Calcium ion deficiency and its influence on structure, critical behavior, magnetic and magnetocaloric effect properties in La0.7Ca0.2-□ Sr0.1MnO3 (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 La0.7Ca0.2-x□xSr0.1MnO3 (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 La0.7Ca0.1□0.1Sr0.1MnO3 composition achieved a maximum entropy change of 3.64 J/kg.K, while La0.7Ca0.2Sr0.1MnO3 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 La0.7Ca0.1□0.1Sr0.1MnO3, reached 3.61 J/kg.K over a 5 K range at 5 T. These findings highlight the potential of Ca2+ vacancies engineering in La0.7Ca0.2-x□xSr0.1MnO3 for developing future magnetic refrigerator applications.