Thermodynamic properties of misfit cobaltite [Bi2-xCa2O4][CoO2]1.7

We synthetized a misfit structure cobaltite [Bi2-xCa2O4][CoO2]1.7 and studied its thermal behavior by simultaneous thermal analysis (STA), low temperature heat capacity measurement in physical property measurement system (PPMS), differential scanning calorimetry (DSC) to measure heat capacity and drop calorimetry to obtain the relative enthalpy increments. The oxygen non-stoichiometry determined by thermogravimetry (STA data combined with weight loss measurement of the material reduced in hydrogen atmosphere) was analyzed in terms of point defect model resulting in two independent reactions, oxygen release and cobalt disproportionation, described by the respective enthalpy and entropy changes. The applied model satisfactorily describes two regimes of oxygen release/incorporation associated with the respective Co valency changes Co4+/Co3+ and Co3+/Co2+. The temperature dependence of the molar heat capacity above room temperature was derived by simultaneous least squares method from DSC and enthalpy data considering the contribution due to oxygen release and deviation from ideal stoichiometry. The low temperature heat capacity was analyzed in terms of a combined Debye-Einstein model.