THERMODYNAMIC BEHAVIOR OF Ca3Co3.93+xO9+δ CERAMICS

Based on the results of differential thermal analysis, differential scanning calorimetry, thermogavimetric analysis, low temperature calorimetry and X-Ray powder diffraction of equilibrated samples as well as on the assessed thermodynamic data phase a section of the Ca–Co–O diagram for pO2 = 0.21 was calculated using the FactSage software. Among the involved phases the misfit cobaltite Ca3Co3.93+xO9+δ is well known for its distinctive thermoelectric properties. As shown by our experiments and reproduced by the proposed thermodynamic model the Ca3Co3.93+xO9+δ phase actually exhibits a narrow homogeneity range with respect to cation composition. The primary focus was put on the heat capacity of Ca3Co3.93O9+δ collected in a broad temperature range and analyzed in terms of a combined Debye - Einstein model. This made it possible to separate two peaks detected at ~ 400 K and ~ 840 K and to evaluate the corresponding entropy and enthalpy changes. The second effect was interpreted as a spin transition from intermediate state to high state.