ANOMALOUS BEHAVIOR OF THE ND1.84CE0.16CUO4 SUPERCONDUCTOR

The as-prepared electron-doped system Nd2-XCeXCuO4 (where X approximate to 0.16) is not superconducting. It becomes superconducting only after removal of a minuscule amount of extraneous oxygen (0.02 of O per unit formula). Mossbauer effect studies were carried out for oxygenated and deoxygenated Nd2-XCeXCu(Co-57)O-4 with X=0.14, 0.16, and 0.18. The spectra show evidence of oxygen anions attaching to the probe Co-57 in apical positions, to form 5- and 6- coordinated species. A conventional procedure for deoxygenation brings about little change in the Mossbauer spectra both above and below the optimal superconducting concentration; however, for X=0.16, a dramatic change is observed - a major fraction of the magnetically split five-coordinated species manifests itself as a paramagnetically relaxed doublet upon deoxygenation, which costitutes a microscopic measure of the superconducting volume fraction. This apparently anomalous behavior at X approximate to 0.16, where the extraneous oxygen is more readily desorbed, may be related to an electronic and/or local structural change in the CuO2 plane.