]. An interesting cobaltite is SrCoO3, aferromagnet with metallic conduction and unusualtransport properties [3].

Ln1 – xAxMnCoO3 (A = alkaline�earth element)solid solutions with hole conduction have been studiedrather extensively [4–6], whereas materials dopedwith high�valence rare�earth cations, such as Ce4+,have received little attention. Most interest has cen�tered on А1 – xCexMnO3 (A = Sr, Ca) solid solutions,which are thought to be candidate cathode materialsfor solid oxide fuel cells [7–9]. А1 – xCexСоO3 solidsolutions are of interest for examining the influence ofheterovalent substitution on their Curie temperatureand transport properties [10].

Analysis of the literature demonstrates that dopingwith Ce4+ has a significant effect on the structural,electrical, magnetic, and transport properties of theSrМO3 (M = Mn, Co) compounds. It is known [11]that, when cooled in a magnetic field, SrMnO3undergoes antiferromagnetic ordering at 278 K.ВSr1 – xCexMnO3 [8] experiences antiferromagneticordering at TN = 290 K and a metal–insulator tran�sition at 315 K. According to Sundaresan et al. [7],Sr1 – xCexMnO3 contains ferromagnetic clusters in theparamagnetic region and is in a spin glass state at low

temperatures. The solid solutions with х = 0.25 and0.35 exhibit a negative magnetoresistive effect, basedon charge ordering near 110 K. Unsubstituted SrCoO3undergoes ferromagnetic ordering at 270–300 K [3].Ferromagnetism was also found in Sr1 – xCexCoO3 – δsolid solutions [10].

Zhang et al. [11] reported the synthesis, structure,and transport properties of the mixed oxideSr0.8Сe0.2Mn0.8Co0.2O3. The presence of cobalt in theoctahedral site of the perovskite�like oxide is of inter�est for gaining insight into the interaction betweenthe antiferromagnetic and ferromagnetic sublattices,which led us to synthesize (Sr0.8Ce0.2)(Mn1 – yCoy)O3(y = 0.3, 0.4) doubly substituted solid solutions andstudy their magnetic properties.

EXPERIMENTAL

Sr0.8Ce0.2Mn1 – yCoyO3 (y = 0.3, 0.4) samples wereprepared by solid�state reactions, using CeO2 (99.99%purity), MnO2 (extrapure grade), and SrCO3 (extra�pure grade) as starting chemicals. Appropriate oxide–carbonate mixtures were thoroughly ground, pressedat 0.3 GPa, and sintered during stepwise heating at100°С intervals with intermediate grindings every 10 h.After calcination, the reaction products were furnace�cooled to room temperature. The initial annealingtemperature was 950°С, and the final temperature was1350°С for the у = 0.3 sample and 1300°C at у = 0.4.The composition of the samples was checked by X�raydiffraction (XRD).

The total oxygen content was evaluated from theweight loss caused by calcination in flowing Н2 at950°С for 2 h. The oxygen stoichiometry in the as�pre�pared samples was determined with allowance for the

Synthesis, Structure, and Magnetic Propertiesof Sr0.8Ce0.2Mn1 – yCoyO3 – δ (y = 0.3, 0.4)

T. I. Chupakhina and G. V. BazuevInstitute of Solid�State Chemistry, Ural Branch, Russian Academy of Sciences,

Pervomaiskaya ul. 91, Yekaterinburg, 620219 Russiae�mail: Chupakhina@ihim.uran.ru

Received July 5, 2010; in final form, February 25, 2011

Abstract—We report the synthesis of Sr0.8Ce0.2Mn1 – yCoyO3 – δ (y = 0.3, 0.4) solid solutions, which have atetragonal perovskite structure (sp. gr. I4/mcm) with unit�cell parameters a = 0.53998(1) nm, c =0.76404(1) nm, and V = 0.22273(4) nm3 at y = 0.3 and a = 0.54054(6) nm, c = 0.7644(6) nm, and V =

0.22335(4) nm3 at y = 0.4. The presence of cobalt in the octahedral sites of the Sr0.8Ce0.2Mn1 – yCoyO3 – δ (y =0.3, 0.4) solid solutions reduces their antiferromagnetic ordering temperature and induces a transition to a spinglass state.

DOI: 10.1134/S0020168511120028

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fact that the Co was reduced to the metallic state dur�ing the calcination and that the residual oxygen wasbonded to divalent manganese, trivalent cerium, andSrO. The Co reduction process was monitored byXRD.

XRD patterns were collected on a DRON�UM1diffractometer (CuKα radiation). The data were ana�lyzed by the Rietveld profile analysis method usingFullProf 2005.

Magnetic measurements were made with a Quan�tum Design MPMS�XL�5 SQUID magnetometer(Magnetometry Center, Institute of Metal Physics,Ural Branch, Russian Academy of Sciences) at tem�peratures from 2 to 300 K in magnetic fields of 0.5 and5.0 kOe after field cooling and zero field cooling.

RESULTS AND DISCUSSION

Synthesis of cerium�substituted perovskite�likeoxides. Difficulties in the preparation of cerium�con�taining mixed oxides of any structure and compositionby a standard ceramic processing technique arise pri�marily from the fact that prolonged annealing at ratherhigh temperatures (near 1350°С) is needed. Lowertemperature annealing almost always leads to the pres�ence of СеО2 (relative intensity of XRD peaks, 1 to 5–7%). In syntheses of the АСеО3 (A = Ba, Sr) cerates, thisproblem can be partially alleviated using precursors(nitrate and citrate routes, PVA additions, Pechini pro�cess, and other approaches). Phase�pure Sr1 – xCexMnO3(x = 0.1–0.4) solid solutions were obtained at 1600°С[7]. Lu et al. [8] reported the preparation ofSr0.8Ce0.2MnO3 by a ceramic processing technique (finalstep: t = 1400°C, τ = 24 h, furnace�cooling). CeO2content was not specified, but the XRD pattern of thematerial contained extra reflections. Mandal et al. [9]prepared Sr1 – xCexMnO3 (х = 0.1–0.45) manganites bya two�step process in an inert atmosphere. After hold�ing in Ar at t = 1400°С, the samples were held severaldays in flowing oxygen at 500°С. The homogeneity ofSr0.9 – xCaxCe0.1MnO3 can be improved by slowheating to 1400°С and cooling at a constant rate [12].Sr1 – xCexCoO3 (x = 0.05–0.45) cobaltites were pre�pared at 6 GPa and 1650–1700°С [10]. TheSr0.8Ce0.2Mn1 – yCoyO3 (y = 0, 0.2) solid solutionsdescribed by Zhang et al. [11] were prepared at1400оС by solid�state reactions, using starting mix�tures mechanically activated in a ball mill for 24 h.

In this study, Sr1 – xCexMn1 – yCoyO3 (y = 0.3, 0.4)solid solutions were prepared both by a standardceramic processing technique and using a citrate pre�cursor route from appropriate metal nitrates. As men�tioned above, this approach proved effective in thesynthesis of perovskites and perovskite solid solutionscontaining cerium on the B site. In this study, the cit�rate route proved ineffective, presumably because thereducing carbonate medium prevented incorporationof tetravalent cerium into the structure of the oxide. Inview of this, the Sr0.8Ce0.2Mn1 – yCoyO3 – δ (y = 0.2, 0.3,

0.4) solid solutions were prepared by a standardceramic processing technique. Our y = 0.2 sample(solid solution described by Zhang et al. [11]) showed,in addition to the major phase, rather strong reflec�tions from CeO2, which prevented us from determin�ing the composition of the major phase with certainty.Increasing the annealing time and synthesis tempera�ture had little or no effect on the amount of unreactedCeO2. It seems likely that cerium incorporation can beensured by mechanical activation of the starting mix�ture [11]. The synthesis temperature and time neededto prepare Sr0.8Ce0.2Mn1 – yCoyO3 – δ (y = 0.3, 0.4) solidsolutions decrease with increasing Co content. In theformer case (у = 0.3), annealing for 10 h at a final tem�perature of 1350°С is needed. In the latter case (у =0.4), annealing for 8 h at 1300°С is sufficient. Giventhat Sr1 – xCexCoO3 can only be synthesized at highpressures, it is reasonable to assume that increasing theCo/Mn ratio facilitates Ce incorporation into the per�ovskite structure.

Structural characteristics of the Sr0.8Ce0.2Mn1 – yCoyO3 – δ (y = 0.3, 0.4) solid solutions. The XRD pat�terns of the Sr0.8Ce0.2Mn0.7Co0.3O3 – δ andSr0.8Ce0.2Mn0.6Co0.4O3 – δ solid solutions (Fig. 1) couldbe indexed in space group I4/mcm, like that of the y =0.2 solid solution [11]. The tetragonal cell parameters inour samples at 290 K are a = 0.53998(1) nm, c =0.76404(1) nm, and V = 0.22273(4) nm3 at y = 0.3

and a = 0.54054(6) nm, c = 0.7644(6) nm, and V =0.22335(4) nm3 at y = 0.4. It is known that, to iden�tify the space group of a perovskite oxide, precisionXRD analysis is needed, preferably with the use ofsynchrotron radiation [13]. Sr0.8Ce0.2MnO3 [7]and Sr0.8Ce0.2Mn0.8Co0.2O3 [11] were reported tohave space group I4/mcm. The structure of theSr1 – xCexCoO3 solid solutions obtained throughcerium doping of strontium cobaltite remains cubic upto х = 0.4. It is worth pointing out here again that thesesolid solutions were obtained at a pressure of 6 GPaand have the same oxygen stoichiometry throughoutthe series [10]. It seems likely that cobalt substitutionfor manganese to y = 0.2 is not conducive to the struc�tural transition to cubic symmetry.

We examined several structural models, and the spacegroup specified corresponds to the smallest agreementfactors. According to Balamurugan et al. [10], Co incor�poration into an oxygen octahedron sharply reduces theс cell parameter (0.7749 nm in Sr0.8Ce0.2MnO3 and0.7697 nm in Sr0.8Ce0.2Mn0.8Co0.2O3) and slightlyincreases the а parameter. This effect is attributable tothe disappearance of the Jahn–Teller distortion inresponse to the increase in the valence of Mn. In oury = 0.3 sample, the two parameters decrease, espe�cially c. At y = 0.4, both parameters slightly increase.Since the MO6 (M = Co, Mn) octahedra are essen�tially undistorted (the M–O1 and M–O2 bondlengths differ insignificantly), the unit�cell parametersseem to depend only on the radii of Mn and Co of dif�ferent valences in octahedral coordination.

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SYNTHESIS, STRUCTURE, AND MAGNETIC PROPERTIES 1363

Magnetic properties of the Sr0.8Ce0.2Mn1 – yCoyO3 – δ(y = 0.3, 0.4) solid solutions.The magnetic susceptibil�ity of Sr0.8Ce0.2Co0.3Mn0.7O2.94 and Sr0.8Ce0.2Mn0.6Co0.4O2.88 was measured in the range 2–300 K after fieldcooling (5 kOe) and zero field cooling. The magnetiza�tion as a function of applied magnetic field was mea�sured at 2 K. The measurement results are presented inFigs. 2–4.

Figures 2 and 3 demonstrate that the magnetic sus�ceptibility of the Sr0.8Ce0.2Mn1 – yCoyO3 – δ (y = 0.3, 0.4)solid solutions is an intricate function of temperature.The χ(T) curves each have two maxima: at 176 and41 K for y = 0.3 and at 196.5 and 20 K for у = 0.4.The data obey the Curie–Weiss law in the range 176–300 K for the y = 0.3 sample and 196.5–300 K for they = 0.4 sample.

The insets in Figs. 2 and 3 present high�tempera�ture (above the first transition) molar magnetic sus�ceptibility data for the two samples. The data are wellrepresented by a Curie–Weiss law. The Weiss constants

�

were determined to be θ = –35.74(2) and –39.44(6) K, and the Curie constants С = 2.42(1) and2.03(1) cm3 K/mol at y = 0.3 and 0.4, respectively. Theeffective magnetic moments μeff for Sr0.8Ce0.2Mn1 – yCoyO3 (y = 0.3, 0.4) are 4.40μB and 4.03μB, respec�tively.

Comparison of the above data with those forSrMnO3 demonstrates that substitution of Ce and Cofor Sr and Mn reduces the temperature and changesthe nature of magnetic ordering. Unsubstituted stron�tium manganite is an antiferromagnet [14]. Ceriumdoping leads to local ferromagnetic ordering in theparamagnetic region, which shows up as a change inthe sign of the Weiss constant and a reduction in Neeltemperature [7, 8]. Partial Co substitution for Mnreduces the ferromagnetic component at high temper�atures, which shows up as further reduction in antifer�romagnetic transition temperature and negative Weissconstants (θ). At a constant degree of Ce substitutionfor Sr (20 at %), the most likely reason for the observed

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Fig. 1. Raw XRD data, calculated profile, and difference plot for (a) Sr0.8Ce0.2Mn0.7Co0.3O2.94 and(b) Sr0.8Ce0.2Mn0.6Co0.43O2.88.

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anomalies is that the Mn and Co differ in oxidationand spin states.

According to XANES data [11], the A site containsonly tetravalent cerium. Therefore, the net magneticmoment is due to cobalt and manganese. In calculat�ing the valence and spin states of the Mn and Co in theSr0.8Ce0.2Mn0.7Co0.3O2.94 and Sr0.8Ce0.2Mn0.6Co0.4O2.88solid solutions, we took into account the oxygen non�stoichiometry and experimentally determined effec�tive magnetic moments. The theoretical effective mag�netic moment of Sr0.8Ce0.2Mn0.7Co0.3O2.94 agrees withthe experimentally determined one provided there areMn ions in two oxidation states (Mn3+ and Mn4+) andtrivalent high�spin cobalt. An increase in Co contentto y = 0.4 increases the oxidation state of the Mn to4+, about two�thirds of the Co is in the oxidation state2+, and the rest is trivalent high�spin cobalt. For anyother combinations of valence and spin states of Coand Mn, the system of linear equations that relate the�oretically predicted and experimentally determinedmagnetic moments, the content of transition metals,and charge characteristics has no solution.

Thus, the first transition (176 and 196.5 K at y = 0.3and 0.4, respectively) attests to antiferromagneticordering degeneracy, observed in Sr0.8Ce0.2MnO3.. Inparticular, whereas the long�range antiferromagnetic

ordering and local ferromagnetism in Sr0.8Ce0.2MnO3is attributable to the presence of Mn3+–O–Mn4+ andMn4+–O–Mn4+ clusters, the behavior ofSr0.8Ce0.2Mn1 – yCoyО3 – δ in a magnetic field is associ�ated with partial frustration of the antiferromagneticlong�range structure because of the presence of cat�ions differing in valence and spin characteristics.

Since our samples are identical in the degree of Cesubstitution for Sr, the distinctions between their mag�netic behaviors are related to the Mn/Co ratio. Bala�murugan et al. [10] attribute the suppression of ferro�magnetism in Sr1 – xCexCoO3 cerium�substituted cobal�tites to the reduction in valence and the increase in thenumber of 3d electrons per cobalt atom. Increasing thecobalt content of Sr0.8Ce0.2Mn1 – yCoyO3 – δ solid solu�tions from y = 0.3 to y = 0.4 increases the oxidationstate of the Mn and reduces the net oxidation state ofthe transition metals owing to the decrease in the over�all valence of the cobalt. These findings suggest thatthe lower TN of the Sr0.8Ce0.2Mn0.7Co0.3O2.94 solid solu�tion is due to the presence of manganese in a mixed�valent state (Mn3+ and Mn4+) and, as a consequence,stronger ferromagnetic correlations.

Cooling to 75–100 K considerably increases themagnetic susceptibility, and the field cooling (FC) andzero field cooling (ZFC) curves deviate from one

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Fig. 2. Temperature dependences of magnetic susceptibility for the Sr0.8Ce0.2Mn0.7Co0.3O2.94 solid solution. Inset: high�temper�ature data and Curie–Weiss fit.

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SYNTHESIS, STRUCTURE, AND MAGNETIC PROPERTIES 1365

another. The λ(T) curves show a second magnetictransition at 27 K for y = 0.3 and at 16 K for y = 0.4.Both the low� and high�temperature magnetic transi�tions can be observed in ac susceptibility measure�

ments. The increase in magnetic susceptibility and thediscrepancy between the FC and ZFC curves are mostlikely due to the development of weak ferromagnetismand a spin glass state. The magnetic field dependenceof magnetization at 2 K (Fig. 4) exhibits hysteresis,which also points to a weak ferromagnetic moment.

Similar behavior in magnetic fields was reported forLaSr2CoMnO7 [15] (the χ(T) curve also has two max�ima), but the Weiss constant of LaSr2CoMnO7 is posi�tive in its paramagnetic region. It seems likely that thisdistinction is directly related to the difference invalence between Ce4+ and La3+.

CONCLUSIONS

The present results demonstrate that Sr0.8Ce0.2Mn 1 – y CoyO3 – δ (y = 0.3, 0.4) solid solutions can beprepared by a standard ceramic processing techniqueand that increasing the Co content of the system isfavorable for Ce incorporation. Cobalt reduces theantiferromagnetic ordering temperature of the mixedoxides in comparison with the cerium�substitutedstrontium manganite Sr0.8Ce0.2MnO3 – δ and induces atransition to a spin glass state.

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Fig. 3. Temperature dependences of magnetic susceptibility for the Sr0.8Ce0.2Mn0.6Co0.4O2.88 solid solution. Inset: high�temper�ature data and Curie–Weiss fit.

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Fig. 4. Magnetization curve of Sr0.8Ce0.2Mn0.6Co0.4O2.88in magnetic fields of up to 50 kOe.

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