Superconductivity and Magnetism in the La2 CuO4-Based Compounds

La2CuO4, crystallizes in an orthorhombic distortion of the tetragonal K2NiF4 structure, a so-called layered perovskite. The important question here concerns the stoichiometry of La2CuO4 prepared by various techniques. Regarding the La and Cu sites, recent neutron diffraction studies of powders have shown that the La:Cu ratio can vary from 2:1 by ≳1% in various preparations., Single crystals have been grown variously from CuO, PbO and Li2O-B2O3 fluxes. The last two fluxes incorporate some Pb and Li, respectively, into the crystals, but the cation ratio is found to be 2:1 within the accuracy of x-ray refinement.It is also known that the oxygen stoichiometry corresponds to La2CuO4 within ≃ 1%. Extensive and precise magnetic susceptibility (χ) and other studies of powder samples showed, however, that the physical properties are extremely sensitive to < 1% changes in the oxygen content. A clear indication from electric and magnetic measurements is that the oxygen stoichiometry is always that, or richer than that, corresponding to a Mott insulator with an exactly half-filled band. Consistent with this, the stoichiometry is more properly written La2-z CuO4-y with small, positive y and z.Several groups found that bulk superconductivity above 30 K can be produced in La2CuO4 by annealing powders or single crystals at high oxygen pressure. The weight gain in this process corresponds to producing La2CuO4.13, and these authors believe the excess oxygens go in as O-2, based on iodometric titration measurements giving the amount of [Cu-O]⊥ in the sample. Jorgensen et al. found that there is a phase separation below 320 K of La2CuO4 and the O2-loaded phase in oxygen-rich samples; the latter structure is also orthorhombic, but the position of the excess oxygens could not be determined unambiguously.