Synthesis and conductivity properties of Gd_0.8Ca_0.2BaCo₂O_5+δ double perovskite by sol–gel combustion

Double perovskite oxides Gd_0.8Ca_0.2BaCo₂O_5+δ have been successfully prepared by EDTA-citrate complexation process. The prepared samples were characterized by means of thermogravimetry–differential analysis, X-ray diffraction, Fourier transform infrared spectroscopy techniques. These analyses suggested that pure Gd_0.8Ca_0.2BaCo₂O_5+δ phase could be formed when the pH value was maintained above three and calcination temperature was higher than 850 °C. The conductivity properties analyzed by means of four probes method, scanning electron microscopy and X-ray photoelectron spectroscopy indicated an improvement in conductivity with a high value of nearly 1200 S/cm, which was due to an increase in the number of small polaron hopping conduction units, Co³⁺–O–Co⁴⁺. Interestingly, the Gd_0.8Ca_0.2BaCo₂O_5+δ samples exhibited a semiconducting behavior through a wide range of temperature below 100 °C and a metallic behavior at 100–500 °C, thereby demonstrating that Gd_0.8Ca_0.2BaCo₂O_5+δ could meet the requirements of a smart material.