Defect-induced insulator-metal transition and negative permittivity in La1-xBaxCoO3 perovskite structure

•Low-frequency plasmonic state and temperature-stable negative permittivity in La1-xBaxCoO3 perovskite structure is achieved.•The partial replacement of La3+ by Ba2+ in La1-xBaxCoO3 system can create oxygen vacancies and Co element with higher oxidation states.•Samples with x≥0.12 manifest metal-like character described by the Drude model approximat.•Zero-crossing behavior of real permittivity is observed in La0.9Ba0.1CoO3 sample.