A novel complex cobalt gallium oxide Ca2Co0.8Ga1.2O4.8:: Synthesis and high-temperature electron transport properties

A new complex oxide with the cation ratio Ca:Co:Ga = 2:0.8:1.2 has been synthesized in air at 1150degreesC. The cobalt atoms adopt oxidation states 2+ and 3+ in equal amounts giving an oxygen content corresponding to the composition Ca2Co0.8Ga1.2O4.8. It crystallizes in F-centered cubic structure with a = 15.0558 Angstrom. Conductivity measurements performed at high temperatures revealed that the temperature increase gives a charge disproportionation of Co3+ species resulting in a small concentration of Co4+ species and thus a small p-type conductivity in the oxide. A decrease of the oxygen pressure promotes oxygen depletion from the oxide and a deterioration of the conductivity. The electric properties are interpreted within a small polaron conduction mechanism. An unusually large mobility activation energy of 0.45 eV can be explained by a large spatial separation of cobalt cations in the structure. (C) 2002 Elsevier Science (USA).