Journal of Alloys and Compounds Influence of ...

Journal of Alloys and Compounds 470 (2009) 67–74

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom

Influence of sintering temperature and oxygen stoichiometry on electrical transport properties of La0.67 Na0.33 MnO3 manganite Y. Kalyana Lakshmi, P. Venugopal Reddy ∗ Department of Physics, Osmania University, Hyderabad 500007, India

a r t i c l e

i n f o

Article history: Received 26 January 2008 Received in revised form 8 March 2008 Accepted 10 March 2008 Available online 23 May 2008 PACS: 61.05.cp 73.63.Bd 74.25.Ha 74.62.−c 75.47.Lx Keywords: X-ray diffraction Nanocrystalline materials Magnetic properties Transition temperature variation Manganites

a b s t r a c t With a view to investigate the influence of sintering temperature and oxygen stoichiometry on electrical and magnetic properties of sodium-doped lanthanum manganite sintered at different temperatures, a series of samples were prepared by the sol–gel route. The samples were characterized by the XRD studies and the data were analyzed using the Rietveld refinement technique and it has been observed that the ¯ space group. The electrical resistivity and thermaterials are having rhombohedral structure with R3c moelectric power studies were investigated both as a function of crystallite size and oxygen content. To understand the conduction mechanism, the electrical resistivity data have been analyzed and it has been concluded that the variation of electrical resistivity in the ferromagnetic region can be explained by electron–electron scattering process (∼T2 ) and two magnon scattering processes, while that in the paramagnetic region is explained by the small polaron hopping mechanism. Similarly, the variation of thermopower in the ferromagnetic region is explained on the basis of electron–magnon scattering. © 2008 Elsevier B.V. All rights reserved.

1. Introduction In recent years, there has been considerable interest in aliovalent-doped LaMnO3 pervoskites (La1−x Ax MnO3 , where A = Ca, Sr, Na, K and Rb) due to their negative colossal magnetoreistance (MR) and close correlation between structural, charge and orbital degrees of freedom [1–6]. The basic properties of mixed-valence manganites depend mainly on the relative amount of Mn3+ and Mn4+ ions [7–11]. The reason for aliovalent doping is that it promotes oxidation of Mn-array, which causes the material to become metallic and ferromagnetic at a defined temperature (the Curie temperature, TC ), which depends on the average Mn oxidation state. It is found that Na-doped manganites exhibit the highest TC making them promising candidates for many important applications. More recently, it has also been found that the point defects, which depend on the oxygen content, play a fundamental role in determining the physical properties [7,8,10–16]. The effect of grain size on structural, magnetic and transport properties in bivalent-doped manganites

∗ Corresponding author. Tel.: +91 40 27682287; fax: +91 40 27090020. E-mail address: [email protected] (P. Venugopal Reddy). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.03.067

has been extensively studied [17–21] and most of the studies indeed suggest that all these properties are markedly affected by the grain size. The data on aliovalent-doped manganites are scanty. Therefore, an attempt has been made to study the influence of varying oxygen stoichiometry and crystallite size on electrical, magnetic and thermoelectric power behavior of La0.67 Na0.33 MnO3 manganite and the results of such an investigation are presented here. 2. Experimental details The polycrystalline materials with the compositional formula, La0.67 Na0.33 MnO3 , were prepared by sol–gel process. High purity starting materials of La2 O3 (99.99%), NaNO3 (99.9%), and MnCO3 (99.9%) were taken in stoichiometric ratio. The pH of solution mixture was maintained between 6.5 and 7. After getting a sol on slow evaporation, a gelating reagent—ethylene glycol was added and heated between 160 and 180 ◦ C to get a gel. This on further heating yields a dry fluffy porous mass and was calcined at 700 ◦ C for 4 h. Finally, the powders were pressed into circular pellets and sintered at four different temperatures in air for 4 h to get the samples of varying crystallite size. Structural characterization was done by powder X-ray diffraction (XRD) using Philips (X pert) diffractometer at room temperature. The valance state of Mn ion and oxygen stoichiometry were determined by redox titration technique while AC susceptibility () measurements were carried out over a temperature range 77–300 K using mutual inductance principle. The electrical resistivity measurements were also carried out using four-point probe method by collecting the data in heating mode over a temperature range 77–300 K. Finally, a dynamic two probe differen-

68

Y. Kalyana Lakshmi, P. Venugopal Reddy / Journal of Alloys and Compounds 470 (2009) 67–74

tial method was employed for the measurement of thermopower [22]. The samples were coated with silver paint between two copper electrodes with adjustable temperature gradient and were monitored using copper-constantan thermocouples. The assembly was placed in closed liquid nitrogen to maintain uniform temperature and to avoid condensation of moisture. The measurements were carried out in the heating mode. The measured thermopower data were corrected by subtracting thermopower values of copper, so as to obtain absolute thermopower values of samples.

3. Results and discussion 3.1. Structural The X-ray powder diffraction (XRD) patterns of all the samples (Fig. 1a) reveal that they are having single phase. The diffraction peaks were indexed to a rhombohedral pervoskite structure hav¯ space group. The crystal structure was further refined by ing R3c a standard Rietveld technique [23] and Fig. 1b shows the observed and calculated XRD patterns of LNMO sample sintered at 800 ◦ C. The unit cell parameters, Mn–O–Mn bond angle and Mn–O bond length were obtained using the Rietveld refinement technique and are presented in Table 1. It can be seen that both the lattice parameters as well as the Mn–O bond length are decreasing with increasing sintering temperature while Mn–O–Mn bond angle is constant. The average crystallite size (S) values were calculated using the XRD data and while doing so, instrumental errors and process-

ing conditions were taken into consideration. The broadening of reflections due to micro-strains was considered to have an angular dependence of the form: ˇStrain = ε tan

(1)

where ˇStrain is the peak shift due to strain ε (=d/d) is the coefficient related to strain and is Bragg angle. The dependence of size effect can be given by the Scherer formula: ˇSize =

K t cos

(2)

where K is the grain shape factor (for a spherical grain K = 0.89), ˚ t is thickness of the is wavelength of Cu K␣ radiation ( = 1.5406 A), crystal. The instrumental broadening effect has been eliminated, by subtracting the value of full width at half-maxima (ˇo ) corresponding to a standard sample (SiO2 ) from ˇSize at respective Bragg peaks. The complete expression for the full width at half maximum is a linear combination of strain and size broadenings and is given by ˇSize,Strain = ε tan + K/t cos

(3)

From this equation, the average crystallite sizes have been calculated and are given in Table 1. It is clear from the table that the crystallite size values are found to increase with increase in sintering temperature and surprisingly the crystallite size of the sample sintered at 1100 ◦ C is abnormally high compared to the other samples of the series. In order to evaluate the oxygen content of the samples, iodometric titrations [24] were carried out and the results are included in Table 1. Interestingly, the oxygen content is found to decrease with increasing sintering temperature, excepting in the case of the sample sintered at 1100 ◦ C. 3.2. Magnetic and electrical characterization

Fig. 1. (a) X-ray diffraction patters of La0.67 Na0.33 MnO3 manganite sintered at different temperatures and (b) a typical plot of XRD pattern of LN-8 sample along with Rietveld refined pattern. The observed data are indicated by a circles, calculated profile by solid line and the below pattern shows the difference between the observed and calculated patterns.

AC susceptibility measurements of all the samples have been carried out over a temperature range 77–300 K (Fig. 2) and based on these results, the ferro to paramagnetic transition temperatures, TC, were obtained and are included in Table 1. It can be seen from the table that TC values are found to increase for the first two samples while they remain constant for the other two samples and, the observed behavior is similar to that reported by Yang et al. [25] (TC values decrease and remain same with increasing grain size). According to these authors, there may exist a critical value of the grain size below which Curie temperature may be affected strongly by the grain size. In the present investigation, as the TC values for two samples are remaining constant it has been concluded that the critical value of the grain size might be about 80 nm. The variation of reistivity with temperature for the samples sintered at different temperatures has been investigated over a temperature range 77–300 K and the behavior is shown in Fig. 3. All the samples are found to exhibit metallic behavior below the metal insulator (M-I) transition temperature (TP ) and the values are given in Table 1. It can also be seen from the table that the peak resistivity (max ) is found to increase while TP values are decreasing with increasing crystallite size excepting in the case of LNMO-11. This behavior is in contrast with the earlier report that TP values increase with increasing particle size [17–19] implying that the decrease of grain size relatively increases the insulating region due to the enhancement of the grain boundary effects. In the case of LNMO-11 sample with the largest grain size (211 nm), resistivity is lower than those of other samples with smaller grain size. It clearly indicates that the sample with largest grain size, the inter connectivity between the grain is better than those of smaller grain size

2.87 2.85 2.80 2.84 39 36 26 31 85.4 740 949 10.6 291 322 325 325 245 220 188 247 46 54 80 211 1.9468 1.9449 1.9415 1.9387 173.5 173.5 173.5 173.5 13.358 13.353 13.337 13.329

S (nm) ´˚ Mn–O bond length (A) Mn–O–Mn bond angle (◦ ) ˚ c (A)

69

samples. Thus the relative contribution of the insulating region connecting the grains is smaller in the sample with a bigger grain size [26]. Apart from this, the observed variation of TP with crystallite size in smaller grain size samples may also be interpreted on the basis of oxygen deficiency. The oxygen deficiency, i.e., the presence of oxygen vacancies, deeply affects the transport properties resulting in both Mn4+ reduction and point defects creation with in the structure thereby shifting TP towards low temperature side. The variation of TP and oxygen content (ı) with crystallite size (S) is shown in Fig. 4. This behavior is in close agreement with the results reported in the case of LCMO samples [27] where TC remained constant while TP decreased with ı. This suggests that the combined effect of crystallite size and oxygen deficiency might be responsible for the observed resistivity behavior [7,8,27]. It is interesting to note from Table 1 that TP and TC are found to have different behaviors with decreasing grain size. In fact, a similar behavior was reported earlier [17,19]. The observed difference between the two transition temperatures may be explained as outlined here. It is well known that two contributions are responsible for the transport properties among CMR materials. One of them is intrinsic and might have originated from the double exchange (DE) interaction between the neighboring Mn ions, while the other one is extrinsic and is due to spin-polarized tunneling between ferromagnetic grains through an insulating grain boundary (GB) barrier. According to DE model, the metal-insulator transition always occurs in the vicinity of TC . However, in the case of granular samples with large number of GBs, the influence of interfaces and the boundaries should be taken into account. Further, as the GB is similar to the amorphous state, the magnetic configuration on the grain surface is more disordered than in the core. In such a situation, the occurrence of anti-ferromagnetic insulating regions on the grain boundary may not modify the magnetic transition temperature, TC. However, the phenomenon may influence the metal-insulator transition TP thereby shifting it to low temperature side [28]

800 900 1000 1100

5.5193 5.5120 5.5012 5.4910

3.3. Conduction mechanism

LNMO-8 LNMO-9 LNMO-10 LNMO-11

˚ a = b (A) Sintering temperature (◦ C) Sample code

Table 1 The structural and experimental data of LNMO samples

TP (K)

TC (K)

Peak ( cm)

% Mn4+

Oxygen content (ı)


With a view to understand the conduction mechanism of Nadoped manganite sintered at different temperatures, the electrical resistivity data in ferromagnetic metallic (T < TP ) region were fitted using the empirical relation: (T ) = 0 + 2 T 2 + 4.5 T 4.5

(4)

where the term 0 represents resistivity due to grain boundary and point-defects [29,30], while the term 2 T2 arises due to electron–electron scattering [31] and the term 4.5 T4.5 term arises due to electron–magnon scattering process in the ferromagnetic phase [18,19]. The best-fit parameters are given in Table 2 and the corresponding plots are shown in Fig. 3. It is clearly evident from the best-fit parameters (Table 2) that the electrical resistivity in these materials might be due to grain/domain boundary, electron–electron and two magnon scattering processes. The values of derived parameters (0 , ␳2 T2 and Table 2 The best-fit parameters of LNMO samples sintered at different temperatures Sample code

0 ( cm)

2 ( cm K−2 )

4.5 ( cm K−4.5 )


35.9576 429.0600 669.8025 5.8653

0.00125 0.01111 0.01454 0.00140

4.22 × 10−10 6.36 × 10−9 1.34 × 10−8 5.94 × 10−11

70


Fig. 2. Temperature dependence of AC susceptibility of La0.67 Na0.33 MnO3 manganite sintered at different temperatures.

4.5 T4.5 ) are not varying systematically with varying crystallite size. The conduction mechanism of manganites at high temperatures (T > TP ) in general is explained by small polaron hopping mechanism:

ing the effective mass of the charge carrier. Due to this the effective band gap increases with increasing oxygen deficiency. Therefore, higher values of activation energies needed for the charge carries to overcome this band gap are justified [7,31].

= ␣ T exp

3.4. Thermoelectric power

E P

(5)

kB T

where ␣ is residual resistivity, while EP is the activation energy, ˛ = 2kB /3ne2 a2 v, here kB is Boltzmann’s constant, e is the electronic charge, n is the number of charge carriers, a is site-to-site hopping distance and v is the longitudinal optical phonon frequency. The activation energy values calculated from the best-fit parameters are given in Table 3 and the corresponding plots are shown in Fig. 5. Further, it is also clear from Table 3 that the values of EP are found to increase with increasing oxygen deficiency. The oxygen deficiency might be responsible for bending of Mn–O–Mn bond angle which inturn might narrow down the bandwidth and enhanc-

In the present investigation thermoelectric power (TEP) measurements were carried out over a temperature range 77–300 K and the variation of S with temperature for all the samples is shown in Fig. 6. It is clear from the figure that S is positive for all the samples and through out the temperature range of investigation. It is interesting to note that all the samples are exhibiting two peaks. Out of these two peaks, the first one is occurring in the temperature region 106–118 K and the occurrence of the peak may be explained on the basis of the spin-wave theory. According to this theory, in ferromagnets and anti-ferromagnets, electrons are scattered by spin waves, giving rise to magnons. In a manner similar to the scattering

Table 3 The fitting parameters of thermoelectric power data in La0.67 Na0.33 MnO3 manganites Sample code

(T0S )1/4

EP (meV)

ES (meV)

WH = EP − ES (meV)

S0 (␮V/K)

S3/2 (␮V/K5/2 )

S4 (␮V/K5 )


787.26 646.54 823.11 87.37

81.1 84.4 115.9 66.2

14.10 12.30 15.54 1.60

67.00 72.10 100.36 64.6

−1.52877 −4.37887 −2.61512 −3.62827

4.44 × 10−3 10.93 × 10−3 6.62 × 10−3 7.01 × 10−3

−1.0593 × 10−8 −3.1235 × 10−8 −1.5223 × 10−8 −2.0839 × 10−8


71

Fig. 3. Electrical resistivity vs. temperature of Na-doped manganite at different sintering temperatures. The solid line represents the best fit to the equation ((T) = 0 + 2 T2 + 4.5 T4.5 .

of phonons resulting in phonon drag effects, the electron magnon interaction produces a magnon drag effect. As the magnon drag effect is approximately proportional to the magnon specific heat, one may expect the variation of S with temperature as T3/2 for ferromagnetic materials [22,32]. In view of these arguments, one may

conclude that the observed peak may be attributed to the magnon drag effect. In contrast to the first peak, the second one is broad and since it is occurring in the vicinity of Tp , it has been assumed that it might have arisen due to metal-insulation transition. On further increase of temperature, the Seebeck coefficient values of all the samples are found to decrease continuously reaching a minimum value. The Seebeck coefficient data in the FM region has been analyzed using the equation: S = S0 + S3/2 T 3/2 + S4 T 4

Fig. 4. Variation of TP and oxygen content (ı) with crystallite size (S).

(6)

where S0 is a constant and accounts the low temperature variation of thermopower. Further, S3/2 is attributed to the electron magnon scattering process, while the origin of S4 is still not clear. However, it has been speculated that S4 arises due to the spin-wave fluctuations in the FM phase [33]. The best fit is shown in Fig. 7 and parameters are tabulated in Table 3. It can be seen from the table that as S3/2 is nearly five orders larger than S4 , one may conclude that it dominates the transport mechanism in the FM metallic region, implying that the transport mechanism might have originated from electron–magnon scattering [33]. Further, the high temperature TEP data of the samples of the present investigation were fitted to SPH model given by the equa-

72


Fig. 5. Variation of ln (/T) vs. inverse temperature (T−1 ) of Na-doped sample at different sintering temperatures. The solid line represents the best fit to the equation = ˛ T exp (EP /kB T).

Fig. 6. Variation of S with T for La0.67 Na0.33 MnO3 sample sintered at different temperatures.

Fig. 7. Variation of thermopower S (␮V K−1 ) with temperature T (K) of Na-doped sample sintered at different temperatures. The solid line gives the best fit to the equation S = S0 + S3/2 T3/2 + S4 T4 .


73

R2 values of both the models that SPH model fits well than VRH model in describing the thermopower data in the high temperature region (TP ). Further, the polaron hopping energy, defined by the equation WH = EP − ES , has been calculated for all the samples and are given in Table 3. Here, EP is the activation energy obtained from the resistivity data. The difference in the activation energies, WH , reflects the charge transport due to hopping of carriers rather than semi-conducting like activated conduction. Further, it is also clear from the table that the calculated values of ˛ are less than 1 and the results strongly supports the validity of using the small polaron hopping mechanism to explain the electrical resistivity as well as thermopower data of samples of the present investigation in the high temperature region [35]. 4. Conclusion In conclusion, the effect of sintering temperature on oxygen stoichiometry, electrical, magnetic and thermoelectric properties of sodium-doped manganite is discussed. The magnetic transition is found to increase with increasing crystallite size whereas, the metal insulator transitions are found to shift towards low temperature side. Finally, it has been concluded that the combined effect of crystallite size as well as oxygen deficiency might be responsible for the observed resistivity behavior. References [1] [2] [3] [4] [5] [6] [7] [8] [9]

Fig. 8. (a) The typical plot of S vs. 1000/T for LNMO-8 sample in the high temperature region and (b) S vs. T−1/4 in the high-temperature region. The solid lines corresponds to the fitting of SPH model and VRH model, respectively. The parameter 2 denotes the quality of the fit.

tion: S=

kB e

[12] [13] [14] [15]

ES KB T + ˛

(7)

T 1/4 0S

,

[16] [17]

where ES is the activation energy obtained from TEP data while ˛ is a sample dependent constant, associated with the entropy transport of charge carriers. In addition, ˛ < 1 suggests the conduction is due to small polaron hopping and ˛ > 2 represents the existence of a large polaron [34]. The high-temperature thermopower data were also fitted to three-dimensional VRH model given by [34]: S∝

[10] [11]

[18] [19] [20] [21] [22] [23] [24]

(8)

[25]

where T0S is the characteristic temperature. The theoretical equations of both the VRH and SPH models were fitted to the experimental data and a typical plot of LNMO-8 is shown in Fig. 8. The fitting parameters ES , ˛ and T0S are listed in Table 3. To judge the quality of both the models, the regression coefficients, R2 have also been calculated and are indicated in the figures. It is clear from

[26]

T

[27] [28] [29]

C. Zener, Phys. Rev. 82 (1951) 403. K.H. Ahn, T. Lookman, A.R. Bishop, Nature 428 (2004) 401. X. Zhu, Y. Sun1, J. Dai, W. Song, J. Phys. D: Appl. Phys. 39 (2006) 2654. S.L. Ye, W.H. Song, J.M. Dai, K.Y. Wang, S.G. Wang, J.J. Du, J. Appl. Phys. 90 (2001) 2943. N. Abdelmoula, A. Cheikh-Rouhou1, L. Reversat, J. Phys. Condens. Matter 13 (2001) 449. S. Bhattacharya, A. Banerjee, S. Pal1, P. Chatterjee, R.K. Mukherjee1, B.K. Chaudhuri, J. Phys.: Condens. Matter 14 (2002) 10221. L. Malavasi, M.C. Mozzati, C.B. Azzoni, G. Chiodelli, G. Flor, Solid State Commun. 123 (2002) 321. L. Malavasi, M.C. Mozzati, P. Ghigna, C.B. Azzoni, G. Flor, J. Phys. Chem. B 107 (2003) 2500. ˜ Z. El-Fadii, E. Martinez, A. J. Vergara, R.J. Ortega-Hertogs, V. Madruga, F. Sapina, Beltra, K.V. Rao, Phys. Rev. B 60 (1999) 1127. J. Topfer, J.B. Goodenough, Chem. Mater. 9 (1997) 1467. E.T. Maguire, A.M. Coats, J.M.S. Skakle, A.R. West, J. Mater. Chem. 9 (1999) 1337. G. Dezanneau, A. Sin, H. Roussel, M. Audier, H. Vincent, J. Solid State Chem. 173 (2003) 216. K. Nakamura, J. Solid State Chem. 173 (2003) 299. B. Raveau, A. Maignan, C. Martin, M. Hervieu, Chem. Mater. 10 (1998) 2641. E. Herrero, J. Alonso, J.L. Martinez, M. Vallet-Regt, J.M. Gonzales-Calbet, Chem. Mater. 12 (2000) 1060. A. Arulraj, R. Mahesh, G.N. Subbanna, R. Mahendiran, A.K. Raychaudhuri, C.N.R. Rao, J. Solid State Chem. 127 (1996) 87. G. Venkataiah, D.C. Krishna, M. Vithal, S.S. Rao, S.V. Bhat, V. Prasad, S.V. Subramanyam, P. Venugopal Reddy, Physica B 357 (2005) 370–379. G. Venkataiah, P. Venugopal Reddy, J. Magn. Magn. Mater. 285 (2005) 343. G. Venkataiah, Y. Kalyana Lakshmi, V. Prasad, P. Venugopal Reddy, J. Nanosci. Nanotechnol. 7 (2007) 2000. L.W. Lei, Z.Y. Fu, J.Y. Zhang, Mater. Lett. 60 (2006) 970. A. Gaur, G.D. Varma, J. Phys.: Condens. Matter 18 (2006) 8837. G. Venkataiah, Y. Kalyana Lakshmi, P. Venugopal Reddy, J. Phys. D 40 (2007) 721. R.A. Young, The Rietveld Method, Oxford University Press, New York, 1993. A.I. Vogel, A text book of Quantitative Inorganic Analysis including Elementary Instrumental Analysis, 4th edn., Longman, London, 1978. J. Yang, B.C. Shao, R.L. Zhang, Y.Q. Ma, Z.G. Sheng, W.H. Song, Y.P. Sun, Solid State Commun. 132 (2004) 83. A. Banerjee, S. Pal, S. Bhattacharya, B.K. Chaudhuri, H.D. Yang, J. Appl. Phys. 91 (2002) 5125. L.E. Hueso, F. Rivadulla, R.D. Sanchez, D. Caeiro, C. Jardon, C. Vazquez-Vazquez, J. Rivas, M.A. Lopez-Quintela, J. Magn. Magn. Mater. 189 (1998) 321. Y.-H. Huang, C.-H. Yan, Z.-M. Wang, C.-S. Liao, G.-X. Xu, Solid State Commun. 118 (2001) 541. D.C. Worledge, G.J. Snyder, M.R. Beasley, T.H. Geballe, J. Appl. Phys. 80 (1996) 5158.

74


[30] M. Viret, L. Ranno, J.M.D. Coey, Phys. Rev. B 55 (1997) 8067. [31] S. Bhattacharya, R.K. Mukherjee, B.K. Chaudhuri, Appl. Phys. Lett. 82 (2003) 4101. [32] S. Das, A. Poddar, B. Roy, S. Giri, J. Alloys Compd. 365 (2004) 94.

[33] R. Ang, Y.P. Sun, J. Yang, X.B. Zhu, W.H. Song, J. Appl. Phys. 100 (2006) 073706. [34] R. Ang, W.J. Lu, R.L. Zhang, B.C. Zhao, X.B. Zhu, W.H. Song, Y.P. Sun, Phys. Rev. B 72 (2005) 184417. [35] J. Yang, Y.P. Sun, W.H. Song, Y.P. Lee, J. Appl. Phys. 100 (2006) 123701.