Effect of Ba substitution on structural, electrical and thermal properties ...

Journal of Alloys and Compounds 619 (2015) 303–310

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Effect of Ba substitution on structural, electrical and thermal properties of La0.65Ca0.35xBaxMnO3 (0 6 x 6 0.25) manganites S.O. Manjunatha a, Ashok Rao a,⇑, T.-Y. Lin b, C.-M. Chang b, Y.-K. Kuo b,1 a b

Department of Physics, Manipal Institute of Technology, Manipal University, Manipal 576104, India Department of Physics, National Dong-Hwa University, Hualien 974, Taiwan

a r t i c l e

i n f o

Article history: Received 31 July 2014 Received in revised form 4 September 2014 Accepted 4 September 2014 Available online 16 September 2014 Keywords: Manganites Rietveld refinement Electrical resistivity Percolation

a b s t r a c t In the present communication, the effect of Ba substitution on the structural, mechanical, electrical transport and thermal properties of La0.65Ca0.35xBaxMnO3 (0.00 6 x 6 0.25) manganites have been investigated. Rietveld refinement of XRD data reveals that all samples are single phase. With increasing Ba content, a transformation of crystal structure from rhombohedral (x = 0.00–0.15) to cubic symmetry (x = 0.25) is observed. In the micro-hardness study, a correlation between the average grain size and micro-hardness is concluded. It is found that all samples exhibit a clear metal–insulator transition, and the transition temperature (TMI) shifts monotonically towards higher temperature. The Seebeck coefficient (S) of the pristine sample is negative in the entire temperature range, whereas for all doped samples the measured S exhibits a crossover from positive to negative values. The analysis of Seebeck coefficient data indicates that the small polaron hopping model is operative in the high temperature region. The room-temperature magnitude of thermal conductivity, j of the studied samples is in the range of 20–35 mW/cm K and such low value is presumably attributed to the Jahn Teller (J-T) effect. An analysis of j using Wiedemann–Franz’s (W–F) law confirms that the total thermal conductivity is mainly associated with the lattice phonons rather than charge carriers. The change in entropy associated with the transition evaluated from the specific heat measurements is found to decrease with increasing x, most likely due to the enhancement of magnetic inhomogeneity in these manganites. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Ever since the discovery of colossal magneto-resistance (CMR) in manganites with general formula RE1xAxMnO3 (RE = La, Pr, Nd and A = Ca, Sr, Ba), enormous work has been done and remarkable magnetic, and magneto-transport properties [1–5] have been reported in these manganites. Recent studies demonstrate that the mixed valence perovskite manganites exhibit a large magneto-caloric effect (MCE) which is seen around the magnetic ordering temperature which has an obvious application in magnetic refrigeration [6–10]. Giri et al. [9] have reported that MCE and relative cooling power (RCP) can be tuned by varying the particle size. Rostamnejadi et al. [10] have shown that La0.67Sr0.33MnO3 manganites are ideal candidates for magnetic refrigeration above room temperature at moderate magnetic fields. In particular, La1xCax MnO3 compounds are important as they have the largest CMR ⇑ Corresponding author. Fax: +91 820 2571071. E-mail addresses: [email protected] (A. Rao), [email protected]. tw (Y.-K. Kuo). 1 Co-corresponding author. http://dx.doi.org/10.1016/j.jallcom.2014.09.042 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.

effect, attributed to the presence of an electronic phase separation between hole-poor anti-ferromagnetic regions and hole-rich ferromagnetic regions [11–14]. Skini et al. [13] has shown that lanthanum deficient manganites have MCE and RCP values close to gadolinium. Lee et al. [14] have demonstrated that sintering temperature plays an important role on MCE and RCP. For these compounds, the two characteristic transition temperatures, metal–insulator transition temperature (TMI) and Curie temperature (TC) are nearly same. In addition, it is found that TMI attains a maximum value with x  0.35 in the La1xCaxMnO3 system [15]. These compounds have a disadvantage that their Curie temperatures, TC (270 K) are well below room temperature. However, partial substitution of Ca by larger sized ions like Sr, Ba and Pb could lead to an increase in TC and an enhancement in MCEs is seen in these compounds [15–20]. Effect of A-site cationic size mismatch on the magneto-resistive properties of manganites has been studied [2,21] and it is revealed that TC is strongly suppressed when the difference in ionic radius between two cations at A sites is large. In fact Reddy et al. [2] have shown that magneto-resistance (MR%) increases with decreasing ionic radii of the dopant at A site and have concluded that these

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materials may be possible candidates for sensor based applications. Recent reports [2,22–24] suggest that A-site doping of ions with a larger difference in ionic radius increases the mismatch factor r2 and strongly influences the transition temperatures TC and TMI. There are several reports on the grain size effect on the structural, electrical and transport properties of these manganites [25–27]. In particular, the physical properties of barium doped manganites, e.g. La0.65Ca0.35xBaxMnO3 are of interest because the difference between ionic size of Ca and Ba is very large and it is observed that TC increases with increasing Ba content [16,17,20]. To the best of our knowledge, there are no reports on MR and thermal properties of Ba doped La0.65Ca0.35xBaxMnO3 compounds. In view of this, a systematic investigation of barium doping at calcium site of La0.65Ca0.35xBaxMnO3 (0.00 6 x 6 0.25) system is undertaken, in order to clarify the effect Ba doping on the structural, electrical transport and thermal properties of this system. It is worth mentioning here that electrical resistivity, thermal conductivity and Seebeck coefficient data enables us to understand the different scattering mechanisms (viz. electron–electron, electron– phonon, electron–magnon and magnon–phonon) which contribute to the transport properties of these materials. 2. Experimental The conventional solid state reaction route was used to prepare a series of bulk samples of nominal composition La0.65Ca0.35xBaxMnO3 (0.00 6 x 6 0.25). Stochiometric proportion of high purity (99.9% pure, Sigma–Aldrich) La2O3, Ca2CO3, Ba2CO3 and MnO2 precursors were weighed and then mixed homogeneously in an agate mortar for about 6 h. The mixture was then calcined at 1000–1250 °C for 24 h with three intermediate grindings to exhaust the carbonates present and to ensure the compositional homogeneity of the samples. The powder was finally pressurized (6 tons) into rectangular bar shaped pellets and then sintered at 1300 °C for 30 h to acquire close packed grains. The samples were characterized using X-ray diffraction (XRD) (Rigaku Miniflex diffractometer with monochromatic Cu Ka radiation) to confirm the crystallinity, purity and single phase formation of the samples of present investigation. The XRD patterns were further analyzed by employing Rietveld refinement technique (using Fullprof program), to estimate the lattice parameters, space groups, type of crystal system, Brag reflections and other related statistics of the samples. The surface morphology of the samples was imaged using scanning electron microscopy (SEM) attached with energy dispersive X-ray (EDAX) analyzer (EVO MA18 with Oxford EDS X-act). EDAX measurements were done to establish the elemental composition of dopants in the prepared samples. Archimedes principle was employed to estimate the densities of the samples in the form of pellet using Xylene as the immersion fluid. The accuracy of mass measurement of the samples was ±0.1 mg. Measured density of the samples is in good agreement (93%) with the theoretical density. The theoretical density of the samples is expected to increase with Ba concentration. However, the measured density of samples for x > 0.15 is found to decrease slightly with x which may be due to the increased porosity and agglomeration in the heavily doped samples. Microhardness of the samples was measured using Vickers indentation technique using Clemex (MMT X7) micro hardness tester. In the present work, 100 gf of indentation load was applied with a residence time of 10 s and rapid load removal. The electrical resistivity measurements were carried out over a temperature range of 5–300 K, with zero filed and applied magnetic field of 8 T using a superconducting magnetic system (Oxford Spectromag). Seebeck coefficient and thermal conductivity measurements were carried out in a closed cycle refrigerator using a heat-pulse technique. Specific heat measurements were performed in temperature range 70–350 K using a high resolution ac calorimeter. The details of the thermal measurements techniques have been described elsewhere [28].

3. Results and discussion 3.1. Structural properties Fig. 1 shows the experimental XRD plots of all samples and room-temperature Rietveld refined patterns for typical sample. The parameters pertaining to the refinement are summarized in Table 1. Sharp peaks are clearly seen in all XRD patterns, indicating the studied samples to be highly crystalline. No impurity peaks can be detected within the experimental limits. With increasing Ba content (x), a structural phase transformation is observed. It is seen

Fig. 1. (a) XRD patterns of La0.65Ca0.35xBaxMnO3 (0.00 6 x 6 0.25) samples. (b) Typical Rietveld refinement plots for x = 0.00 and x = 0.05 samples.

that, the crystal structure remains rhombohedral for x 6 0.15 and thereafter a structural transformation to cubic structure is observed. This crystal structure transformation is also confirmed by the tolerance factor t g ¼ pffiffiðhrA iþrO Þ , where hrAi, hrMni, and rO 2ðhr Mn iþr O Þ

are the ionic radii of A-site, Mn-site and oxygen ion, respectively. It is well-known that an orthorhombic structure is realized for tg < 0.96, rhombohedral for 0.96 < tg < 1, and cubic as tg moves close to 1. In present study, it is found that tg increases from 0.96 to nearly 1 with increasing x, consistent with the experimental observation that the crystal structure of studied compounds transforms from rhomobohedral to cubic symmetry. This is attributed essentially to the replacement of a smaller Ca2+ ion (1.34 Å) by a larger Ba2+ ion (1.61 Å), which tends to conquer the high symmetry structure by decreasing the distortion of MnO6 oxygen octahedron. Thus we can conclude that the size of dopant ion has an apparent influence on the structural symmetry of these manganites under present investigation. The surface morphology of the samples has been investigated using SEM and the typical micrographs are presented in Fig. 2. The SEM micrographs reveal that all samples, except pristine and x = 0.05, show a granular morphology with small sized pores present in the samples. The mean grain size, as estimated from the SEM images, is found to decrease with increasing Ba concentration. EDAX measurement is performed to ensure the purity and composition of the samples. The results (atomic percentage of elements present) indicate that all samples are formed without loss of any ingredients and with no trace of any extra elements within the detection limits.

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S.O. Manjunatha et al. / Journal of Alloys and Compounds 619 (2015) 303–310 Table 1 Rietveld refined crystallographic parameters of La0.65Ca0.35xBaxMnO3 (0 6 x 6 0.25) compounds. Composition x System Space group

0 R-3C

0.05 Rhombohedral R-3C

0.1

0.15

R-3C

R-3C

0.25 Cubic Pm3 m

Lattice parameters a (Å) c (Å) V (Å3)

5.4474 13.3392 342.809

5.4602 13.3688 345.186

5.4748 13.3947 347.700

5.4810 13.4113 348.918

3.8805 – 58.438

Position parameter of oxygen X Tolerance factor (tg) r2 (Å2)

0.43584 0.958 0.00009

0.43605 0.962 0.003203

0.44356 0.967 0.00595

0.44263 0.972 0.00833

0.50000 0.981 0.0134

Goodness of fit, v2 Average grain size (lm) from SEM studies Crystallite size (nm) from XRD studies

1.01 8.96 34.47

0.884 6.196 31.67

0.839 3.69 27.09

0.988 2.10 26.77

1.25 2.21 28.35

Micro-hardness (GPa)

6.354

5.534

1.6994

1.411

1.774

Fig. 2. Typical SEM micrographs of La0.65Ca0.35xBaxMnO3 (0.00 6 x 6 0.25) samples with x = 0.0, 0.1, 0.15, 0.25 recorded in backscattered electron mode.

3.2. Micro-hardness Results of micro-hardness studies, evaluated using Vickers indentation technique, are summarized in Table 1. The hardness of the samples depicts a similar trend as observed in grain size, viz. the micro-hardness evaluated for the present samples is found to decrease with Ba content up to x = 0.15 and thereafter increase with x. The observed correlation between micro-hardness and grain size may be attributed to the fact that the decrease in grain size facilitates the creation of mechanical strains in the system

due to the oxygen deficiencies at the grain boundaries, which in turn reduces the micro-hardness in the samples. Beyond x = 0.15, we observe an increase in micro-hardness which is attributed to the increase in grain size. 3.3. Electrical resistivity and magneto-resistance measurements Fig. 3 represents the temperature dependent electrical resistivity for all samples in the temperature range 5–300 K. For the pristine sample, the metal–insulator transition is observed at

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and e-phonon scattering processes. Fig. 4(a) depicts the low-temperature resistivity behavior where symbols indicate the experimental data and lines indicate the fitted curves. The fitting parameters are tabulated in Table 2. From this analysis, it can be inferred that the electrical transport mechanism in the low temperature metallic region is governed by the predominant e–e scattering process as q2  q4.5. The variation of electrical resistivity with temperature in the high temperature regime (T > TMI) can be analyzed using two models, viz. variable range hopping and small polaron hopping models [27]. Variable range hopping model (VRH) is given by the relation,

qPM ðTÞ ¼ q0 exp



T0 T

14 ð2Þ

where T0 = 16a3/kBN(EF) is a characteristic constant and N(EF) is the density of states at the Fermi level. On the other hand, the adiabatic small polaron hopping model (ASPH) is given by the relation, Fig. 3. Variation of resistivity as a function of temperature for La0.65Ca0.35xBax MnO3 (0.00 6 x 6 0.25) compounds.

the temperature (TMI) of about 266 K. It is important to mention that TMI is a genuine I–M transition which basically arises due to the competition between double exchange and super-exchange mechanisms. In addition to this, we also observe a broad humplike peak at a temperature well below TM1 (243 K). In the present study, we observed this behavior for all samples. The hump-like peak is an extrinsic effect which arises from the grain boundaries. It is noted that such a feature has also been reported in other manganites [26,27]. We will now explain the reason for hump like structure. The hump like structure is a result of large mismatch between the rare-earth site and this produces strain in the lattice [27]. For the fundamental requirement of the fact that the lattice should be in equilibrium, the extra strain is transferred to grain boundaries and thus the lattice acquires minimum energy configuration. This gives rise to the hump like structure. We also see that the hump like structure becomes borderer with Ba-content. This is due to the fact that increasing Ba-content enhances the mismatch and thus induces extra strain to the lattice; thereby a hump-like structure is seen to become broader with increase in Ba-content. It is evident from the inset of Fig. 3 that TMI increases monotonically with increasing x. The shift in TMI can be attributed to the replacement of smaller Ca2+ ions by larger Ba2+ ions. The electrical properties of manganites are mainly governed by the double exchange mechanism (simultaneous hopping of electron between Mn ions through intermediate oxygen ions) [29]. This real charge transfer process is very sensitive to the Mn3+–O–Mn4+ bond state and angle. Doping of Ba2+ ions decreases the distortion of MnO6 octahedron which enhances the overlap integral between Mn and oxygen ions, which results in the increase of conduction band width and in turn the observed shift in TMI. We now consider the behavior of electrical resistivity in three temperature regimes, viz. metallic regime (T < TMI), insulating regime (T > TMI) and competing regime (T  TMI). From Fig. 3, it is evident that for T < TMI the temperature dependent resistivity exhibits metallic behavior for all samples. In order to understand the conduction mechanism in the metallic region (T < TMI), we have fitted the experimental data with the well-known empirical equation [27]

qðTÞ ¼ q0 þ q2 T 2 þ q4:5 T 4:5

qPM ðTÞ ¼ q0 T exp



Ea kB T

 ð3Þ

where Ea is the activation energy for hopping conduction and q0 is a constant. In the present study, we have tried both models and it is found that the resistivity data are better fitted by the ASPH model. Fig. 4(b) shows the plot of ln(q/T) vs. 1/T for all samples under present investigation. The obtained fitting parameters are listed in Table 3.

ð1Þ

where q0 is the residual resistivity arising from grain boundary scattering, q2 is due to electron–electron scattering (e–e), and q4.5 is resistivity component due to mixed effects of e–e, e-magnon

Fig. 4. Resistivity vs. temperature curves analyzed (a) in the low temperature (metallic) regime (b) in the in high temperature (insulating) regime for the La0.65Ca0.35xBaxMnO3 (0.00 6 x 6 0.25) samples. The solid lines represent the best fits according to Eqs. (1) and (3).

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S.O. Manjunatha et al. / Journal of Alloys and Compounds 619 (2015) 303–310 Table 2 Fitting parameters of low temperature resistivity data of La0.65Ca0.35xBaxMnO3 (0 6 x 6 0.25) compounds.

q2 (X mm/K2)

q0 (X mm)

x

0 0.05 0.10 0.15 0.25

q4.5 (X mm/K4.5)

0T

8T

0T

8T

0T

8T

0.58 0.92 1.21 0.330 0.207

0.29 0.37 0.51 0.15 0.09

5.64  105 6.81  104 8.20  104 2.33  104 9.44  105

3.03  105 3.54  104 4.24  104 1.2  104 5.51  105

1.07  1011 1.62  1010 1.06  1010 7.32  1011 5.36  1011

1.61  1012 1.18  1010 1.17  1010 4.03  1011 2.80  1011

Table 3 Activation energy calculated from high temperature resistivity data and the fitting parameters of percolation analysis of La0.65Ca0.35xBaxMnO3 (0 6 x 6 0.25) compounds. Formula

qPM = q0Texp(Ea/kBT) (ASPH model)

LCBMO

Ea (meV)

x = 0.00 x = 0.05 x = 0.10 x = 0.15 x = 0.25

q(T) = qFM f + qPM (1f) (Percolation model) R2

0T

8T

0T

8T

143.7 166 178.9 119.5 70.2

55.4 86 75.1 69.6 59.9

0.997 0.998 0.999 0.998 0.989

0.997 0.999 0.997 0.998 0.998

It is important to note here that none of the models given above can explain the electrical properties near TMI, where both the ferromagnetic metallic (FMM) and paramagnetic insulating (PMI) phases co-exist. Recently, a phenomenological model, namely the percolation model, has been reported to quantitatively analyze the electrical transport properties in the whole range of temperature [27,30–32]. In the percolation model, it is assumed that the FMM and PMI regions are electrically linked in series and there exists a competition between these phases around the metal– insulator transition temperature. To elucidate the role of these co-existing phases, we have analyzed the present electrical data using the percolation model based on the phase segregation mechanism [27,30–32]. According to the model, due to the competition between FM and PM phases, the resistivity can be formulated as,

qðTÞ ¼ qFM ðTÞ þ qPM ðTÞð1  f Þ

ð4Þ

where f is the volume fraction of the FM phases and (1f) is the volume fraction of the PM phase. In the present case, qFM and qPM are obtained from Eqs. (1) and (3), respectively. Volume fraction of FM and PM phases satisfy the Boltzmann distribution,

f ¼

1 1 þ exp



U kB T



(K)

T mod C

R2

28913.04 12971.01 6659.42 7239.13 –

263 267 281 285 –

0.999 0.999 0.999 0.998 –

U0 kB

over a wide temperature range. This observation confirms the validity of the percolation model in the presently studied system. The temperature dependent electrical resistivity with external magnetic field of 8T shows that all the compounds La0.65Ca0.35x BaxMnO3 exhibit a reduction in resistivity with the application of magnetic field of 8 T. The applied magnetic field promotes the delocalization of charge carriers and leads to the local ordering of the electron spins which suppress the electrical resistivity [24]. It is found that with applied magnetic field, the broad peak in q(T) shifts towards higher temperatures. This may be due to the increase in magnetic ordering under applied magnetic field. On the contrary, the anomalous feature at the metal–insulator transition is suppressed with the application of external magnetic field. The temperature dependent magneto-resistance (MR) with an application of 8 T external magnetic field is shown in Fig. 5. It is worth noting that the percentage of MR is decreasing on either side of TMI for the pristine sample which is commonly observed in manganites [24]. However with Ba doping we notice that the percentage of MR is increasing with decreasing temperature and attains a value as large as about 60% at low temperatures. Similar trend has been also reported elsewhere [24]. We now discuss these findings

ð5Þ

  T where U  U 0 1  T mod is the energy difference between FM and C

PM states and T mod denotes the temperature in the vicinity where C resistivity has a maximum value. It can be noticed from this model that the system shows a metallic behavior when f > fC or (1f) < fC and exhibits an insulating behavior when f < fC or (1f) > fC, where is the percolation threshold. The experimental data have been fitted in the temperature range 50–300 K using Eqs. (1), (3) and (4) and the results are summarized in Table 3. During the process of fitting, two parameters T mod and U0 are to be adjusted to achieve the best fit. Other C parameters such as q0, q2, q4.5 and Ea are as obtained from the metallic (T 6 TMI) and insulating (T P TMI) regions. Goodness of fit between the experimental and calculated data is evaluated by calculating the v2 value for each fit. The fitting parameters T mod and C U0 are also tabulated in Table 3. It is seen that the volume fraction f is equal to one (FM phase) well below the Curie temperature (TC) and slowly approaches to zero (PM phase) as the temperature increases. The transition from FM to PM state takes place slowly

Fig. 5. Percentage of magneto-resistance (MR) vs. temperature for La0.65Ca0.35x BaxMnO3 (0.00 6 x 6 0.25) samples at 8 T.

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in terms of various types of magneto-resistance. It is known that two terms contribute to magneto-resistance; one is the intrinsic MR arising from the suppression of spin fluctuations and the other is extrinsic MR due to inter-grain spin polarized tunneling across the grain boundaries [24]. The intrinsic MR in general has the maximum value at the transition temperature while the extrinsic MR increases with decrease in temperature. We can thus conclude that the pristine sample is exhibiting intrinsic MR while the doped samples show extrinsic MR.

suggesting that the small polaron model is applicable in the insulating regime. In addition, the obtained a values are found to be less than 1 for all samples, we thus conclude that the small polarons are responsible for thermoelectric transport in these compounds. The metallic regime is analyzed by employing the empirical equation containing impurity, electron–magnon scattering, and spin wave fluctuation terms which is given by the following equation [35]:

3.4. Seebeck coefficient

where S0 is a temperature-independent constant arising from impurity, S3/2T3/2 corresponds to electron–magnon scattering and S4T4 corresponds to spin wave fluctuations in the FM region. The fitting parameters are tabulated in Table 4 and the fitted curve to Eq. (7) of the pristine sample is presented in the inset (i) of Fig. 6. Further, it is noted that the parameter S3/2 is many orders of magnitude larger than S4, implying the electron–magnon scattering dominates the thermoelectric transport in the metallic regime for these manganites.

The variation of Seebeck coefficient (S) in the temperature range 10–350 K is presented in Fig. 6. It can be seen in Fig. 6 that the value of S for the pristine sample is negative in the entire temperature range, where all Ba doped samples show a crossover from a small positive value of S at low temperatures to a negative value at higher temperatures. This positive to negative crossover is possibly due to the change in the nature of charge carriers from hole to electron with increasing temperature. The sign of S can be explained using a qualitative model [33]. According to this model, in absence of a static JT distortion, it is predicted that the charge carriers will be electrons. On the other hand, presence of large JT distortion, the charge carries behave like holes. Therefore, it is expected that pristine sample has less JT distortion and with Ba-doping JT distortion increase. All samples exhibit a pronounced slope change at TMI as indicated by arrows in Fig. 6. This behavior is consistent with that observed in other manganites [34,35]. With increasing Ba content, TMI is found to shift towards higher temperatures. Besides, the value of S increases with Ba content thereby indicating there is a weakening of metallicity with doping. These findings in S(T) characteristics are consistent with that observed in electrical resistivity measurements. An effort has been made to analyze the contribution of various mechanisms to the thermoelectric transport in the insulating as well as metallic regime. The insulating regime is analyzed using the polaron model [35] which is given by,

SðTÞ ¼

  kB ES þa e kB T

ð6Þ

where ES is the activation energy for Seebeck coefficient and a is a constant related to the kinetic energy of the polarons. The calculated values of ES and a are tabulated in Table 4 and a representative fitting result for the pristine sample using Eq. (6) is shown in the inset (ii) of Fig. 6. By comparing Ea and ES, it is noticed that Ea  ES

SðTÞ ¼ S0 þ S3=2 T 3=2 þ S4 T 4

ð7Þ

3.5. Thermal conductivity The results of temperature dependent thermal conductivity (j) measurements in the temperature from 10 to 350 K are presented in Fig. 7. The room-temperature values of total thermal conductivity of the series compounds lie in the range of 20–35 mW/cm K. Such a low value of j is presumably due to the presence of Jahn– Teller effect in these samples [36,37]. Besides, it is seen that the magnitude of thermal conductivity decreases monotonously with increase in x. Such an observation is attributed to the increased scattering produced by the formation of Jahn–Teller polarons created by increased distortion of MnO6 octahedra via Ba doping [34]. It is noted that all samples exhibit a step-like feature at TMI, as indicated by arrows in Fig. 7. Again, the anomaly is shifting towards higher temperatures with increasing x, consistent with electrical resistivity and Seebeck coefficient measurements. In general, the total thermal conductivity of solids is composed with two parts, i.e. contributions from electronic (charge carriers) and lattice (phonons) terms. The contribution of charge carriers to the thermal conductivity can be evaluated by employing Wiedemann–Franz’s (W–F) law, je q/T = L, where L (Lorentz number) = 2.45  108 W X K2. The contribution of je to the total j is estimated to be less than 1% and thus we conclude that the measured j of studied compounds is mainly due to phonon thermal conductivity (jph) with a negligibly small contribution of je. 3.6. Specific heat

Fig. 6. Temperature dependent Seebeck coefficient plots of La0.65Ca0.35xBaxMnO3 (0.00 6 x 6 0.25) compounds. Inset (i) and (ii) shows the fits in the metallic and insulating regions of the pristine sample, respectively.

The variation of specific heat (CP) in the temperature range 80–350 K is presented in Fig. 8. It can be seen that a pronounced peak corresponding to the ferromagnetic to paramagnetic transition is observed in all studied samples. The position of peak is shifting towards higher temperatures and the peak becomes progressively broader with x. The observed behavior is consistent with the S and j studies. A smooth background drawn tangential between the low and high temperature ends is subtracted from the data of each sample to estimate the magnetic contribution of specific heat (DCP) during the transition. The change in entropy (DS) is evaluated by integrating the area under the DCP/T verses T plots shown in the inset of Fig. 8. The magnitude of DS in the unit of R (ideal gas constant) is tabulated in Table 4, which is found to decrease with increasing Ba content. Such a result indicates an increase of magnetic inhomogeneity with the substitution of Ba on the Ca sites in these compounds. The possible origin of this magnetic inhomogeneity could be the occurrence of

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Table 4 Fitting parameters obtained from Eq. (7), activation energies from electrical resistivity (Ea) and Seebeck coefficient (ES), and entropy change estimated from specific heat measurements for La0.65Ca0.35xBaxMnO3 (0 6 x 6 0.25) compounds. x 0.00 0.05 0.10 0.15 0.25

S0 (lV/K) 7.889 0.649 1.261 1.127 0.595

S3/2 (lV/K3/2) 3

3.254  10 1.307  103 3.431  104 1.500  103 7.689  104

S4 (lV/K4)

Ea (meV)

ES (meV)

a

DS (R)

4.244  109 2.239  109 1.048  109 2.427  109 1.628  109

143.7 166 178.9 119.5 128.2

7.816 4.981 5.375 20.704 10.47

0.00358 0.00225 0.00239 0.0057 0.00295

0.8175 0.3112 0.2699 0.2630 0.1972

Fig. 7. Temperature dependent thermal conductivity plots of La0.65Ca0.35xBaxMnO3 (0.00 6 x 6 0.25) compounds. The anomalous features at TMI are indicated by arrows.

content. The electrical resistivity measurements show that the TMI shifts towards higher temperatures with the increase in Ba content. The existence of CMR effect is studied by measuring resistivity with different applied magnetic fields. The MR% under 8 T is about 58% at transition for the pristine sample and decreases with increasing x. Analyses of the electrical transport data at temperatures T < TMI (metallic region) confirms that the electrical transport is governed by the predominant electron–electron scattering process, whereas the adiabatic small polaron hopping (ASPH) model is applicable in the high-temperature T > TMI (insulating region) range. The electrical resistivity data in the entire temperature range (50–300 K) is analyzed using the phenomenological percolation model based on the phase segregation mechanism. It is inferred from the value of U0 that the pristine sample is more conductive than the rest of the samples. It is seen that the value of U0 is very large for the pristine sample compared to the Ba doped samples, indicating a pronounced grain boundary effect in Ba doped samples. Analysis of the Seebeck coefficient data reveals that the small polaron hopping mechanism is operative at high temperatures. The low temperature region is explained by considering the impurity, electron–magnon scattering, and spin wave fluctuation terms. It is found that the electron–magnon scattering is dominating for the thermoelectric transport below TMI. The magnitude of room-temperature thermal conductivity of these compounds lies in the range of 20–35 mW/cm K. The electronic thermal conductivity (je) is evaluated using W–F law and estimated to be less than 1%. Thus we conclude that the heat conduction is mainly due to phonons in these manganites. Finally, the decrease in entropy change and the broadening of the CP anomalies with x can be attributed to the enhancement of magnetic inhomogeneity with Ba doping. Acknowledgments

Fig. 8. Temperature dependent specific heat plots of La0.65Ca0.35xBaxMnO3 (0.00 6 x 6 0.25) compounds. The inset shows the plot of DCP/T vs. T in the vicinity of the transition.

anti-ferromagnetic interaction as a result of Ba doping. The enhancement of magnetic inhomogeneity may have the effect of weakening the double-exchange mechanism and then decrease the entropy change at Curie temperature. 4. Conclusions Structural, magneto-electrical and thermal properties of Badoped La0.65Ca0.35xBaxMnO3 (0.00 6 x 6 0.25) compounds have been carried out. The XRD patterns indicate that all compounds in present study are single phase in nature. It is seen that the lattice parameters and the unit cell volume increases with increasing Ba

One of the authors (M.S.O) acknowledges Manipal University for providing the financial support to pursue Ph.D. We thank Dr. Shivakumara C, Solid State and Structural Chemistry Unit, IISc, Bangalore, for providing necessary help in XRD analysis. Authors are grateful to Material processing laboratory, NITK, Surathkal for providing density and hardness measurement facilities. The authors are thankful to Dr. Rajeev Rawat and Mr. Sachin Kumar, UGC-DAE, CSR Indore for resistivity and MR measurement. The transport and thermal measurements are supported by the Ministry of Science and Technology of Taiwan under Grant No. MOST 103-2112-M-259-008-MY3 (Y.K.K.). References [1] S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R. Ramesh, L.H. Chen, Science 264 (1994) 413. [2] G. Lalitha Reddy, Y. Kalyana Lakshmi, N. Pavan Kumar, S. Manjunath Rao, P. Venugopal Reddy, J. Magn. Magn. Mater. 362 (2014) 20. [3] T.V. Ramakrishnan, J. Phys. Cond. Matt. 19 (2007) 125211. [4] Y. Kalyana Lakshmi, S. Manjunathrao, P. Venugopal Reddy, Mater. Chem. Phys. 143 (2014) 983. [5] Neeraj Panwar, Indrani Coondoo, S.K. Agarwal, Mater. Lett. 64 (2010) 2638. [6] M.H. Phan, S.C. Yu, J. Magn. Magn. Mater. 308 (2007) 325.

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