The miscibility gap of ${\mathrm{La}}_2$${\mathrm{CuO}}_{4+\mathrm{\delta }}$ was investigated by studying three electrochemically oxygenated samples with different oxygen contents, using variable-temperature neutron powder diffraction. The low-temperature lattice parameters of the two coexisting orthorhombic phases (antiferromagnetic, Bmab and superconducting, Fmmm) were found to be in good agreement with those determined for high-pressure annealed samples, indicating that the two techniques yield essentially identical samples, at least in this concentration range. A sample with δ≊0.032 remained phase separated up to 415 K and, above this temperature, was found to be single-phase tetragonal (F4/mmm). The phase separation temperature and the orthorhombic-to-tetragonal phase transition temperature were obtained for all samples, allowing the shape of the miscibility gap to be determined.

• Received 5 November 1993

DOI:https://doi.org/10.1103/PhysRevB.49.6239