Neutron diffraction investigation of phase separation in La2CuO4+y single crystals

La₂CuO_4+y single crystals with y=0.03 and 0.04 are investigated with a high-resolution neutron diffractometer in the temperature range 10<T<293 K. Althouogh the excess oxygen concentration y falls in the range of the dissolution gap, the crystal with y=0.03 does not undergo phase separation (PS)—the oxygen distribution in this crystal remains uniform over the entire experimental temperature interval. Macroscopic PS is observed in the crystal with y=0.04. An analysis of the dependence of the widths of the reflection orders on the interplanar separation gives the sizes of the coherent regions of the two coexisting phases: (970±30) Å in the direction of the tetragonal axis c and (1460 645) Å in the basal plane.