ORIGINAL PAPER

Synthesis and characterization of layered perovskite oxides(La, Sr)n + 1(Ti, Cr)nO3n + 1 (n01, 2)

Sushma Sharma & Arun Mahajan & Suram Singh &

Rajinder Singh & Devinder Singh

Received: 18 May 2012 /Revised: 20 June 2012 /Accepted: 6 July 2012 /Published online: 19 July 2012# Springer-Verlag 2012

Abstract We report here the synthesis of layered perov-skite oxides of the composition La0.5Sr1.5Ti0.5Cr0.5O4 andLaSr2TiCrO7 by conventional solid-state reaction method.Results of XRD analysis show that the phases crystallizewith tetragonal unit cell in the space group I4/mmm. Bothphases behave as insulators and the linearity of log ρversus T−1/4 plot in the temperature range 150–350 Ksuggests that the electronic conduction occurs by Mott’svariable range hopping mechanism. The antiferromagneticinteractions observed for the samples arise from Cr3+–O–Cr3+ superexchange interaction.

Keywords Layered perovskite oxides . XRD . Electricalproperties . Magnetic properties

Introduction

Multilayered transition metal oxides have been the focusof research efforts for the last two decades due to theirability to exhibit a wide variety of interesting functionalproperties, such as high Tc superconductivity, oxide ionconductivity, and high-performance magnetoresistance andthermoelectric characteristics. These oxides, known asRuddlesden–Popper (RP) phases, can be represented bythe general formula An+1BnO3n+1, where A is usually arare-earth, alkaline-earth, or alkali ion and B can be a 3Dor 4D transition metal ion [1, 2]. These phases generallycrystallize with tetragonal or orthorhombic unit cell in thespace group I4/mmm or Fmmm [3–6]. All members of theseries are built of ABO3 perovskite blocks of corner-

sharing BO6 octahedra, these blocks being infinite inplane xy and having n layers in direction z. Adjacentblocks are separated by AO rock salt blocks, so that theformula unit may rightfully be read as [(ABO3)n(AO)].While the n01 phases are 2D in structure, the 3D char-acteristic increases with an increase in n. These phases aregenerally electrical insulators and antiferromagnetic inphysical behavior. The transport properties of variousmembers within a given series are governed primarily bythe identity and valence of the transition metal ion, thewidth n of the ABO3 perovskite slabs, the B–O–B bondangle, and the oxygen content. For example, LaNiO3 (n0∞)is metallic, whereas GdNiO3 is an antiferromagnetic semi-conductor as a result of a decrease in the Ni–O–Ni bondangle with decreasing size of the lanthanide ion. La2NiO4, inwhich the corner-sharing NiO6 octahedral units in the abplane are sandwiched between the LaO rock salt layers, issemiconducting below 500 K. The lowest electrical resistiv-ity observed in an n01 phase of any RP family is of the orderof 10−2 Ω cm. Resistivity generally decreases further inhigher members of the family because of the increasing 3Dcharacter [5].

The parent members Sr2TiO4 and Sr3Ti2O7 of RP seriescrystallize with tetragonal unit cell in the space group I4/mmm [1, 2]. Wide possibilities for substitutions at both Srand Ti positions play an important role in determining theirphysical properties and show a great potential for diverseapplications. Though substituted Sr2TiO4 and Sr3Ti2O7

phases have been widely studied, there is no report of theoxides with simultaneous substitution of La at the Sr siteand Cr at the Ti site. In view of this, we report our attempt toobtain RP phases La0.5Sr1.5Ti0.5Cr0.5O4 (n01) and LaSr2Ti-CrO7 (n02) from solid-state reaction method. Their crystalstructure has been determined by powder X-ray diffractom-etry. The electrical resistivity and magnetic susceptibility ofthe phases have been studied as functions of temperature.

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

Ionics (2013) 19:505–509DOI 10.1007/s11581-012-0772-2

Experimental

The phases La0.5Sr1.5Ti0.5Cr0.5O4 and LaSr2TiCrO7 wereprepared from the starting materials La2O3, SrCO3, TiO2,and Cr2O3 (all are Aldrich-make, purity>99.9 %). Prior touse, La2O3 was heated at 1,000 °C to remove moisture,SrCO3 was heated at 200 °C, while TiO2 and Cr2O3 wereused as received. The reactant oxides/carbonates wereweighed corresponding to the stoichiometries of the desiredphases, mixed, and homogenized by grinding in cyclohex-ane with an alumina mortar and pestle. The mixtures werepressed into pellets with 10 mm in diameter and 1 mm inthickness by hydraulic press under 20 MPa, and then cal-cined at 1,200 K in static air atmosphere in an electric tubefurnace for about 36 h. The calcined pellets were grounded,and again pressed into pellets, and then sintered at 1,623 Kin static air atmosphere in the electric tube furnace for about76 h with a number of intermediate grindings and pelletiz-ings. Finally, the samples were cooled down slowly to roomtemperature in the furnace. The final black colored products,after pulverization, were used for further investigations.

Room temperature X-ray diffraction data of the phaseswere recorded on Bruker AXS diffractometer type D 76181(Karlsruhe, Germany) using CuKα radiations. The data werecollected at scanning speed of 1°/min in the 2θ range of 10–80°. The experimental XRD data are given in Tables 1 and 2,while the patterns are plotted in Figs. 1 and 2. The totalamount of various constituent cations was estimated by PerkinElmer atomic absorption spectrometer 700.

The electrical resistivity of the pellets of these phasessintered at 1,450 K was recorded by the four probe methodin the temperature range 150–350 K within error limits of±0.01 %. Thin copper wires were attached to the surface ofthe pellet for the purpose of electrodes with silver epoxy.The densities of the sintered pellets were determined by theArchimedes method. The magnetic susceptibility of thepolycrystalline phases was measured by the Faraday tech-nique in the temperature range 100–300 K using Hg[Co(SCN)4] as calibrant. All magnetic susceptibility valueswere corrected for diamagnetism of the constituent ions.

Results and discussion

The structures of the new phases reported here were identi-fied from initial X-ray diffraction data and could be indexedwith a tetragonal unit cell with the I4/mmm space group. Theunit cell parameters and space group were determined andtested by using the program “Checkcell” [7] and are listed inTables 1 and 2. The values of both parameters a and c aresmaller than those of Sr2TiO4 (a03.88 Å and c012.60 Å)[1] and Sr3Ti2O7 (a03.90 Å and c020.38 Å) [2], whichcould be due to the difference in ionic radii of Ti4+ and Cr3+

Table 1 Powder X-ray diffraction data of La0.5Sr1.5Ti0.5Cr0.5O4 (spacegroup: I4/mmm), a03.8541(3) Å, c012.5326(3) Å

h k l dobs (Å) dcal (Å) Iobs Ical

1 0 1 3.6775 3.6838 11 18

0 0 4 3.1290 3.1331 14 15

1 0 3 2.8299 2.8327 100 100

1 1 0 2.7247 2.7252 84 85

1 1 2 2.4993 2.4991 8 14

1 0 5 2.1020 2.1012 17 8

0 0 6 2.0893 2.0887 19 21

1 1 4 2.0572 2.0562 31 47

2 0 0 1.9288 1.9270 42 43

2 1 1 1.7097 1.7075 7 4

1 1 6 1.6596 1.6578 18 13

2 0 4 1.6433 1.6414 10 6

1 0 7 1.6260 1.6237 12 6

2 1 3 1.5950 1.5933 38 22

0 0 8 1.5683 1.5665 7 3

2 1 5 1.4188 1.4202 16 12

2 2 0 1.3653 1.3626 14 8

1 1 8 1.3615 1.3581 10 8

2 1 7 1.2444 1.2417 8 4

3 0 3 1.2304 1.2279 11 4

3 1 0 1.2214 1.2187 12 5

2 0 8 1.2185 1.2155 11 5

Table 2 Powder X-ray diffraction data of LaSr2TiCrO7 (space group:I4/mmm), a03.8800(4) Å, c020.2594(3) Å

h k l dobs (Å) dcal (Å) Iobs Ical

1 0 1 3.8141 3.8107 12 8

0 0 6 3.3794 3.3765 12 11

1 0 5 2.7992 2.8023 100 100

1 1 0 2.7431 2.7436 91 79

1 1 4 2.4121 2.4124 8 13

1 0 7 2.3211 2.3199 9 5

1 1 6 2.1292 2.1293 22 36

0 0 10 2.0267 2.0259 16 21

2 0 0 1.9390 1.9400 48 51

1 1 8 1.8618 1.8608 6 4

2 1 1 1.7282 1.7288 6 2

2 0 6 1.6814 1.6821 8 5

1 0 11 1.6649 1.6638 8 4

1 1 10 1.6295 1.6297 13 11

2 1 5 1.5936 1.5951 35 23

2 1 7 1.4885 1.4882 5 2

1 1 12 1.4383 1.4378 5 2

2 0 10 1.4009 1.4011 13 14

2 2 0 1.3714 1.3718 14 9

3 0 1 1.2905 1.2907 5 1

506 Ionics (2013) 19:505–509

and Sr2+ and La3+ ions. The lattice constant c of La0.5Sr1.5-Ti0.5Cr0.5O4 and LaSr2TiCrO7 phases showed a dramaticvariation compared to a. The systematic increase in c withincreasing n is clearly due to the addition of perovskitelayers between the rock salt-like (La,Sr)O layers. The atom-ic positions of the phases La0.5Sr1.5Ti0.5Cr0.5O4 and LaSr2-TiCrO7 have been estimated respectively from analogy withSr2VO4 [8] and Sr3V2O7 [9] with tetragonal symmetrywithout refinement. The calculated diffraction patterns(Tables 1 and 2) of the phases were determined with theprograms “Diamond” (method of Klaus Brandenburg 1998)and “Mercury 2.3” based on the atomic positions, cellparameters, and space group I4/mmm. The agreement be-tween the calculated and observed relative reflection

intensities (Tables 1 and 2) of the phases is in generalsatisfactory, suggesting that the phases with the compositionLa0.5Sr1.5Ti0.5Cr0.5O4 and LaSr2TiCrO7, having Sr2TiO4

and Sr3Ti2O7 type structures, respectively, have beenformed.

The densities of the sintered pellets of the phases wereabout 90 % theoretical density. The smaller value of exper-imental density than the X-ray density could be due to thepresence of porosities in these polycrystalline materials. Theroom temperature resistivity (ρ300) is 4.35×104 Ω cm forLa0.5Sr1.5Ti0.5Cr0.5O4 and 1.44×10

4 Ω cm for LaSr2TiCrO7.The temperature dependence of electrical resistivity is givenin Fig. 3, where log ρ is plotted against temperature (T). Theplot shows that the temperature coefficient of resistivity of

Fig. 1 X-ray diffraction pattern of La0.5Sr1.5Ti0.5Cr0.5O4

Fig. 2 X-ray diffraction pattern of LaSr2TiCrO7

Ionics (2013) 19:505–509 507

both phases is negative, suggesting that the materials areinsulators with no anomalous features in the entire temper-ature range of 150–350 K. The insulator behavior is attrib-uted to the superexchange coupling of electrons. Similarelectric transport behavior has also been shown by theparent RP phases Sr2TiO4 and Sr3Ti2O7. The results suggestthat electrical resistivity decreases with an increase in n. Thedecrease in ρ with an increase in n is consistent with anincrease in the density of charge carriers. Since the numberof adjacent perovskite layers rises with increasing n, anincrease in Ti/Cr band width is also expected, arising froman increase in Ti/Cr–O–Ti/Cr overlap along the crystallo-graphic c direction. A similar type of results was alsoreported by Zhang et al. [4]. Various equations based upondifferent mechanisms of conduction, such as those applica-ble in the case of the Arrhenius model, the polaron hoppingmodel, and the variable range hopping model have been

applied to the data of electrical resistivity of the phases.We find that the conduction process in the phases is bestdescribed using Mott’s variable range hopping model. Inthis model ρ(T) is expressed as [10]

ρ ¼ ρo exp BT�1 4=� �

where B is the characteristic energy of hopping. The log ρversus T−1/4 plot for the phases is shown in Fig. 4. Thelinearity of plots shows that the electrical conduction inthese phases occurs by Mott’s variable range hopping mech-anism which is generally observed in such perovskite-related phases [11–13]. The characteristic energy of hopping(B), calculated from the slopes of plots of log ρ versus T−1/4,is equal to 69.23 and 61.11 K1/4 for La0.5Sr1.5Ti0.5Cr0.5O4

and LaSr2TiCrO7, respectively. The comparatively largervalue of B for the La0.5Sr1.5Ti0.5Cr0.5O4 suggests that insu-lator behavior is more pronounced in this phase.

The temperature dependence of inverse molar magneticsusceptibility is shown in Fig. 5. The linearity of plots suggeststhat the Curie–Weiss law is followed in the temperature regionof investigation. The application of Curie–Weiss law to themagnetic susceptibility data of the phases gives negative valuesof θ (Table 3) which is consistent with antiferromagnetic inter-actions. The antiferromagnetic behavior of the phases could bedue to Cr3+–O–Cr3+ superexchange interaction. The Curie

150 200 250 300 3503

4

5

6

7

8

log

ρ(O

hm

cm

)

Temperature (K)

La0.5

Sr1.5

Ti0.5

Cr0.5

O4

LaSr2TiCrO

7

Fig. 3 Plot of log ρ versus T (Kelvin) of La0.5Sr1.5Ti0.5Cr0.5O4 andLaSr2TiCrO7

0.23 0.24 0.25 0.26 0.27 0.28 0.293

4

5

6

7

8

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

log

ρ(O

hm

cm

)

La0.5

Sr1.5

Ti0.5

Cr0.5

O4

LaSr2TiCrO

7

Fig. 4 Plot of log ρ versus T−1/4 of La0.5Sr1.5Ti0.5Cr0.5O4 andLaSr2TiCrO7

50 100 150 200 250 300

100

150

200

250

300

350

χ m

-1 (

emu

/mol

)-1

Temperature (K)

La0.5

Sr1.5

Ti0.5

Cr0.5

O4

LaSr2TiCrO

7

Fig. 5 Plot of inverse molar susceptibility c�1m

� �versus T (Kelvin) of

La0.5Sr1.5Ti0.5Cr0.5O4 and LaSr2TiCrO7

Table 3 Density and magnetic parameters of La0.5Sr1.5Ti0.5Cr0.5O4

and LaSr2TiCrO7

S. no. Compound Experimentaldensity(g cm−3)

X-raydensity(g cm−3)

μeff (B.M.) θ (K)

1 La0.5Sr1.5Ti0.5Cr0.5O4 4.988 5.618 3.72 −32

2 LaSr2TiCrO7 5.051 5.727 3.12 −52

508 Ionics (2013) 19:505–509

constants (C) were determined from the slope (C−1) of c�1m

versus temperature (T) plot. The effective magnetic moments

(μeff) per Cr site, as estimated from the relations μeff ¼ 2:84ffiffiffiffiffiffi2C

pin the case of La0.5Sr1.5Ti0.5Cr0.5O4 and μeff ¼ 2:84

ffiffiffiffiC

pfor LaSr2TiCrO7, are given in Table 3. The values of μeff forboth phases are smaller than the spin-only magnetic moment ofCr3+ (3.87 B.M.), suggesting the presence of antiferromagneticinteractions in the magnetic structure of these phases. A similartype of results were also reported earlier in such perovskite-likeoxides [6, 12, 14, 15]. The greater negative value of θ and muchsmaller value of μeff (3.12 B.M.) for LaSr2TiCrO7 than the spin-only value of Cr3+ compared to La0.5Sr1.5Ti0.5Cr0.5O4 shows thatantiferromagnetic interactions are more dominant in the former.

Conclusions

The RP-type phases La0.5Sr1.5Ti0.5Cr0.5O4 and LaSr2Ti-CrO7, prepared by the ceramic method, crystallize withtetragonal unit cell in the space group I4/mmm. The electri-cal conduction in both phases occurs by variable rangehopping mechanism. The Weiss constant (θ) suggests thatboth phases are antiferromagnetic.

Acknowledgments Thanks are due to Prof. A. Ramanan, Departmentof Chemistry, Indian Institute of Technology, New Delhi, for providingsoftwares for structure determination. We are also thankful to Prof.

Ramesh Chandra, Institute Instrumentation Centre, Indian Institute ofTechnology, Roorkee, for recording XRD data.

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