Enhancement of the magnetic and magnetocaloric properties by Na substitution for Ca of La_0.8Ca_0.2MnO₃ manganite prepared via the Pechini-type sol–gel process

We report in this work the results of synthesis and characterization of La_0.8Ca_0.2−xNa_xMnO₃ (0 ≤ x ≤ 0.2) prepared by the Pechini sol–gel method. We have investigated different effects of replacing Ca by Na on the magnetic properties and magnetocaloric effect of La_0.8Ca_0.2MnO₃ compounds using X-ray diffraction and magnetization measurements. We have observed an evolution from an orthorhombic to a rhombohedral symmetry caused by the addition of Na and that the rhombohedral phase is stable for x ≥ 0.1. The partial substitution of Ca by Na leads to a monotonic increase in the Curie temperature (T_C) of the samples from 105 K for x = 0 to 330 K for x = 0.2. A second-order nature of the magnetic phase transition is established in all the prepared compounds with the help of the Banerjee criterion. At an applied field of 5T, the maximum magnetic entropy change (∆S_M^max) shows an enhancement with increasing Na⁺ concentration. Moreover, the relative cooling power (RCP) values are found to vary between 203 and 300.5 J/kg in this series of materials. In order to compare the magnetocaloric performances of the prepared samples, we have used several figures of merit such as the temperature-averaged entropy change (TEC), refrigerant capacity (RC) and normalized refrigerant capacity (NRC). Accordingly, the addition of Na ion greatly increases TEC, RC and NRC and thereby improves the magnetocaloric performance in La_0.8Ca_0.2−xNa_xMnO₃ (0 ≤ x ≤ 0.2). These metrics can help in identifying materials with high performance and clearly suggest that the sol–gel-made samples with x ≥ 0.1 can be customized for use in magnetic cooling applications near room temperature.