This study explores the effects of partially substituting \(Ca^{2 + }\) with \(Sr^{2 + }\) on the structural, magnetic, and magnet-transport properties of the mixed valence perovskite manganites La0.7Ca0.18– xSrxBa0.12Mn0.95Sn0.05O3 (0≤x≤0.04), synthesized using the solid-state reaction (SSR) method. The substitution of \(Ca^{2 + }\) by the larger \(Sr^{2 + }\) cation is expected to generate internal chemical pressure within the perovskite lattice, leading to modifications in the \(Mn - O - Mn\) bond geometry and consequently affecting the double-exchange interactions responsible for the magnetoresistance effect. X-ray diffraction analysis indicates that all samples adopt an orthorhombic crystalline structure with Pbnm space group. The substitution of \(Ca^{2 + }\) with larger \(Sr^{2 + }\) cation increases the average A-site ionic radius \(r_{{\text{A}}}\) , resulting in an increase of the \(Mn - O - Mn\) bond angle and a decrease in the \(Mn - O\) bond distance. These structural adjustments enhance the electron bandwidth \(W\) and strengthen the ferromagnetic double-exchange \(\left( {DE} \right)\) interaction. Consequently, the Curie temperature ( \(T_{{\text{C}}}\) ) increases from 148.7 to 233.3 K, and the metal-insulator transition ( \(T_{{{\text{MI}}}}\) ) shifts toward higher temperatures. Temperature-dependent ac susceptibility at various frequencies indicates the presence of magnetic relaxation associated with domain wall motion and dynamic magnetic responses within the ferromagnetic ordered state. The hysteresis loops exhibit ferromagnetic behavior at 1.8 K and paramagnetic state at 300 K. Furthermore, a systematic decrease in magnetoresistance \(\left( {MR} \right)\) from 38 to 20.3% is observed with increasing \(Sr\) content. These findings demonstrate that minor \(Sr\) -doping at the \(Ca\) -site is an effective tool for tuning the competition between lattice distortion and magnetic interactions, directly influencing the electronic and magneto-transport performance of these manganites.
