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

In the case of Hg-bearing cuprate high temperature superconductors, th e great flexibility of rock salt layers permits the introduction of su itable cations to substitute for Hg in the Hg-O layers. In the present investigations, we have studied the structural transitions effected t hrough 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 PbxBa 2Ca2 Cu3O8+b (x = 0.05, 0.10 and 0.20; the samples with these concentr ations have been de signated as sample A, B and C respectively), curio us long period polytype-like structures get formed. Some of the long-p eriod polytypes (LPP) observed in the present investigation were found to possess c-periodicities of similar to 31.58 Angstrom, similar to 6 3.16 Angstrom, similar to 107 Angstrom, similar to 139 Angstrom for sa mple A; similar to 228 Angstrom, similar to 246 Angstrom for sample B; and similar to 189 Angstrom for sample C. It is shown that, in additi on to the intergrowth of short period basic structures, a new mechanis m elucidated in the present study involving ordering of Tl, Pb atoms a t 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 simil ar to 228 Angstrom and 246 Angstrom resulting from the transition due to TI, Pb substitution for Kg, which are integral multiples of the c p eriodicities of their basic building blocks, can only become explicabl e in terms of the long range ordering of Tl and Pb atoms.