Calorimetric study of CaCu₃Ti₄O₁₂, a ceramic with giant permittivity

We conducted an investigation into the thermodynamic properties of two stoichiometric CaCu₃Ti₄O₁₂ (CCTO) samples prepared by solid-state reaction and soft chemistry methods to probe the stability of the material relative to simpler oxide constituents (e.g., CaO, CuO, and TiO₂) over a wide temperature range. Thermodynamic functions (i.e., heat capacity, formation enthalpies, entropies, and Gibbs free energies) have been measured from near absolute zero to 1100 K using calorimetric methods, including drop solution, low-temperature adiabatic relaxation, and differential scanning calorimetry. In addition, the thermodynamic characteristics of the magnetic-phase transition from the antiferromagnetic to the paramagnetic state are reported. It has been shown that CCTO is very stable relative to constituent oxides and calcium titanate at room temperature and higher, independent of the synthesis route. The enthalpic factor is dominant in the thermodynamics of CCTO, with the entropic factor having almost no effect on the stability of the compound relative to other oxide assemblages. The recommended values for the standard molar enthalpy of formation from constituent oxides and from elements at 298.15 K are -122.1 ± 4.5 and -4155.7 ± 5.2 kJ/mol^-1, respectively. The mean of the third law entropy at 298.15 K is 368.4 ±0.1 J/mol^-1/K^-1. Based on the thermodynamic data reported, the study confirms the possibility of CCTO decomposition in a reducing atmosphere or CO₂ under conditions recently observed in experiments.