Pr3+ doping at the A-site of La0.67Ba0.33MnO3 nanocrystalline material: assessment of the relationship between structural and physical properties and Bean-Rodbell model simulation of disorder effects

Ma Oumezzine; Herbet Bezerra Sales; Ahmed Selmi; E K Hlil

doi:10.1039/c9ra03494c

Pr³⁺ doping at the A-site of La_0.67Ba_0.33MnO₃ nanocrystalline material: assessment of the relationship between structural and physical properties and Bean-Rodbell model simulation of disorder effects

RSC Adv. 2019 Aug 15;9(44):25627-25637. doi: 10.1039/c9ra03494c. eCollection 2019 Aug 13.

Authors

Ma Oumezzine¹, Herbet Bezerra Sales², Ahmed Selmi¹, E K Hlil³

Affiliations

¹ Laboratory of Physical Chemistry of Materials, Faculty of Sciences of Monastir, University of Monastir 5019 Monastir Tunisia oumezzine@hotmail.co.uk.
² UEMA/CCT, Universidade Federal de Campina Grande - UFCG Campina Grande PB Brazil.
³ Institut Néel, CNRS, Université J. Fourier B.P. 166 38042 Grenoble France.

Abstract

Bulk nanocrystalline samples of (La_1-x Pr _x )_0.67Ba_0.33MnO₃ (0.075 ≤ x ≤ 0.30) manganites with a fixed carrier concentration are prepared by the sol-gel based Pechini method. Rietveld refinement of the X-ray diffraction patterns, shows the formation of single-phase compositions with rhombohedral symmetry. Upon Pr³⁺ doping at the A-site, the unit cell volume and the B-O-B bond angles are reduced. FTIR spectra present a prominent absorption peak of the in-phase stretching mode (B_2g mode) rising from the vibration of the Mn-O bond. Raman spectra at room temperature reveal a gradual shift toward lower frequencies in (E_g) phonon mode with increasing Pr³⁺ concentration. The M(T) measurements shows a clear ferromagnetic (FM)-paramagnetic (PM) phase transition with increasing temperature. An increase in resistivity and activation energy and a decrease in the metal-semiconductor transition (T _M-SC) and Curie temperatures (T _C) was observed as a consequence of Pr³⁺ doping. The results are discussed according to the change of A-site-disorder effect caused by the systematic variations of the A-site average ionic radius 〈r _A〉 and A-site-cation mismatch σ ², resulting in the narrowing of the bandwidth and the decrease of the mobility of e_g electrons. The magneto-transport behavior in the whole measured temperature and a magnetic field can be described by a percolation model, which is in agreement with the limited experimental data of the samples for x = 0.075, 0.15 and 0.30. The experimental results confirm that A-site substitution with Pr³⁺ destroys the Mn³⁺-O^2--Mn⁴⁺ bridges and weakens the double exchange (DE) interaction between the Mn³⁺ (t³ _2ge¹ _g, S = 2) and Mn⁴⁺ (t³ _2ge⁰ _g, S = 3/2) ions. On the other hand, the Bean and Rodbell model has been successfully used to simulate the magnetization data of the samples with x = 0.15 and x = 0.22. The random replacement of La³⁺ by Pr³⁺ is shown to induce more disorder in the system, which is reflected in the increase of the fitted disorder parameter and spin value fluctuation. At a temperature close to room temperature, the maximum magnetic entropy change (ΔS _Max) and the relative cooling power (RCP) of La_0.52Pr_0.15Ba_0.33MnO_2.98 are found to be, respectively, 1.34 J kg^-1 K^-1 and 71 J kg^-1 for a 1.5 T field change.