Investigation of the impact of A-site cation disorder on the structure, magnetic properties, and magnetic entropy change of trisubstituted divalent ions in La_0.7(Ba,Ca,Sr)_0.3MnO₃ manganite

This study investigates the effect of A-site disorder, characterized by the average ionic radius (〈r_A〉) and the cation mismatch (σ²), on the structural, magnetic, critical behavior, and magnetic entropy changes in La_0.7(Ba,Ca,Sr)_0.3MnO₃ manganites with trisubstituted Ba, Ca, and Sr. The sol-gel method was used to prepare polycrystalline samples. All series of compounds crystallize in rhombohedral symmetry with the R3̄c space group. A linear relationship between lattice parameters, unit cell volume, and 〈r_A〉 was observed. This reveals an unusual behavior in the correlation between 〈r_A〉 and σ² concerning magnetic properties, which is attributed to the complex simultaneous trisubstitution of divalent ions. Energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) were utilized to validate the chemical composition of compounds. All the samples crystallized in rhombohedral symmetry, and the lattice parameters increased continuously with increasing 〈r_A〉. A-site disorder causes distortions in the Mn-O bond length and Mn-O-Mn bond angle in the MnO₆ octahedral structure, which influences the double-exchange interaction and electronic bandwidth (W). The Curie temperature (T_C) increases linearly with increasing W. The critical behavior around T_C for all the samples was investigated by determining the values of the critical exponents (β, γ, and δ) using the modified Arrott plot (MAP) method. The estimated critical exponents show that the unconventional model establishes a short-range ferromagnetic order. The maximum magnetic entropy change (−ΔS_M) was obtained with the lowest 〈r_A〉 and σ² value. The analysis of the critical behavior and universal curve indicates a second-order phase transition (SOPT) nature for all samples.