SUPPRESSION OF SUPERCONDUCTIVITY IN HG-1223 AND HG-1234 BY PARTIAL REPLACEMENT OF HG BY CARBON

Hg based 1223 and 1234 phases were prepared by a high-pressure high-te mperature technique using precursors obtained by the decomposition of BaO2, CaCO3 and CuO in oxygen flow at 925 degrees C. These phases were found to exhibit much lower T-c's (64 K for 1223 phase, and no superc onductivity for the 1234 phase) than the corresponding Hg based sample s prepared by different precursors and this even after heat treatments in oxygen flow. The structural refinement of this 1223 phase carried out by using synchrotron X-ray powder data revealed a large deficiency on the Hg site; the occupancy factor for this site was found to be 0. 69(1). This may be due to the replacement of some of the Hg cations by C. By IR spectroscopy the characteristic frequencies corresponding to the vibrations of the CO3-2 group were found in these samples. Large amounts of intergrowth defects were detected by electron microscopy an d no superstructure spots were found in the diffraction patterns. The replacement of Hg by C in the structures of the Hg based superconducto rs induces a decrease of the concentration of the extra oxygen in the (Hg, C) layer bonded to the Hg atoms. Since it is this oxygen which ox idizes the (CuO2) layers, its decrease causes a reduction of these lay ers, which is responsible for the decrease of the superconducting tran sition temperatures.