Thermodynamic study of the sodium chromite NaCrO₂

A thermodynamic study was undertaken on two kinds of NaCrO₂(s) powder, a commercial and an experimental one synthetized. Heat capacity measurements were carried out in the temperature range of 25–600 °C using a differential scanning calorimeter. Gibbs energy of formation versus temperature was deduced using these experimental data, the enthalpy of formation from literature and the entropy at room temperature deduced from ab initio calculations combining with a quasi-harmonic statistical model. The equation obtained is given by: Δ_fG°(T) (NaCrO₂) (kJ/mol O₂)=−876.094+0.191xT (±2%). The temperature of phase transition measured by differential thermal analysis at (819 ± 4) °C is slightly higher than the temperature in the literature attributed to the decomposition of NaCrO₂ into Cr₂O₃ and Na₂O. However, the X-ray diffraction patterns showed only NaCrO₂ phase which didn't confirm the decomposition reaction.