Structural Evolution, Defect Chemistry, and Microstructural Tuning of Novel La2-xCaxMn2-yCoyO6-δ Double Perovskite Oxides Synthesized via Facile Sol–Gel Route

Lanthanum-based double perovskite oxides (DPOs) have attracted considerable attention owing to their structural tunability, defect chemistry, and redox-active transition-metal centers, making them promising candidates for energy-related applications. In the present study, a series of novel La2-xCaxMn2-yCoyO6-δ (x = y = 0.1, 0.3, 0.5, 0.7) DPOs were synthesized via a citrate–nitrate sol–gel method, enabling homogeneous precursor mixing and precise control over microstructural evolution. X-ray diffraction confirmed the formation of a phase-pure monoclinic crystal structure with space group C2/c (No. 15) for all DPO compositions, while systematic variations in lattice parameters and unit-cell volume demonstrated the influence of dual-site Ca/Co substitution on the crystal structure. Williamson–Hall (W–H) and size–strain plot (SSP) analyses revealed a progressive reduction in crystallite size and lattice strain, accompanied by an increase in dislocation density with increasing dopant concentration, confirming effective microstructural tuning. FTIR spectroscopy further verified the formation of the double perovskite framework through the characteristic metal–oxygen vibration at approximately 602 cm−1, while EDX analysis confirmed the successful incorporation of the constituent elements without detectable impurities. The compositional dependence of the oxygen-defect characteristics indicates that dual-site substitution provides an effective strategy for tailoring the structural and defect chemistry of La2-xCaxMn2-yCoyO6-δ. The combined structural stability, controlled microstructure, and defect engineering highlight the potential of these materials for oxygen electrocatalysis, solid oxide fuel cell cathodes, and other energy-conversion and environmental applications.