Tuning oxygen content via La3+ incorporation to enhance structural and electrical transport properties of Sr0.725Y0.275CoO3-δ

La 3+ doping in Sr 0.725 (Y 1- x La x ) 0.275 CoO 3- δ ceramics eliminates the detrimental Y 2 O 3 secondary phase and triggers a tetragonal (x < 1, I4/mmm ) to cubic (x = 1, Pm-3m ) phase transition. XPS (X-ray Photoelectron Spectroscopy) confirms increased Co 4+ content and reduced oxygen vacancies (oxygen content rises from 2.662 to 2.918). This significantly lowers resistivity due to decreased vacancy scattering and enhanced Co 4+ –O–Co 3+ hole hopping. The temperature coefficient of resistance (TCR) shifts from negative to positive. Critically, the x  = 0.025 composition achieves TCR < −3%·K −1 and optimal resistivity within the 230–330 K operational window. This work demonstrates synergistic optimization of high TCR (<-4%·K −1 ), low resistivity, and single-phase stability via La 3+ doping, enhancing its viability for uncooled infrared detectors.