On the structural transition of the Hg–Ba–Ca–Cu–O HTSC phase to long-period polytypic structures as a result of Tl, Pb Doping On Hg sites

Abstract

In the case of Hg-bearing cuprate high temperature superconductors, the great flexibility of rock salt layers permits the introduction of suitable cations to substitute for Hg in the Hg-O layers. In the present investigations, we have studied the structural transitions effected through substitution of Tl and Pb in the Hg-O layers. It has been found that for the doped compositions corresponding to Hg0.50Tl0.50-x Pb x Ba2Ca2 Cu3O8+δ (x = 0.05, 0.10 and 0.20; the samples with these concentrations have been designated as sample A, B and C respectively), curious long period polytype-like structures get formed. Some of the long-period polytypes (LPP) observed in the present investigation were found to possess c-periodicities of ∼31.58 Å, ∼63.16 Å, ∼107 Å, ∼139 Å for sample A; ∼228 Å, ∼246 Å for sample B; and ∼189 Å for sample C. It is shown that, in addition to the intergrowth of short period basic structures, a new mechanism elucidated in the present study involving ordering of TI, Pb atoms at Hg sites in the Hg-O layers, will have to be invoked for making the genesis of LPP, resulting from structural transitions originating due to Tl, Pb doping on Hg sites, intelligible. Thus the LPPs with c∼228 Å and 246 Å resulting from the transition due to Tl, Pb substitution for Hg, which are integral multiples of the c periodicities of their basic building blocks, can only become explicable in terms of the long range ordering of Tl and Pb atoms.