Phase separation to antiferromagnetic and superconducting domains in oxygen-rich single-crystal ${\mathrm{La}}_2$${\mathrm{CuO}}_{4+\mathit{x}}$: Magnetic and structural neutron-diffraction studies

We report on magnetic and structural neutron-diffraction studies of single-crystal ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4+\mathit{x}}$ versus temperature. The low-temperature diffraction data are consistent with the coexistence of two structurally related phases: nearly stoichiometric, antiferromagnetic (AF) domains, with average domain diameter ∼500 Å in the ${\mathrm{CuO}}_2$ plane, and oxygen-rich, metallic, and superconducting domains. The ordered magnetic moment in the AF domains does not saturate at 10 K, and is on average, μ=(0.3±0.05)${\mathrm{\mu }}_{\mathit{B}}$/Cu atom. The phase-separation transition to these two phases occurs at ${\mathit{T}}_{\mathrm{PS}}$=260±5 K, which is about 15 K higher than the Néel temperature, ${\mathit{T}}_{\mathit{N}}$=245±3 K. The value of ${\mathit{T}}_{\mathrm{PS}}$ is identical within the errors with that (259±2 K) found previously from specific-heat measurements on the same crystal. The structural properties of the single crystal are discussed and compared to those of polycrystalline oxygen-rich ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4+\mathit{x}}$.