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

The as-prepared electron-doped system Nd2-XCeXCuO4 (where X approximat e to 0.16) is not superconducting. It becomes superconducting only aft er removal of a minuscule amount of extraneous oxygen (0.02 of O per u nit 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. Th e spectra show evidence of oxygen anions attaching to the probe Co-57 in apical positions, to form 5- and 6- coordinated species. A conventi onal procedure for deoxygenation brings about little change in the Mos sbauer spectra both above and below the optimal superconducting concen tration; 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 fracti on. 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.