Synthesis, structure and superconducting properties of the (Hg_1 xRe_x)Sr₂(Nd_1 yCa_y)Cu₂O₆₊ system

Abstract

The synthesis, structure and superconducting properties of the (Hg 1−x Re x )Sr 2 (Nd 1−y Ca y )Cu 2 O 6+δ system (with 0 x 0.2 and 0.2 y 0.6) have been investigated. Near single phase compositions with x = 0.2 and 0.2 y 0.6 may be prepared by solid state reaction at room pressure. The maximum T c = 100 K in this system is found for the nominal composition (Hg 0.8 Re 0.2 )Sr 2 (Nd 0.6 Ca 0.4 )Cu 2 O 6+δ . Rietveld analysis of time-of-flight neutron powder diffraction data from a sample with the refined composition Hg 0.78 Re 0.22 Sr 2 Nd 0.59 Ca 0.41 Cu 2 O 6.88 indicates that the rhenium cation in the (Hg,Re)O δ layer of this material is hexavalent and six coordinate. On the basis of x-ray and neutron powder diffraction data, the Hg and Re cations in the (Hg,Re)O δ layer of Hg 0.78 Re 0.22 Sr 2 Nd 0.59 Ca 0.41 Cu 2 O 6.88 appear to be randomly distributed, in contrast to the ordered nature of these cations in similar materials prepared by high pressure methods. The irreversibility field of essentially optimally doped Hg 0.78 Re 0.22 Sr 2 Nd 0.59 Ca 0.41 Cu 2 O 6.88 indicates no enhancement of the kind reported in similar materials prepared under high pressure. We suggest that this difference may be associated with (dis)ordering of the Hg and Re cations in the (Hg,Re)O δ layer of these materials.