We have studied the effect of a partial substitution of Ca2+ by Na+ on the structural, magnetic and magnetocaloric properties of the mixed valence perovskites $${\text{La}}_{{{0}{\text{.75}}}} {\text{Ca}}_{{{0}{\text{.25 - }}x}} {\text{Na}}_{x} {\text{MnO}}_{{3}}$$La0.75Ca0.25 -xNaxMnO3 (0 ≤ x ≤ 0.10), prepared by the flux method. X-ray diffraction studies revealed that our samples crystallize in the orthorhombic structure with a Pbnm space group. Rietveld analysis showed that MnO6 octahedron has a little distortion and that θMn–O–Mn bond angles increase with increasing Na+ content. The field cooled (FC) and zero field cooled (ZFC) magnetizations indicated that our samples have a paramagnetic-ferromagnetic phase transition, when decreasing temperature. A small deviation was observed between FC and ZFC curves. This deviation can be explained by the competition between the ferromagnetic and antiferromagnetic interactions, leading to a glassy behavior. The Curie temperature, TC, increased from 268 K for x = 0 to 278 K for x = 0.10. The magnetizations isotherms of our compounds, with a second-order phase transition, were investigated. For the x = 0.10 sample, the maximum of the magnetic entropy change was found to be 3.15 and 6.44 J kg−1 K−1, under an applied magnetic field of 2 and 5 T, respectively. Moreover, it has important relative cooling power values of about 134.82 and 317.79 J kg−1, under the same applied magnetic field. In addition, rescaled entropy data collapsed into the same curve, which indicates the universal behavior for the magnetocaloric effect (MCE) in our compounds. These materials could be potential candidates as working substances in magnetic refrigeration.

中文翻译：

---

La0.75Ca0.25–xNaxMnO3 (0 ≤ x ≤ 0.10) 锰酸盐中的大磁热效应

我们研究了 Na+ 部分取代 Ca2+ 对混合价钙钛矿的结构、磁性和磁热性质的影响 $${\text{La}}_{{{0}{\text{.75}}} } {\text{Ca}}_{{{0}{\text{.25 - }}x}} {\text{Na}}_{x} {\text{MnO}}}_{{3}} $$La0.75Ca0.25 -xNaxMnO3 (0 ≤ x ≤ 0.10)，通过助熔剂法制备。X 射线衍射研究表明，我们的样品在具有 Pbnm 空间群的正交结构中结晶。Rietveld 分析表明 MnO6 八面体有一点变形，θMn-O-Mn 键角随着 Na+ 含量的增加而增加。场冷却 (FC) 和零场冷却 (ZFC) 磁化强度表明，当温度降低时，我们的样品具有顺磁-铁磁相变。在 FC 和 ZFC 曲线之间观察到小的偏差。这种偏差可以通过铁磁和反铁磁相互作用之间的竞争来解释，导致玻璃态。居里温度 TC 从 x = 0 时的 268 K 增加到 x = 0.10 时的 278 K。研究了具有二级相变的化合物的磁化等温线。对于 x = 0.10 的样品，在 2 T 和 5 T 的外加磁场下，磁熵变的最大值分别为 3.15 和 6.44 J kg-1 K-1。此外，在相同的外加磁场下，它具有约 134.82 和 317.79 J kg-1 的重要相对冷却功率值。此外，重新调整的熵数据折叠成同一条曲线，这表明我们化合物中磁热效应 (MCE) 的普遍行为。