La₂CuO_{4 + δ}: Synthesis Under High Oxygen Pressure and Study of Phase Relations and Energetics

High oxygen pressures have been achieved in a piston-cylinder apparatus using a double capsule assembly consisting of a sealed outer Au capsule, containing an oxygen source (KMnO₄), and an inner, open Pt capsule containing the sample. Using this technique, La2Cu04 was annealed at 800 °C, 5-25 kbar for 2-4 h. Transposed temperature drop calorimetry at 704 °C was used to determine the enthalpy of oxidation, and weight loss measurements characterized the oxygen nonstoichiometry, δ, in La₂CuO_{4 + δ}in the high-pressure, oxygen-annealed samples. For samples analyzed at room temperature, x-ray diffraction measurements show that beyond δ ≈ 0.10-0.13, additional oxygen is accommodated in a perovskite-like LaCuO_3-αphase. An analysis of the thermochemical measurements indicates that the nature of holes in La₂CuO_4+scould change in the range of δ ≈ 0.03-0.06.^16,17It is further suggested that the observed change in the thermochemical behavior in the range of δ ≈ 0.03-0.06 could be the driving influence behind the spinodal decomposition of La₂CuO_4+δat low temperatures (Dabrowski et al.¹⁰).