PHASE-SEPARATION TO ANTIFERROMAGNETIC AND SUPERCONDUCTING DOMAINS IN OXYGEN-RICH SINGLE-CRYSTAL LA2CUO4- MAGNETIC AND STRUCTURAL NEUTRON-DIFFRACTION STUDIES(X )

We report on magnetic and structural neutron-diffraction studies of si ngle-crystal La2CuO4+x versus temperature. The low-temperature diffrac tion data are consistent with the coexistence of two structurally rela ted phases: nearly stoichiometric, antiferromagnetic (AF) domains, wit h average domain diameter approximately 500 angstrom in the CuO2 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, mu = (0.3 +/- 0.05)mu(B)/Cu atom. The phase-separation trans ition to these two phases occurs at T(PS) = 260 +/- 5 K, which is abou t 15 K higher than the Neel temperature, T(N) = 245 +/- 3 K. The value of T(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 La2CuO4+x.