Bi-induced structural distortion and magnetic phase transition in Pr_0.7-x Bi _x Ca_0.3MnO₃: insights into...

Pr_0.7-x Bi _x Ca_0.3MnO₃ (x = 0.0, 0.1, 0.2) were successfully synthesized using the conventional solid-state reaction route from stoichiometric mixtures of binary oxides at high temperatures. Structural characterization by X-ray diffraction combined with Rietveld refinement confirms that the samples with x = 0.0 and x = 0.1 are single-phase compounds crystallizing with a orthorhombic structure with the Pnma space group. Furthermore, the reconstructed two- and three-dimensional electron density maps reveal pronounced charge localization and anisotropic bonding pathways induced by Bi substitution, highlighting the strong coupling between lattice distortion and the evolution of magnetic and electronic properties. Magnetic measurements performed under an applied field of 50 mT show that the parent compound Pr_0.7Ca_0.3MnO₃ exhibits antiferromagnetic behavior at low temperatures, whereas Bi substitution suppresses the antiferromagnetic charge-ordered state and promotes dominant ferromagnetic interactions, leading to a magnetically inhomogeneous non-collinear state for x ≥ 0.1. Arrott plot analysis evidences a Bi-induced crossover from a first-order to a second-order magnetic phase transition, while modified Arrott plots identify the 3D-Heisenberg universality class as the most appropriate model, highlighting the dominance of short-range isotropic ferromagnetic interactions and the strong influence of Bi substitution on the magnetic exchange correlations. The magnetocaloric properties were investigated from isothermal magnetization measurements around the Curie temperature T _C. Under a magnetic field variation of ΔH = 5 T, the maximum magnetic entropy changes (-ΔS ^max _M ) were estimated to be 0.80, 0.47, and 0.48 J kg^-1 K^-1 for x = 0.0, 0.1, and 0.2, respectively. Meanwhile, the corresponding relative cooling power (RCP) values reach 9.7, 30.4, and 29.9 J kg^-1. Although the obtained magnetocaloric parameters remain lower than those of benchmark magnetic refrigerants do, the enhanced RCP observed for the Bi-substituted compounds indicates that Bi doping plays a favorable role in tuning the magnetocaloric response. These findings provide new insight into the interplay between structural distortion, magnetic ordering, and magnetocaloric properties, and demonstrate that Bi substitution is an effective strategy for modulating the low-temperature magnetocaloric behavior of Pr_0.7-x Bi _x Ca_0.3MnO₃ manganites.