Tuning the microstructure and magnetocaloric properties of Na+-substituted La0.67Ca0.33−xNaxMnO3 porous nanospheres

This study investigated the effects of Na ⁺ substitution on the microstructure and magnetocaloric properties of La _0.67Ca _0.33−xNa _xMnO ₃ (x = 0, 0.05, 0.10, 0.15, 0.20) porous nanospheres synthesized by the solvothermal method. The structural analysis showed that Na ⁺ substitution increased the unit cell volume and distorted the lattice structure of La _0.67Ca _0.33−xNa _xMnO ₃ porous nanospheres. Na ⁺ doping also reduced the diameter uniformity of the porous nanospheres and caused them to relax and disintegrate. The magnetocaloric measurements revealed that Na ⁺ doping increased the Curie temperature values. The La _0.67Ca _0.23Na _0.1MnO ₃ composition exhibited a maximum magnetic entropy change of 3.57 J/kg.K, while La _0.67Ca _0.18Na _0.15MnO ₃ achieved the largest relative cooling power of 308.18 J/kg under an applied field H = 5 T. The critical parameters of the La _0.67Ca _0.33−xNa _xMnO ₃ porous nanospheres were examined using the modified Arrott plot, Kouvel-Fisher method and critical isotherm, indicating significant changes in critical parameters due to Na ⁺ doping. The results suggested that Na ⁺-substituted La _0.67Ca _0.33−xNa _xMnO ₃ porous nanospheres could be promising candidates for magnetic refrigeration near room temperature.