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

Abstract

Bulk nanocrystalline samples of (La_1−xPr_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 σ², 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⁺ (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.