Synthesis and characterization of mixed phases in the Ca–Co–O system using the Pechini method

Abstract The synthesis of mixed phases in the Ca–Co–O system using the Pechini method was investigated. The phases CaCo 2 O 4 , Ca 3 Co 4 O 9 and Ca 3 Co 2 O 6 , which can be expressed in terms of the molar ratios of the binary oxides (CaO:Co 2 O 3 ) as (1:1), (3:2) and (3:1) respectively, were obtained in pure form. Powder X-ray diffraction data showed they are comparable with materials obtained by conventional solid-state synthesis. The mild synthesis conditions of the Pechini method allowed the evolution of the Ca–Co–O phases to be studied as a function of temperature and heating time. Both variables have a strong influence on the composition of the products, indicating that both kinetic and thermodynamic factors play a role, with the former being of more importance at lower temperatures. The (1:1) phase, previously not identified in the phase diagram of the Ca–Co–O system, was obtained preferentially across the composition range below ca. 650 °C. The (3:2) phase was preferred until ca. 800 °C. At even higher temperatures, the (3:1) phase is obtained. The upper thermal stability limit of mixed phases in the Ca–Co–O system increases with the Ca:Co ratio. An updated phase diagram for the Ca–Co–O system is proposed.