Pr 3+ doping at the A-site of La 0.67 Ba 0.33 MnO 3 nanocrystalline material: assessment of the relationship between structural and physical properties and Bean-Rodbell model simulation of disorder effects.

Bulk nanocrystalline samples of (La 1- x Pr x ) 0.67 Ba 0.33 MnO 3 (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 Pr 3+ 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 Pr 3+ 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 Pr 3+ 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 Pr 3+ destroys the Mn 3+ -O 2- -Mn 4+ bridges and weakens the double exchange (DE) interaction between the Mn 3+ (t 3 2g e 1 g , S = 2) and Mn 4+ (t 3 2g e 0 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 La 3+ by Pr 3+ 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 (Δ S Max ) and the relative cooling power (RCP) of La 0.52 Pr 0.15 Ba 0.33 MnO 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.