Calcium ion deficiency and its influence on structure, critical behavior, magnetic and magnetocaloric effect properties in La_0.7Ca_0.2-x□_xSr_0.1MnO₃ (x = 0.00, 0.05, and 0.10) manganites

This study investigated the influence of Ca²⁺ ion vacancies on the structural, magnetic, and magnetocaloric properties of La_0.7Ca_0.2-x□_xSr_0.1MnO₃ (x = 0.00, 0.05, and 0.10) synthesized by the solid-state reaction method. Structural analysis revealed a single-phase orthorhombic crystal structure with the Pnma space group for most samples, except for LV-10-SSR, which exhibited a secondary Mn₃O₄ phase due to excessive Ca²⁺ vacancies. The presence of Mn²⁺, Mn³⁺, and Mn⁴⁺ ions, as verified by XPS measurements, facilitated the formation of the Mn₃O₄ phase. The introduction of vacancies significantly influenced lattice parameters, lattice volume, average grain size, crystallite size, density, and MnO₆ octahedral distortion, characterized by the Jahn-Teller effects. These distortions altered Mn–O bond lengths and Mn–O–Mn bond angles, directly impacting the material's magnetic properties. Magnetic properties measurements showed that Ca²⁺ ion vacancies alter the ferromagnetic-to-paramagnetic transition temperature and magnetization. Specifically, LV-10-SSR confirmed magnetic transitions near 50 K, indicative of Mn₃O₄ and Mn²⁺ presence. Critical behavior analysis using modified Arrott plot, Kouvel-Fisher, and critical-isotherm methods reveals that x = 0.00 and 0.05 exhibit long-range magnetic ordering, while x = 0.10 exhibits short-range magnetic ordering. The La_0.7Ca_0.1□_0.1Sr_0.1MnO₃ composition achieved a maximum entropy change of 3.64 J/kg.K, while La_0.7Ca_0.2Sr_0.1MnO₃ exhibited the highest relative cooling power of 401.63 J/kg under a 5 T magnetic field. Additionally, the maximum temperature-averaged entropy change (TEC) for La_0.7Ca_0.1□_0.1Sr_0.1MnO₃, reached 3.61 J/kg.K over a 5 K range at 5 T. These findings highlight the potential of Ca²⁺ vacancies engineering in La_0.7Ca_0.2-x□_xSr_0.1MnO₃ for developing future magnetic refrigerator applications.