Effect of Structural Disorders on Magnetocaloric Effect in $\hbox {La}_{2/3-{\rm x}}\hbox {Gd}_{\rm x}\hbox {Sr}_{1/3-{\rm y}}\hbox {Ba}_{\rm y}\hbox {MnO}_{3}$

Cation disordering in ABO₃ perovskite manganites plays the import role on their magnetism and transport properties. In the present, single phase of La_2/3-xGd_xSr_1/3-yBa_yMnO₃ ploycrystals are synthesized with fixing the averaged A-site ionic size of 1.247 Å. Subtle structural variations have been studied by Rietveld refinements using X-ray powder diffraction data. The unit cell symmetry remains unchanged with doping while the lattice parameters, Mn-O bond length, the Mn-O-Mn bond angle vary with the dopants. With increasing the Ba content, Mn-O bond length decreases while Mn-O-Mn bond angle increases, which indicates that the double-exchange interaction increases with Ba content. However, the ferromagnetic-to-paramagnetic transition temperature, T_c, decreases with Ba content, which confirms the strong suppression effect of cations disordering on magnetic ordering. The first-order to second-order magnetic phase transition is manipulated by a rival ship between A-site size-disorder degree and electron-lattice coupling for the sample system. In this system, we easy adjust the Tc above the room temperature. The transition temperature may be easily broadened by adjusting the cations disordering. It may be used as a promising candidate for magnetic refrigerant near room temperature.