Structural and Electrical Transport Properties of La0.67Ba0.33Mn1-yTiyO3 Ceramics

Zalita Zainuddin1,a and Abdul Halim Shaari2,b 1School of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia

43600 UKM Bangi, Selangor, Malaysia

2Department of Physics, Faculty of Science, Universiti Putra Malaysia 43400 UPM Serdang, Selangor, Malaysia

azazai@ukm.my (corresponding author), bahalim@science.upm.edu.my

Keywords: Manganite; electrical transport; hopping

Abstract. The structural and electrical transport properties of La0.67Ba0.33Mn1-yTiyO3 manganite, with y = 0.00, 0.05, 0.10, 0.15, 0.20, 0.40 and 0.60, prepared using the solid state reaction technique have been investigated. The X-ray diffraction spectra of the Ti substituted samples showed the formation of single phase compound with Pm3m cubic structure except for the y = 0.60 sample, which showed La2Ti2O7 phase formation. Lattice parameter increased with Ti content and then decreased at y = 0.60. Resistivity versus temperature study showed that only samples with y = 0.05 and 0.10 exhibited both metallic and semiconductor-like behaviour with the metal-insulator transition temperature, Tp of 167 K and 43 K, respectively. At higher Ti concentration the samples only showed the semiconducting behaviour. At T < Tp the resistivity curves followed the ρ = ρo1 +

ρ1T2 relation and for T > Tp, the curves can be fitted with the nearest neighbour hopping (NNH),

variable range hopping (VRH) or/and the small polaron hopping (SPH) models.

Introduction

The Mn3+/Mn4+ mixed valence perovskite manganite system, Ln1-xAxMnO3, where Ln is an element from the lanthanide group and A is a divalent ion, has stimulated an increasing interest due to their variety of phases such as ferromagnetic metallic (FMM), antiferromagnetic insulator (AFI), ferromagnetic insulator (FMI), cluster glass and spin glass, which emerged due to the unique coupling among charge, spin, orbital and lattice degree of freedom of the 3d electrons in this system. Manganite materials exhibit the colossal magnetoresistance (CMR) behaviour near Curie temperature, TC. Extensive research activities experimentally and theoretically on the properties of this material were being done by researchers worldwide for better understanding of the principles lying behind all those phenomena and expecting that its improved feature could be useful in the technological industries. The magnetic exchange, structure properties and electronic transport of manganites crucially depend on the Mn3+/Mn4+ ratio and the effective ionic radius of the A-site cations. These properties are believed to exist due to the competition between the super exchange (SE) and the double exchange (DE) mechanism [1]. Further studies showed that other mechanism also influences the phase transition in manganites, including the electron-electron interaction arising from the Jahn-Teller (JT) distortion [2], the orbital ordering [3], the electron-electron correlations and the coupling between spin and orbital structure.

In our previous work we reported on the magnetic, electrical transport and impedance spectroscopy properties of the La0.67Sr0.33Mn1-yTiyO3 (0.00 ≤ y ≤ 0.60) compounds [4]. In this work we studied the structural and electrical transport properties of the La0.67Ba0.33Mn1-yTiyO3 system with Ti substitution at the Mn site. Ba substitution at La sites promotes a higher symmetry crystal structure and exhibited a long range ferromagnetic order [5] with Curie temperature above room temperature. The La1-xBaxMnO3 (LBMO) system is also less studied compared to the Ca and Sr doped compound. On the other hand a non-magnetic material Ti is a good choice as a substitute ion due to its good solubility hence it will not affect the chemical stability of the original system.

Advanced Materials Research Vol. 501 (2012) pp 86-90Online available since 2012/Apr/12 at www.scientific.net© (2012) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.501.86

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Experimental Details

Polycrystalline La0.67Ba0.33Mn1-yTiyO3 (LBMT) samples with y = 0.00, 0.05, 0.10, 0.15, 0.20, 0.40 and 0.60 have been prepared using the conventional solid state reaction of a stoichiometric mixture of La2O3, BaCO3, MnO2 and TiO2, with at least 99 % purity. The powders were wet milled with acetone as the mixing medium to homogenize the mixture and enhanced the solid solution during calcinations. The dried mixture was then ground and calcined in air at 900 oC for 12 hours for carbonate decomposition. The powders was reground and pressed into pellet to ensure maximum particle packing and uniformity for maximum interaction during the sintering process. Finally the samples were sintered at 1300 oC in air for 24 hours and slow cooled to room temperature. The phase and crystal structure analysis of the final products were done using Philips 7602 EA Almelo /X’pert Pro Pw 3040 X-ray diffractometer (XRD) with CuKα radiation with a scanning rate of 4o/min. An Oxford Instruments Model 7353, energy dispersive X-ray (EDX) was used to determine the chemical composition of the samples. Four point probe measuring equipment was used to measure the temperature dependent of the resistance.

Results and Discussion

The quantitative compositional percentage data of the elements obtained from the Energy Dispersive X-ray, EDX confirmed the expected La:Ba:Mn:Ti ratios for the prepared La0.67Ba0.33Mn1-yTiyO3 samples. The X-ray diffraction (XRD) patterns at room temperature for the Ti substituted samples are shown in Fig. 1(a). The entire spectra displays single phase compound without detectable secondary phase except for higher Ti composition (y = 0.60), which shows the existence of *La2Ti2O7 (00-027-1182) phase. The number of diffraction peaks, the peaks relative intensities and width vary slightly when Ti content is increased. This indicates the Ti incorporation at the Mn site in the La0.67Ba0.33Mn1-yTiyO3 system.

All of the samples were indexed with the cubic Pm3m structure to evaluate the lattice parameter, a. The calculated a for all samples are plotted in Fig 1(b). a increases as the Ti content increases and then decreases when y = 0.60. The increment of the lattice parameters indicates that the Ti ions have entered and substituted the Mn ions at the B sites, thus lattice deformations are likely to occur due to the larger atomic radius of Ti4+ (0.605 Å) than Mn4+ (0.530 Å). a decreases when y = 0.60, which may be due to the rearrangement of the atoms in the lattices.

Fig. 1. (a) XRD patterns for different y composition and (b) lattice parameter versus Ti concentration for La0.67Ba0.33Mn1-yTiyO3.

20 30 40 50 60 70 80 90

Intensity (a.u.)

2θ (degree)

001 111

011

003202112012

002103 y = 0.60

y = 0.40

y = 0.20

y = 0.15

y = 0.05

y = 0.10

y = 0.00

***

3.89

3.90

3.91

3.92

3.93

3.94

3.95

3.96

3.97

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Lattice parameter, a

(Å)

Ti concentration, y

(a) (b)

Advanced Materials Research Vol. 501 87

Fig. 2 shows the temperature dependence of resistivity with different Ti concentration. Due to the limitation of our equipment we can only measure the temperature until up to 275 K. The parent materials show only metallic behaviour but with a small amount of Ti doping of 0.05 and 0.10, the samples show both the metallic and semiconducting behaviour with the metal-insulator transition temperature, Tp of 167 K and 43 K respectively. In this compound when Ti ion substitutes the Mn ion, a disorder is introduced in the charge transfer mechanism where some of the double exchange links between the Mn3+-O2--Mn4+ is replaced by the Mn3+-O2--Ti4+ network [6]. At a certain temperature range the electron transfer, from the Mn3+ site to the intervening oxygen ion together with another simultaneous electron transfer from the oxygen to the Mn4+ ion site, will be frustrated because the non magnetic Ti cannot participate in the charge transfer process. Tp, as well as the resistivity strongly depends on Ti composition. Ti doping decreases the Tp and increases the resistivity with several orders of magnitude greater than the resistivity of the parent compound. Samples with higher Ti doping, y > 0.15 exhibits semiconductor behaviour at all measured temperatures. The metallic Mn-O-Mn network has collapsed due to the addition of the Ti ions, occupying the Mn site, which depopulated the hopping electrons. The lower amount of hole electronic carriers decreases the electronic conduction. Taguchi et al. [7] found that in (La0.1Ca0.9)(Mn1-xTix)O3 perovskite oxides, Ti4+ ions makes the cation-anion-cation overlap integrals weaken thus increasing the resistivity.

Fig. 2. The resistivity versus temperature graph for La0.67Ba0.33Mn1-yTiyO3 with different y. Two empirical equations are used to fit the resistivity data in the metallic region at lower

temperature (T < Tp), to explain the nature of electronic transport, which are

ρ(T) = ρ01 + ρ1T 2 (1)

ρ(T) = ρ02 + ρ2T 2.5 (2)

where ρ0 the is resistivity due to the domain, grain boundary and other temperature independent scattering mechanism. ρ1T

2 is the electrical resistivity causes by the electron-electron scattering process and ρ2T

2.5 arises due to the electron-magnon scattering process [8]. Fig. 3 shows the log resistivity versus temperature plot, fitted with both equations, for samples with y = 0.00 and 0.05, with the best fitting parameters of ρ01, ρ1, ρ02, ρ2 and linear correlation coefficient, R2. Both fittings show nearly parallel lines with the experimental resistivity in the T < Tp region, however the lines deviate when the temperature is further increased with larger deviation for fitting lines of Eq. 2, which shows that the temperature dependent electrical resistivity is governed by the electron-electron scattering process which is associated with the spin fluctuations, rather than the electron-magnon scattering process.

-3

-1

1

3

5

7

9

20 60 100 140 180 220 260

log resistivity,

ρ (

Ωcm

)

Temperature, T (K)

y = 0.00

y = 0.05

y = 0.10

y = 0.15

y = 0.20

y = 0.40

y = 0.60

Tp = 167 K

Tp = 43 K

88 Solid State Science and Technology XXVI

Fig. 3: The best fit curves at lower temperature using Eq. 1 (straight lines) and Eq. 2 (dotted lines) with ρ01, ρ1, ρ02, ρ2 and R2 values for La0.67Ba0.33Mn1-yTiyO3 with (a) y = 0.00 and (b) y = 0.05.

A few models can be used to explain the semiconducting region of the resistivity where T > Tp.

The nearest neighbour hopping (NNH) model or known as the thermally activated conduction (TAC) law, is expressed by

ρ = ρο exp (Ea/kBT) (3)

where ρο is a pre-factor which is a constant, Ea is energy gap (or activation energy) typically 0.1 eV, kB is Boltzmann constant and T is temperature. In this process the donor sites are randomly distributed in space, and charge carriers hop to the nearest empty donor sites. This simple activation law may indicate the opening of a gap at the Fermi level above TP. A deviation from linearity of the ln resistivity versus inverse temperature curve can be observed when temperature increases above Tp. The non-linear temperature curve signifies a temperature-dependent activation energy that is a feature of small lattice polaron hopping (SPH) conduction where the polaron hops rapidly between two sites with the same identical configuration. The SPH resistivity equation of Mott is

ρ = ροT exp (EP/kBT) (4) where EP is the polaron activation energy. Temperature at which the slope of the linear curve changes from linearity is at temperature θD/2 which is the characteristic of Debye temperature [9]. θD can be defined by the relation hνph = kBθD where h is Planck constant and νph is the optical phonon frequency. The high temperature, T > Tp resistivity data could be explain using the the SPH model for T > θD/2. Mott’s variable range hopping (VRH) model for three-dimensional (3D) system at Tp < T < θD/2 is given by

ρ = ρο exp ((T0/T) 1/4 (5)

where T0 = 16α 3/kB N(EF) is a constant. α is the inverse of the localization length of the trapped charge carriers, kBT0 is the Mott’s activation energy and N(EF) is the density of states at Fermi level which represents the effective band of electron hopping [8]. All of the parameters obtained from the three models are summarized in Table 1. It is found that Ep increases with Ti content while other parameters do not have any certain trend with Ti variation. Sample with y = 0.60 is best fitted with the nearest neighbour hopping model with Ea = 190 MeV.

0.00

0.01

0.02

0.03

0.04

60 100 140 180 220 260 300

Resistivity,

ρ(Ω

cm)

Temperature, T (K)

(a)ρ

02= 8.24 Ω cm

ρ2

= 0.21 x10-4 Ω cm K-2

(R2 = 0.9956)

ρ01

= 7.38 Ω cm

ρ1

= 2.92 x10-4 Ω cm K-2

(R2 = 0.9997)

0

2

4

6

8

10

12

14

16

60 100 140 180 220 260 300

Resistivity,

ρ(Ω

cm)

Temperature, T (K)

(b)

ρ02

= 3.11 Ω cm

ρ2

= 0.09 x10-4 Ω cm K-2

(R2 = 0.9938)

ρ01

= 3.05 Ω cm

ρ1

= 1.22 x10-4 Ω cm K-2

(R2 = 0.9944)

Advanced Materials Research Vol. 501 89

Table 1. Tp, Ea, Ep, θD, νph = kBθD/h, T0 and N(EF) values obtained from the best fitting of the resistivity data with the NNH, SPH and VRH model for La0.67Ba0.33Mn1-yTiyO3 with different y.

y

Tp

(K) Ea

(meV) Ep

(meV) θD

(K) νph = kBθD/h

(Hz) T0

(K) N(EF)

(eV -1cm-3) 0.05 167 - 58 408 8.50 ×1012 3.17 ×105 6.41 × 1021 0.10 43 - 118 264 5.50 ×1012 3.68 ×107 5.52 × 1019 0.15 - - 128 360 7.49 ×1012 1.50 ×108 1.36 × 1019 0.20 - - 173 368 7.67 ×1012 4.11 ×108 4.94 ×1018 0.40 - - 210 342 7.13 ×1012 1.37 ×08 1.48 ×1020 0.60 - 190 - - - - -

Conclusion

La0.67Ba0.33Mn1-yTiyO3 samples prepared in this work are single phase with Pm3m cubic structure except for y = 0.60. Lattice parameter increases with Ti content and then decreases at y = 0.60. Samples with y = 0.05 and 0.10 exhibited both metallic and semiconductor-like behaviour. The samples exhibit semiconducting behaviour at higher Ti concentration. At T < Tp the resistivity curves for y = 0.00 and 0.05 follows the ρ = ρo1 + ρ1T

2 relation. The resistivity curves can be fitted with the nearest neighbour hopping (NNH), variable range hopping (VRH) and/or the small polaron hopping (SPH) models at T > Tp.

References

[1] C. Zener, Interaction between d-shells in the transition metals. ii ferromagnetic compounds of manganese with perovskite structure. Phys. Rev. 82 (1951) 403-405.

[2] A.J. Millis, P.B. Littlewood, and B.I. Shraiman, Double exchange alone does not explain the resistivity of La1-xSrxMnO3. Phys. Rev. Lett. 74 (1995) 5144-5147.

[3] R. Maezono, S. Ishihara and N. Nagaosa, Orbital polarization in manganese oxides, Phys. Rev. B 57 (1998) R13993- R13996.

[4] Z. Zalita, S. A. Halim, K.P. Lim, Z.A. Talib, Z. Hishamuddin & C.P. Walter, Magnetic, Electrical Transport and Impedance Spectroscopy Studies on Ti Substituted La0.67Sr0.33MnO3 Ceramics. Sains Malaysiana. 38(5) (2009) 673–678 .

[5] S.V. Trukhanov, Magneticand magnetotransport properties of La1-xBaxMnO3-x/2 perovskite manganites, J. Mater. Chem. 13 (2003) 347-352.

[6] N. Kallel, K. Frohlich, S. Pignard, M. Oumezzine and H. Vincent, Structure, magnetic and magnetoresistive properties of La0.7Sr0.3Mn1-xSnxO3 samples (0 ≤ x ≤ 0.20). J Alloy Compd. 399 (2005) 20-26.

[7] H. Taguchi, M. Sonoda, M. Nagao and H. Kido, Role of tetravalent ion in metal-insulator transition in (La0.1Ca0.9)(Mn1-xTix)O3. J. Solid State Chem. 126 (1996) 235-241.

[8] G. Venkataiah and P.V. Reddy, Electrical behavior of sol-gel prepared Nd0.67Sr0.33MnO3 manganite system J. Magn. Magn. Mater. 285 (2005) 343-352.

[9] N.F. Mott and E.A. Davis, Electronics Processes in Noncrystalline Materials, Oxford, Clarendon, 1971.

90 Solid State Science and Technology XXVI

Solid State Science and Technology XXVI 10.4028/www.scientific.net/AMR.501 Structural and Electrical Transport Properties of La0.67Ba0.33Mn1-yTiyO3 Ceramics 10.4028/www.scientific.net/AMR.501.86

DOI References

[2] A.J. Millis, P.B. Littlewood, and B.I. Shraiman, Double exchange alone does not explain the resistivity of

La1-xSrxMnO3. Phys. Rev. Lett. 74 (1995) 5144-5147.

http://dx.doi.org/10.1103/PhysRevLett.74.5144 [5] S.V. Trukhanov, Magneticand magnetotransport properties of La1-xBaxMnO3-x/2 perovskite manganites,

J. Mater. Chem. 13 (2003) 347-352.

http://dx.doi.org/10.1039/b208664f [8] G. Venkataiah and P.V. Reddy, Electrical behavior of sol-gel prepared Nd0. 67Sr0. 33MnO3 manganite

system J. Magn. Magn. Mater. 285 (2005) 343-352.

http://dx.doi.org/10.1016/j.jmmm.2004.07.051