Tuning magnetocaloric properties in La0.8-xGdxSr0.2MnO3 ceramics by Gd3+ doping: A combined study via first-principles calculations and sol-gel synthesis

La 0.8- x Gd x Sr 0.2 MnO 3 (LGSMO, 0.05≤ x  ≤ 0.15) ceramics were synthesized via the sol-gel method and sintered at 950 °C for 10 h. The XRD and TEM analyses indicate a systematic evolution of the unit cell parameters with Gd 3+ doping, though the rhombohedral perovskite structure (R-3c) remains stable across the entire series. The La 0.8- x Gd x Sr 0.2 MnO 3 ceramics exhibited an average grain size ranging from 177 to 249 nm. The chemical composition and successful incorporation of Gd 3+ into the structure were confirmed by XPS and EDS analyses. Furthermore, The XPS analysis confirmed the +3 oxidation states of La and Gd, and a mixed Mn 3+ /Mn 4+ valence state. All samples exhibited a paramagnetic phase (PM)-ferromagnetic phase (FM) transition near the Curie temperature (Tc), as characterized by Banerjee's criterion (Arrott plots) and the magnetization normalization. A monotonic decrease in Tc from 306 K ( x  = 0.05) to 259 K ( x  = 0.15) is observed with increasing Gd 3+ content, a trend that, according to density functional theory (DFT), originates from the modified double-exchange interaction. The DFT calculations of the density of states for the LGSMO ceramics reveal that the Mn 3d orbitals shift progressively toward the Fermi level with increasing Gd 3+ content. This shift indicates a reduced bonding capability of the Mn 3d orbitals, which elucidates the underlying cause for the observed variations in the Tc and magnetocaloric properties. A Curie temperature of 306 K and a peak entropy change of 4.82 J/(kg K) were obtained for the La 0.75 Gd 0.05 Sr 0.2 MnO 3 ( x  = 0.05) ceramic.