Effect of Gd doping on structural, electrical, magnetic, and thermal properties of La_0.7-xGd_xBa_0.3MnO₃ (x = 0, 0.1, 0.3, and 0.5)

The development of materials with tailored magnetic, electrical, and thermal responses is vital for next-generation electronic devices. Here we examine how Gd substitution alters the structural, electrical and thermal properties in La_0.7-xGd_xBa_0.3MnO₃ (LGBMO) pellets with x = 0.0, 0.1, 0.3, 0.5. X-ray diffraction confirms a single-phase rhombohedral structure (R-3c) for all the samples. Progressive Gd doping reduces the average A-site ionic radius <r_A> and increases A-site disorder (σ²), which in turn decreases the Mn-O-Mn bond angle and unit-cell volume. SEM micrographs reveal reduction in grain size as Gd content increases and produce visible porosity at x = 0.5. Temperature-dependent resistivity (ρ(T)) shows two metal-insulator transitions (MITs), both shifting to lower temperature with increasing Gd content. For x = 0.3 and 0.5, ρ(T) exhibits a low-temperature minimum followed by a pronounced upturn on further cooling characteristic of Kondo-like behavior. Magnetization (M(T)) measured at 50 Oe evidences a paramagnetic-to-ferromagnetic transition for compositions up to x ≤ 0.3 and the Curie temperature decreases from 332 K (x = 0)) to 132 K at x = 0.3. In the x = 0.5 sample, zero-field-cooled curve shows a cusp at T_f = 44 K (freezing temperature) and a clear bifurcation between field-cooled and zero-field-cooled curves appears above T_f, indicating magnetic inhomogeneity and a spin-glass-like state. Thermopower (S(T)) measurement show that the Seebeck coefficient changes sign from hole-like to electron-like in the pristine and x = 0.1 samples, whereas the x = 0.3 composition remains electron-like across the entire temperature range, consistent with n-type thermoelectric transport in the ferromagnetic metallic state. Together, the S(T) behaviours, along with the positive temperature slope of the thermal conductivity, point to small-polaron dominated transport in the paramagnetic phase, where vibronic coupling suppresses carrier mobility and phonon mean free paths.