Substitutions in Hg-cuprate superconductors
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Several substitutions have been attempted in the Hg layer of the Hg 1212 and 1223 cuprate superconductors. A new series of (Hg, Bi)1212 superconducting compounds has been synthesized. The highest Tc of the series (92 K) is found for the composition (Hgo.67Bio.33)Sr2(Yo.5Cao.5)Cu207_s, with lattice constants a = 3.811 A and c = 12.002 A. X-ray diffraction and SEM with EDX analysis confirm that Hg is indeed incorporated in the structure. High-pressure oxygenation at 250 bar and 400 °C changes the magnetization curve, adding a second transition at ~42 K; the 92 K transition remains unchanged. High-pressure oxygenation of Pb-substituted composition of the 1223 system, (Hg0.67Pbo.33)Ba2Ca2Cu308, also leads to the formation of low temperature phase with Tc = 67 K. The low Tc phase is associated with the incorporation of extra oxygen, possibly in the Hg layer.
I. INTRODUCTION The discovery of new mercury cuprate superconductors (HgBa 2 Cu0 4 , HgBa 2 CaCu 2 0 6 , and HgBa2Ca 2 Cu 3 0 8 , in particular) by Putilin et al.,1'2 Meng et al.,3 and Schilling et al.4 has given rise to a great deal of interest in these compounds. Their structures are very similar to the corresponding thallium compounds except that there is little oxygen in the Hg-layer. Each of these compounds has a Tc significantly higher than the corresponding Tl-compound, which had been the highest so far. Furthermore, it was shown by Chu et al.5 that the Tc of HgBa 2 Ca 2 Cu 3 0 8 could be raised even higher to 153 K under hydrostatic pressure of 150 kbar. A "1201" Hg-Bi compound, Hg0.5Bi0.5Sr2-^ La x CuO 5 _a, with Tc of 27 K has just been reported by Pelloquin et al.6 A Tl-analogue of 1212 compounds of the series, (Tl0.5Hgo.5)Sr2(Y1_xCax) Cu 2 O7-g, was recently reported by Liu etal.,1 who reported Tc = 92 K for x = 0.7. Recently, Iqbal et al} synthesized Pb-substituted compounds of the series (Hg, Pb)Ba 2 Ca n -!Cu n 0 2n +2 (n = 2 and 3). We have succeeded in making a related series (Hgi- x Bi x ) Sr 2 (Y > ,Cai- ) ,)Cu 2 07-5. Several of these are superconducting. The highest Tc of 92 K is obtained for x = 0.33 and y = 0.5. Unlike HgBa 2 Ca n _ 1 Cu n O 2n+2 , these compounds have oxygen in the mercury layer as in the T1O layer of TlBa 2 CaCu 2 0 7 . This changes the coordination of the mercury ions from 2-fold to 4- or 6-fold, and the effect of this change on the stability of the compound and on Tc is worth investigating. We have also investigated the effect of incorporation of J. Mater. Res., Vol. 9, No. 11, Nov 1994 http://journals.cambridge.org
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additional oxygen by high-pressure oxygenation in the (Hg, Bi) and (Hg, Pb) superconductors. II. EXPERIMENTAL METHODS A. Sample preparation Samples of nominal compositions of (Hgo.67Bio.33) Sr 2 (Y!_ x Ca x )Cu 2 0 7 with x = 0, 0.5, and 1.0 and (Hg0.67Pbo.33)Ba2Can-1Cun02n+2 (n = 3) were prepared in a multistep process. The Bi-compound was made starting with oxides and carbonates, whereas the starting materials for the Pb-compound were nitrates. The mixture was prereacted up to 900 °
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