Stability and thermodynamic functions of lanthanum nickelates

Enthalpy of formation, entropy, and heat capacity of La3Ni2O7, La4Ni3O10, and LaNiO3, which belong to the Ruddlesden-Popper homologous series were investigated using equilibration with the gas phase, adiabatic calorimetry, and differential scanning calorimetry. All three compounds remain almost stoichiometric in their stability range, while the established energetic trends are consistent with decreasing thermal stability in the order La3Ni2O7 > La4Ni3O10 > LaNiO3. The enthalpy of the structural phase transition in La3Ni2O7 at 490 K was determined. The derived thermodynamic functions were used to construct the phase stability diagram of the La–Ni–O system in terms of temperature and oxygen pressure.