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

摘要

Bulk nanocrystalline samples of (La1-xPrx)(0.67)Ba0.33MnO3 (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 (TM-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 sigma(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+-O2--Mn4+ bridges and weakens the double exchange (DE) interaction between the Mn3+ (t32ge1g, S = 2) and Mn4+ (t32ge0g, 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 (Delta S-Max) and the relative cooling power (RCP) of La0.52Pr0.15Ba0.33MnO2.98 are found to be, respectively, 1.34 J kg(-1) K-1 and 71 J kg(-1) for a 1.5 T field change.