ORIGINAL PAPER

Synthesis and characterization of layered perovskite oxidesLa1+xSr2−xMnCrO7 (x=0.2, 0.5)

Devinder Singh & Rajinder Singh & Sushma Sharma &

Meena Sharma

Received: 8 February 2012 /Revised: 14 May 2012 /Accepted: 27 May 2012 /Published online: 6 July 2012# Springer-Verlag 2012

Abstract Intergrowth perovskite type complex oxides ofcomposition La1.2Sr1.8MnCrO7 and La1.5Sr1.5MnCrO7 havebeen synthesized by ceramic method. Rietveld profile anal-ysis shows that the phases crystallize with tetragonal unitcell in the space group I4/mmm. Both the phases behave asinsulators in the high temperature region and the linearity oflog ρ versus T−1/4 plot in the temperature range 150–300 Kshows that the electronic conduction occurs by a 3D vari-able range hopping mechanism. The phases show insulator-metal transition at low temperature which could be due tothe mixed valence state of Mn3+/Mn4+ by double exchangemechanism. The ferromagnetic interactions observed for thesamples arises from double exchange interaction betweenMn3+ and Mn4+ and Cr3+ and Mn3+ ions.

Keywords Layered perovskite oxides . Rietveld XRDanalysis . Insulator-metal transition . Magnetic properties

Introduction

A rich variety of oxides classified as Ruddlesden–Popper(RP) phases are described as intergrowth structures havingthe general formula (AO)(ABO3)n, where A is usually arare-earth, alkaline-earth, or alkali ion and B can be a 3Dor 4D transition-metal ion. The crystal structure of RPphases can be described by the stacking of finite n layersof perovskite ABO3 between rock salt AO layers along thecrystallographic c direction [1]. The corner-sharing BO6

octahedra form infinite sheets in the ab plane where strongelectronic interactions can occur. With increasing n, aniso-tropic properties and an increase in dimensionality may beexpected in RP phases. The n01 member of this series(A2BO4) exhibits a quasi-two-dimensional K2NiF4-typestructure, with only one layer of corner sharing BO6 octa-hedra along the c direction. The n0∞ member (ABO3)assumes the three-dimensional distorted perovskite struc-ture. In the second member of the RP family, A3B2O7, twoinfinite BO6 sheets are connected in the c direction betweenthe rock-salt layers. The unit cell structure of A3B2O7 isshown in Fig. 1. The physical properties of various memberswithin a given series are governed primarily by the identityand valence of the transition-metal ion, the width n of theABO3 perovskite slabs, the B–O–B bond angle and theoxygen content.

Ruddlesden–Popper (RP) phases of composition (La/Sr)3Mn2O7 have been the subject of immense interest inrecent years due to their exhibition of properties like colos-sal magnetoresistance in a variety of systems, particularlythose involving mixed valent cations in the perovskite layer[2–6]. It is well-known that Mn-site doping in perovskitemanganites is very interesting because it dramaticallychanges the magnetotransport properties due to the crucialrole of Mn ions in manganites. The doping effect in the Mnsite by metallic cations such as Al3+ and Fe3+ ions in thecharge-ordered (CO) manganites is not evident at low dop-ing level, but the CO state is destroyed at moderate dopingcontent (x>0.03) [7]. In contrast, the doping of Cr3+, Co3+

and Ti4+ in the Mn site destroys the CO state readily andrenders the material ferromagnetic [8]. In our earlier work,we have reported the synthesis and physical properties ofLaSr2MnCrO7 [9]. The phase crystallizes with tetragonalunit cell in the space group I4/mmm. It shows insulator-

D. Singh (*) : R. Singh : S. Sharma :M. SharmaDepartment of Chemistry, University of Jammu,Jammu 180 006, Indiae-mail: drdssambyal@rediffmail.com

Ionics (2013) 19:499–504DOI 10.1007/s11581-012-0752-6

metal transition at low temperature due to the double ex-change (DE) interaction mechanism, i.e., the hopping ofeg electrons between Mn3+ and Mn4+ ions mediated byoxygen anions. The dominant magnetic interactions inthe phase LaSr2MnCrO7 are ferromagnetic. Since thechange of La/Sr content in this phase leads to mixedvalent state of Mn, it was thought interesting to synthe-size two new Ruddlesden–Popper phases with n02(A3B2O7 type) of composition La1.2Sr1.8MnCrO7 andLa1.5Sr1.5MnCrO7 and study their electrical and magnetic

properties as a function of temperature. These phaseshave been synthesized by ceramic method. The crystalstructure of these phases has been determined by Riet-veld profile of the X-ray diffraction data. The electricalresistivity and magnetic susceptibility have been studiedas functions of temperature and the results are analysed.

Experimental

The phases La1.2Sr1.8MnCrO7 and La1.5Sr1.5MnCrO7

were prepared from the starting materials La2O3, SrCO3,Mn2O3 and Cr2O3 (All are Aldrich-make, purity>99.9 %). Prior to use, La2O3 was heated at 1,000 °Cto remove moisture, SrCO3 was heated at 200 °C, whileMn2O3 and Cr2O3 were used as received. The reactantoxides/carbonates were weighed corresponding to thestoichiometries of the desired phases, mixed and ho-mogenized by grinding in cyclohexane with an aluminamortar and pestle. A 5 % of SrCO3 was added inexcess to compensate for its loss at high temperature.The mixtures were pressed into pellets with 10 mm indiameter and 1 mm in thickness by hydraulic pressunder 20 MPa, and then calcined at 1,200 K in staticair atmosphere in an electric tube furnace for about36 h. The calcined pellets were grounded, and againpressed into pellets, and then sintered at 1,563 K instatic air atmosphere in an electric tube furnace forabout 76 h with a number of intermediate grindingsand pelletizings. Finally, the samples were cooled downslowly to room temperature in the furnace. The finalblack coloured products, after pulverization, were usedfor further investigations.

The total amount of various constituent cations was esti-mated by Perkin Elmer atomic absorption spectrometer 700.The samples were subjected to room temperature powder X-ray diffraction study on a Phillips diffractometer type PW

B

O

A

Fig. 1 Unit cell structure of Ruddlesden–Popper phase A3B2O7

Fig. 2 Rietveld refinementprofiles: observed (red lines),calculated (black lines) anddifference (green lines) for thefit to the XRD pattern ofLa1.2Sr1.8MnCrO7

500 Ionics (2013) 19:499–504

1820 using CuKα radiations in 2θ range of 10–70°. The datawere analyzed with the Rietveld analysis program DBWS-9807 for the structure determination [10].

The electrical resistivity of the pellets of these phasessintered at 1,450 K was recorded by Keithley 6517Aelectrometer in the temperature range 10–300 K usingLeybold closed cycle helium cryostat. Collinear four-probes were made on the surface of pellet with silverpaste. Thin copper wires were attached to the fourprobes with help of silver paste for the purpose ofelectrodes. The magnetic susceptibility of the polycrys-talline phases was measured by Faraday technique inthe temperature range 100–300 K using Hg[Co(SCN)4]as calibrant. All magnetic susceptibility values werecorrected for diamagnetism of the constituent ions.

Results and discussion

The X-ray diffraction data of the phases could be indexedwith tetragonal unit cell in the space group I4/mmm. Notrace of any extra peaks due to constituent oxides or n01 orn03 phases was found, suggesting the formation of singlephase compounds. The Rietveld analysis of the X-ray dif-fraction data of the phases was done starting with the modelof the well-known RP-type n02 phase, Sr3Ti2O7 [1]. Thestructures were refined with tetragonal unit cell in the spacegroup I4/mmm, assuming a pseudo-Voight (pV) peak shapefunction. The Rietveld refinement of the powder diffractiondata of the phases La1.2Sr1.8MnCrO7 and La1.5Sr1.5MnCrO7

is illustrated in Figs. 2 and 3. The refinement values, struc-tural parameters and R-factors along with the estimated

Fig. 3 Rietveld refinementprofiles: observed (red lines),calculated (black lines) anddifference (green lines) for thefit to the XRD pattern ofLa1.5Sr1.5MnCrO7

Table 1 Structural parametersof La1.2Sr1.8MnCrO7

Lattice constants: a03.854 (12)Å, c020.111 (24)Å, cellvolume0298.72 Å3,Rwp05.25 %, Rexp03.72 %and S (goodness of fit)01.41

Positional co-ordinates of La, Sr, Mn, Cr and O

Atom Site x y z B (Å2) Occupancy

La/Sr(1) 2b 0.0 0.0 0.5 0.98 (4) 1

La/Sr(2) 4e 0.0 0.0 0.3110 (4) 0.90 (5) 1

Mn/Cr 4e 0.0 0.0 0.0914 (6) 0.96 (4) 1

O(1) 2a 0.0 0.0 0.0 1.24 (10) 1

O(2) 4e 0.0 0.0 0.1750 (5) 1.06 (12) 1

O(3) 8g 0.0 0.5 0.0990 (7) 1.18 (9) 1

Selected bond lengths (Å) Selected bond angles (°)

Mn/Cr-O(1) 1.838 (12) O(1)-Mn/Cr-O(2) 179.99 (50)

Mn/Cr-O(2) 1.681 (16) O(2)-Mn/Cr-O(3) 85.46 (43)

Mn/Cr-O(3) 1.933 (1) O(1)-Mn/Cr-O(3) 94.54 (39)

La/Sr(2)-O(2) 2.735 (13) Mn/Cr-O(1)-Mn/Cr 179.99 (31)

La/Sr(2)-O(2) 2.740 (1) Mn/Cr-O(3)-Mn/Cr 170.93 (56)

La/Sr(2)-O(3) 2.644 (11) La/Sr(1)-La/Sr(2)-O(2) 180.00 (21)

La/Sr(1)-O(1) 2.725

La/Sr(1)-O(3) 2.771 (10)

La/Sr(1)-La/Sr(2) 3.801 (8)

Ionics (2013) 19:499–504 501

standard deviation (ESD) of the last significant number forthe phases are given in the Tables 1 and 2. In both thephases, no loss of oxygen has been noticed in Rietveldanalysis. The goodness of fit, S, values 1.41 and 1.46 forthe phases are reasonable for assigning the structure to thephases on the basis of the Rietveld analysis. The results ofX-ray diffraction studies show that the phases with compo-sition La1.2Sr1.8MnCrO7 and La1.5Sr1.5MnCrO7 have beenformed with the RP-type (n02) structure.

Selected interatomic bond lengths and bond angles havealso been calculated from the structural parameters, whichare tabulated in Tables 1 and 2. The analysis of bond lengthsand bond angles shows that the average co-ordination ge-ometry about the transition metal (Mn/Cr) site is somewhatirregular, which is often observed in the RP-type phases [11,12]. One of the reasons for this asymmetry in the presentcase seems to be the presence of the mixed valence state ofthe manganese ion (Mn3+/Mn4+) in the lattice. The mixedvalence state of Mn could be due to the increased lanthanumin the phases. Although there is only marginal difference inthe ionic size of La3+ and Sr2+ ions, gradual rise in cellvolume is observed with increase in lanthanum content.

The temperature dependence of electrical resistivity isgiven in Fig. 4, where log ρ is plotted versus temperature(T). The plot shows that in the high temperature range thetemperature coefficient of resistivity is negative suggestingthat both the phases behave as insulators in this temperaturerange. The insulating behaviour arises from localization ofcharge carriers. The linearity of log ρ versus T−1/4 plot(Fig. 5) in the high temperature range 150–300 K showsthat the electronic conduction in the insulator region occurs

by a 3D variable range hopping mechanism. Similar resultswere also reported by Zhang et al. [13]. However, as tem-perature falls the temperature coefficient of resistivityabruptly becomes positive and the phases become metallicin nature. The I-M transition temperature ( Tmax

ρ ) for

La1.2Sr1.8MnCrO7 and La1.5Sr1.5MnCrO7 was found to be52 and 58 K, respectively. The presence of mixed valencestate of Mn3+/Mn4+ in these phases leads to creation ofmobile charge carriers by way of electron transfer fromMn3+ to Mn4+ state at low temperature by double exchangemechanism resulting in I-M transition [4–6, 14]. Perusal ofFig. 4 shows that the I-M transition temperature shifts tohigher side in the phases with increased lanthanum concen-tration. The results suggest that metallic behaviour shift tohigher temperature with enhanced concentration of Mn3+

ion in the mixed valence state of manganese.

Table 2 Structural parametersof La1.5Sr1.5MnCrO7

Lattice constants: a03.8562 (8)Å, c020.1289 (14)Å, CellVolume0299.32 Å3,Rwp05.65 %, Rexp03.87 %and S (goodness of fit)01.46

Positional co-ordinates of La, Sr, Mn, Cr and O

Atom Site x y z B (Å2) Occupancy

La/Sr(1) 2b 0.0 0.0 0.5 0.91 (5) 1

La/Sr(2) 4e 0.0 0.0 0.3016 (7) 0.89 (7) 1

Mn/Cr 4e 0.0 0.0 0.1010 (5) 0.99 (4) 1

O(1) 2a 0.0 0.0 0.0 1.27 (10) 1

O(2) 4e 0.0 0.0 0.1706 (4) 1.16 (9) 1

O(3) 8g 0.0 0.5 0.097 (6) 0.98 (6) 1

Selected bond lengths (Å) Selected bond angles (°)

Mn/Cr-O(1) 2.033 (10) O(1)-Mn/Cr-O(2) 180.00 (44)

Mn/Cr-O(2) 1.801 (13) O(2)-Mn/Cr-O(3) 92.39 (210)

Mn/Cr-O(3) 1.930 (5) O(1)-Mn/Cr-O(3) 87.61 (208)

La/Sr(2)-O(2) 2.637 (16) Mn/Cr-O(1)-Mn/Cr 180.00 (23)

La/Sr(2)-O(2) 2.784 (3) Mn/Cr-O(3)-Mn/Cr 175.22 (414)

La/Sr(2)-O(3) 2.808 (88) La/Sr(1)-La/Sr(2)-O(2) 180.00 (31)

La/Sr(1)-O(1) 2.727

La/Sr(1)-O(3) 2.744 (86)

La/Sr(1)-La/Sr(2) 3.994 (14)

3

4

5

6

7

0 50 100 150 200 250 300 350

T (K)

log

ρ(o

hm c

m)

12

Fig. 4 Plot of log ρ versus temperature (K) of La1.2Sr1.8MnCrO7 (1)and La1.5Sr1.5MnCrO7 (2)

502 Ionics (2013) 19:499–504

The temperature dependence of the inverse molar mag-netic susceptibility for the phases is shown in Fig. 6. Thevalues of magnetic moment (μeff), estimated from hightemperature region (250–300 K), are of the order of 6.21and 6.36 Bohr Magneton (B.M.) for La1.2Sr1.8MnCrO7 andLa1.5Sr1.5MnCrO7, respectively, which are in good agree-ment with reported phase LaSr2MnCrO7 (6.03 B.M.) [9].The higher values of μeff than the reported LaSr2MnCrO7

could be due to the larger Mn3+ content in the proposedphases. The theoretical spin only magnetic moment for eachof these phases has also been calculated assuming thatmanganese ion is present in the mixed valence state Mn3+/Mn4+ taking high spin configuration of Mn3+ and Mn4+

ions. The theoretical values of magnetic moment have beencalculated from the relationship [15]

μcal ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

ðn1μ21 þ n2μ2

2Þ þ 1μ23

q

where n1 and n2 are the relative molar fractions of manga-nese ion present in 3+ and 4+ oxidation states, while μ1, μ2and μ3 are the theoretical magnetic moment contributions of

Mn3+, Mn4+ and Cr3+ ions. The μcal values for La1.2Sr1.8Mn-CrO7 and La1.5Sr1.5MnCrO7 come to be 5.64 and 5.87 (B.M.)respectively. The Weiss constant (θ) obtained from the hightemperature linear region is positive for both the phases.The larger μeff and positive θ values suggest that ferro-magnetic interactions are dominant in the magnetic struc-ture of these phases. Zhang et al. also reported that thedoping of Cr in bilayered LaSr2Mn2O7 leads to ferro-magnetic interactions [13].

The ferro-paramagnetic transition at low temperature inboth the phases could be due to double exchange (DE)interaction between Mn3+ and Mn4+ ions. Since the Cr3+

ion has the same electronic configuration (t32geog) as the Mn4+

ion, there may exist a ferromagnetic DE interaction betweenCr3+ and Mn3+ ions just as between Mn4+ and Mn3+ ions.The proposal has been proved by some experimental results[16]. The greater value of θ for La1.5Sr1.5MnCrO7 than thatof La1.2Sr1.8MnCrO7 indicates the increased ferromagneticinteractions of the total exchange interaction in the former. Itmay be due to fact that the increase of La content give rise tothe appearance of Mn3+ ions which leads to increase in theferromagnetic DE interaction between Mn3+ and Mn4+ andthat between Mn3+ and Cr3+ ions.

The Ferromagnetic Curie temperature (TC), obtainedfrom the molar magnetic susceptibility (cM ) versus temper-ature plot (Fig. 7), was found to be 135 and 148 K forLa1.2Sr1.8MnCrO7 and La1.5Sr1.5MnCrO7, respectively.When comparing the Tc values with the Tmax

ρ values, Tcwas about 80–90 K higher than the Tmax

ρ . Namely, for the

phases, no coincidence can be seen between the metal-insulator transition and the magnetic transition, as common-ly observed for n0∞ type ferromagnet [17–20]. This couldbe interpreted as being the result of the anisotropic electrontransfer and the exchange interactions in the a-axis and c-axis directions [21, 22]. It is generally considered that, in

0

2

4

6

8

10

12

14

16

18

20

80 120 160 200 240 280 320

Temperature (K)

χ m-1

(em

u/m

ol)

-1

1

2

Fig. 6 Plot of inverse molar magnetic susceptibility ( c�1m ) versus

temperature (K) for La1.2Sr1.8MnCrO7 (1) and La1.5Sr1.5MnCrO7 (2)

0

1

2

3

4

5

80 120 160 200 240 280 320

Temperature (K)

χm

(em

u/m

ol)

0

0.2

0.4

0.6

0.8

80 120 160 200 240 280 320

Temperature (K)

χ m (

emu

/mo

l)

Fig. 7 Plot of molar magnetic susceptibility (cm) versus temperature(K) for La1.5Sr1.5MnCrO7. Inset shows the cm versus temperature (K)plot of La1.2Sr1.8MnCrO7

3

3.5

4

4.5

5

5.5

0.23 0.25 0.27 0.29

T-1/4 (K-1/4)

log

ρ (

Oh

m c

m)

12

Fig. 5 Plot of log ρ versus T−1/4 of La1.2Sr1.8MnCrO7 (1) andLa1.5Sr1.5MnCrO7 (2)

Ionics (2013) 19:499–504 503

layered perovskite, in the motion of doped holes andhence the exchange interaction would be anisotropic inthe a–b axis (in-plane) and c axis (out-of-plane) direc-tion. Asano et al. [21, 22] suggested that in-plane (a–b)exchange interaction is stronger than out-of-plane (caxis), and that the probability of electron hopping (holetransfer) in the in-plane is much higher than that in theout-of-plane direction. Hence, the lower critical temper-ature (Tmax

ρ ) we observed results from the out-of-plane

exchange interaction, and the higher critical temperature(TC) results from in-plane exchange interaction. Thus,the hopping conduction observed in the phases at T>150 K in the present study indicates that in-plane ex-change interaction is predominantly responsible for thecarrier localization. In addition, experimental results ofthe samples obtained in the present study are explainedby the interpretation of Asano et al. [21, 22] for aniso-tropic conduction in these materials.

Conclusions

The layered perovskite oxides La1.2Sr1.8MnCrO7 andLa1.5Sr1.5MnCrO7 have been synthesized by the conven-tional solid state reaction method. Rietveld analysis of theX-ray diffraction data shows that the phases crystallize withtetragonal unit cell in the space group I4/mmm. The phasesundergo insulator-metal transition due to presence of mixedvalence state of Mn3+/Mn4+ by double exchange mechanismand the electrical conduction in these phases occurs byMott’s variable range hopping mechanism. The dominantmagnetic interactions in both the phases are ferromagneticwhich arises from double exchange (DE) interaction be-tween Mn3+ and Mn4+ and Cr3+ and Mn3+ ions.

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