Structural, Magnetic and Magnetocaloric Properties of La-deficient La_0.77-xSr_xCa_0.2MnO₃ Perovskites

La-deficient La_0.77-xCa_0.2Sr_xMnO₃ (0≤x≤0.1) polycrystalline samples are synthesized using the sol-gel technique. The crystal structures of all the samples are single orthorhombic phase with Pbnm space group. Rietveld analysis of x-ray diffraction patterns shows that the Mn-O-Mn bond angle θ_Mn-O-Mn increases whereas the Mn-O bond length d_Mn--O decreases monotonically with increasing Sr²⁺ content, which results in a rich overlap between Mn 3d and O 2p orbitals and leads to a systematic increase of the Curie temperature in this compound. It is found that the magnetic entropy change has a maximum at x=0.06 in La³⁺-deficient La_0.77-xCa_0.2Sr_xMnO₃. This may result from competition between the super-exchange interactions (Mn⁴⁺-O^2--Mn⁴⁺) and double-exchange interactions (Mn³⁺-O^2--Mn⁴⁺) originating from the appearance of superfluous Mn⁴⁺ ions by substitution of Sr²⁺ for La³⁺ in this series. Large magnetic entropy changes of 2.32 and 1.83Jkg^-1K^-1 in the x= 0.06 and x= 0.1 samples at their T_Cs of 271K and 303K upon a low magnetic field (10kOe) make these materials promising candidates at near room temperature.