NEUTRON AND SYNCHROTRON X-RAY POWDER-DIFFRACTION STUDY OF LA2CUO4+DELTA

A detailed synchrotron x-ray and neutron powder-diffraction study has been carried out in the temperature range 10 500 K on an oxygen-rich superconducting (Tc 35 K) sample of La2CuO4 prepared under an oxygen pressure of 3 kbar. The value of was determined to be 0.03 0.01 by a combination of thermogravimetric and mass-spectrometric techniques. Two phase transitions were found, generally similar to those reported in a recent neutron-diffraction study of this system by Jorgensen et al. [Phys. Rev. B 38, 11 337 (1988)]. At 297 K, the sample was single phase with orthorhombic symmetry (space group Bmab, a=5.357, b=5.389, c=13.175), but with a significant amount of anisotropic broadening of the x-ray-diffraction peaks. Upon cooling a reversible phase transformation was found to set in at 290 K, characterized by the appearance of a second orthorhombic phase in coexistence with the first, with slightly different b and c, but identical a, unit-cell dimensions. Between 290 and 250 K, the amount of second phase increased steadily until the two phases were present in roughly equal proportions. No further change was observed down to 10 K. At the latter temperature, the symmetries of the two phases were consistent with space groups Bmab and Fmmm, respectively, as reported by Jorgensen et al.; surprisingly, however, it is the Bmab phase that evolves below 290 K, as shown by the temperature dependence of the unit-cell parameters. Upon heating above 297 K, there was a phase transformation from orthorhombic to tetragonal symmetry (space group I4/mmm) at about 410 K. No discontinuity in the temperature dependence of the a dimension could be detected between 10 and 410 K. Details of the structures at 297 and 10 K were determined by the Rietveld refinement technique. © 1990 The American Physical Society.