NEUTRON-DIFFRACTION STUDY OF PHASE-SEPARATION IN LA2CUO4-CRYSTALS(DELTA SINGLE)

Single crystals of La2CuO4+delta with delta = 0.03 and delta = 0.04 ha ve been studied in the temperature region 10 K less than or equal to T less than or equal to 293 K using a high resolution neutron diffracto meter. In both crystals, the concentration of excess oxygen is inside the miscibility gap. However, the La2CuO4.03 crystal does not undergo a phase separation - homogeneous oxygen distribution was preserved ove r the entire temperature region studied, while in the La2CuO4.04 cryst al, the phase separation phenomenon, connected with excess oxygen diff usion was observed. Analysis of the diffraction peak width as a functi on of interplane spacing allowed us to determine the dimensions of the coherent regions of coexisting phases: 975 +/- 22 Angstrom in the dir ection of the tetragonal axis of the high temperature phase, and (1460 +/- 160) Angstrom in the perpendicular direction. Our data provide di rect evidence that both the oxygen-rich and oxygen-poor regions have t he same Cmca space symmetry.