Research articles
Effect of rare-earth substitution at La-site on structural, electrical and thermoelectric properties of La0.7−xRExSr0.3MnO3 compounds (x = 0, 0.2, 0.3; RE = Eu, Gd, Y)

https://doi.org/10.1016/j.jmmm.2017.11.007Get rights and content

Highlights

  • Lattice parameters decrease with rare-earth doping.

  • TCR increases with rare-earth doping.

  • All the samples show sign reversal in thermo-power.

  • At high temperatures, SPH model is applicable for resistivity and thermo-power data.

Abstract

In the present communication, we present results on the effect of rare-earth (RE) substitution at La-site on the structural, electrical and thermoelectric properties of La0.7−xRExSr0.3MnO3 compounds. The lattice parameters are observed to decrease with RE-doping which is attributed to the fact that the substituted RE ions (RE = Eu, Gd and Y) are smaller than that of La ion. In high temperature semiconducting regime, small polaron hopping (SPH) model is valid, whereas, variable hopping model is valid in low temperature metallic region. The resistivity in the entire temperature range follows percolation model. All the samples exhibit sign reversal in thermopower, S. From temperature dependent S data, it is seen that SPH model is applicable in high temperature regime.

Graphical abstract

Temperature variation of TCR% for (a) La1−xEuxSr0.3MnO3, (b) La1−xGdxSr0.3MnO3 and (c)La1−xYxSr0.3MnO3 samples.

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Introduction

Last two decades have witnessed an enormous work on pervoskite manganites of the type RE1−xAxMnO3 (RE represents trivalent rare-earth elements like La, Pr, Gd, etc. and A stands for divalent alkaline-earth cation like Ca2+ and Sr2+) [1], [2]. In particular, more attention has been paid on colossal magneto resistance (CMR) and exceptionally large magneto-calorie effect (MCE) demonstrated by these compounds, as these properties are important as far as applicability of these compounds is concerned [3], [4].

Notably, La1−xSrxMnO3 is one of the most interesting hole doped systems [5], [6], [7]. The parent compound LaMnO3 is a well-known A-type antiferromagnetic insulator and due to its non-collinearity, it has a small ferromagnetic component. Among the various La1−xSrxMnO3 compounds, La0.67Sr0.33MnO3 is of unique significance as the CMR is known to be more pronounced when concentration of Sr is about 30% wt% [8], [9], [10]. As observed in manganites, La0.67Sr0.33MnO3 shows the typical metal to insulator transition at characteristic temperature, TMI. This compound also exhibits a high spin polarization. In addition to this it also exhibits a ferro to para-magnetic transition at a characteristic temperature called Curie temperature (TC) [11], [12], [13], [14]. Due to simultaneous occurrence of metal to insulator transitions (TMI), along with strong Jahn Teller interactions [15], [16], the compound La0.66Sr0.33MnO3 (LSMO) has been extensively studied. Some of the application areas include solid oxide fuel cells (SOFC) [17], [18], potential use in oxygen sensor [19], and resistance switching for multi-bit process [20].

La0.7Sr0.3MnO3 compounds have limitation as their TC and TMI are much above the room temperature. Hence its applicability at ambient temperatures is limited. Keeping this in mind several authors in the past have doped rare earth and alkaline site in LSMO system so as to increase the magnetic transition temperature (TC) thereby exhibiting high magneto-resistance at room temperature [21] which can further elevate the magneto-calorie effect thereby making it suitable for applications near room temperatures. Recent investigations reveal that doping of Al and Ti for Mn in La0.7Sr0.3MnO3 lowers the magnetic ordering temperature from 364.5 K to below 300 K [22]. When barium is doped with Sr as in La0.7Sr0.3−xBaxMnO3 the TC of the material progressively decreases from 350 to 320 K (for x = 0.0 to 0.33) as the concentration of Ba is increased [23]. The metal to insulator transition temperature (TMI) also decreases from 269 to 249 K (for x = 0.1) and almost remain constant with further increase in barium [23]. Reports suggest that TMI is suppressed by application of external pressure in La1−xSrxMnO3 system of manganites [24]. Anomalies in magneto-resistance and thermal expansion have been observed in La0.7Sr0.3MnO3 system [25]. With the addition of silver-oxide in the compound Na0.5La0.2Sr0.3MnO3:xAg2O, magneto-resistance shoots up from 22% (for x = 0) to 47% (for x = 0.3) in applied magnetic field of 1 T. The temperature coefficient of resistance (TCR) increases from 2.9% (for x = 0) to 7.4% (for x = 0.3) [26].

There are several reports on Eu-doping at La site in LSMO system. However these studies show conflicting results [27], [28], [29], [30]. Reports by Dhahri et al. [28] show that with Eu-doping the structure of La0.67−xEuxSr0.33MnO3 remains rhombohedral; on contrary other reports suggest that orthorhombic structure is observed for pristine and Eu-doped compounds [29], [30]. On the other hand Vadnala et al. [27] have shown that pristine compound of La0.7Sr0.3MnO3 exhibits rhombohedral structure, however upon Eu-doping a structural transformation to orthorhombic structure is observed. Similarly there are contradictory reports on Mn-O bond-length and Mn-O-Mn bond angles. Vadnala et al. [27] and Raju et al. [29] have reported that both the bond angle and bond length decreases with Eu-doping, however, Dhahri et al. [28] have shown that former increases with Eu-content and latter is seen to decrease. Keeping these in mind, it was desirable to undertake such studies. In addition to Eu-doping, we have also studied Y and Gd substitution in La0.7Sr0.3MnO3 as Y+3 and Gd+3 ionic radii are smaller than that of La+3 radius. The motivation for undertaking thermoelectric measurement is the fact that it gives information on the nature of charge carriers. In this communication we report structural, electrical and thermo-electric properties of La0.7−xRExSr0.3MnO3 (x = 0, 0.2, 0.3; RE = Eu, Y, Gd).

Section snippets

Experimental details

The samples were prepared using high purity ingredients (≥99.9%) like La2O3, Mn2O3, SrCO3, RE2O3 (RE = Y, Gd, Eu) in stoichiometric ratio using standard solid state reaction method. The powders (about 5 g) were evenly mixed in agate mortar-pestle for 6 h and were then calcinated at 900 °C, 1100 °C, 1200 °C for 24 h each with several intermediate grindings for homogenization. Thereafter the powder was pressed into pellets by applying a load of 5 tons. Typical dimensions of pellets are

Structural properties

The XRD data for all the samples was analyzed using Rietveld method by means of FULLPROF software. Fig. 1 depicts XRD results for representative samples. In Fig. 1 the solid circles denote the observed values; solid lines represent the refined data and the vertical lines represent the Bragg peaks respectively. Rietveld analysis indicates that all the compounds have rhombohedral structure with R-3C space group. It is seen from Fig. 1 that some of the Bragg peaks decrease with the rare-earth

Conclusions

RE substituted samples of La0.7−xRExSr0.3MnO3 were prepared using solid state reaction technique. All the samples were single phased and lattice parameters are observed to decrease with increase in RE content which is essentially due to the fact that the substituted RE ionic sizes are smaller than that of La ions. Goldschmidt tolerance factor is observed to decrease with increase in RE content indicating that RE doping enhances distortion in the lattice. Pristine as well as RE doped samples

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