Europium substitution for lanthanium in LaBaMnO - The structural and electrical properties of La0.7-xEuxBa0.3MnO3 perovskite

Polycrystalline samples of La0.7-xEuxBa0.3MnO3 (x = 0.05, 0.10 and 0.15) are prepared using the Pechini sol gel method. The diffraction peaks are narrow and intense in all X ray diffraction (XRD) patterns, indicating that the studied samples are well crystallized. The variation of the conductivity with temperature shows two metal-insulator transitions. The temperatures of these transitions are Eu content independent. It is also found that the conductivity decreases with Eu concentration. Such behavior is related to a reduction of double exchange mechanism and to the structural features. At higher temperatures, electronic conduction is found to be dominated by thermally activated hopping of small polaron. The deduced activation energy increases when increasing europium content from E-a = 80 meV for x = 0.05 to E-a = 178 meV for x = 0.15. The analysis of the ac-conductivity reveals different behaviors which could be related to the grain size disorder. The conductivity spectra are composed by two parts. The first one concerns low frequency range (f < 10 kHz), where the conductivity is frequency independent and thermally activated. The second part was split in three regions. In the first one, (f<f(c)= 125 kHz), the conductivity follows the omega(S) law but in the second region, (f(c) < f < 1000 kHz), the spectra is characterized by the appearance of a peak which shifts to higher frequency indicating the presence of a relaxation phenomenon. In the third region, (f>1000 kHz), the conductance is temperature independent and it decreases with increasing frequency. The study of the variation of the exponent 's' as a function of temperature and europium concentration confirms that hopping model is the dominating mechanism in the system and proves that the material turns from metal into semi-insulating behavior when increasing europium content. The ac-conductivity analysis reveals different behaviors which could be related to the grain size disorder. (C) 2016 Elsevier B.V. All rights reserved.