Crystal structure, magnetic susceptibility and thermopower of superconducting and non-superconducting Nd_1.85Ce_0.15CuO_4+y

An experimental study of superconducting and non-superconducting Nd1.85Ce0.15CuO4+y, including structure determination by neutron powder diffraction, recording of oxygen changes by gas volumetry, and susceptibility and thermoelectric measurements, is reported. Difference neutron diffraction patterns from samples prepared on-line at the spectrometer show that the structures of superconducting and non-superconducting samples are identical within the limits set by the statistical errors of our data. Simultaneous gas volumetric measurements reveal that Δy<0.03 (1) when the sample is oxidized from the superconducting to the non-superconducting state. Structural refinements confirm that Nd1.85Ce0.15CuO4+y has the T′-type tetragonal structure reported in the literature, but additional oxygen may be located on the apical O(3) oxygen site of the T-type structure, with a total oxygen content of 4+y=4.03 (5). Consistent with this result, we find very small values of the thermoelectric power indicating that Nd1.85Ce0.15CuO4+y is close to the formal threshold, yc=0.075, between electron and hole conduction, but surprisingly, the thermoelectric power of the superconducting sample is positive, while it is negative in the non-superconducting sample below 210 K. © 1992.