STRUCTURAL AND TRANSPORT MEASUREMENTS IN LA1.8SR0.2NIO4+DELTA

Lattice constants, electrical resistivity, and magnetization have been measured as a function of temperature on single crystals of La1.8Sr0.2NiO4+delta, a nickelate analog of lanthanum strontium cuprate, the 40-K superconductor. Diamagnetism was observed at 4.2 K, but not zero resistance. The oxygen concentration was carefully controlled to probe the region close to stoichiometric lanthanum strontium nickelate, from delta = 0.001 to 0.003. The high-temperature tetragonal structure persists down to near 100 K, with semiconductorlike resistivity showing activation energies between 32 and 54 meV, depending on delta. Below 100 K the structure transforms into a low-temperature orthorhombic phase, often displaying distinct, cycling-dependent changes in activation energies. In some specimens the resistance occasionally fell by several orders of magnitude, but recovered to previous values within a few degrees. X-ray-diffraction rocking curves reveal striking changes in the mosaic spread of these specimens as a function of both time and temperature, which helps explain the irreproducibility of the activation energies and the resistance anomalies. Although no further structural changes are observed, the paramagnetic susceptibility goes through a maximum at 20 K. Consistent with previous reports, the magnetic remanence becomes finite below 20 K and the susceptibility decreases, sometimes becoming diamagnetic above 4.2 K. The activation energy in this phase abruptly decreases to only 5 meV, while the resistivity exceeds 10(10) OMEGA cm. No evidence of zero resistance was found.