THE HG-BEARING CUPRATE SUPERCONDUCTORS - THE RELATIONSHIP BETWEEN THE OXYGEN-CONTENT AND T-C IN HG-1212 AND THE EFFECT OF PRESSURE ON THE STRUCTURE AND T-C IN HG-1223

HgBa2CaCu2O6+delta and HgBa2Ca2Cu3O8+delta, which are the n = 2 and n = 3 members of the homologous series HgBa(2)R(n-1)Cu(n)O-2n+2+delta, are superconducting with maximum T-c of 127 and 133 K, respectively. The structural refinements based on powder neutron diffraction data show that the delta values corresponding to the maximum T-c are: 0.22(2) for n = 2 and 0.41(2) for n = 3. The study of the relationship between the oxygen content, the structure, and T-c for Hg-1212 shows that the T-c(delta) is a parabolic function, namely T-c goes through a maximum at delta = 0.22 and thus Hg-1212 exists in the underdoped and overdoped state. The structural mechanism associated with the parabolic variation of T-c is the displacement of the apical oxygen. This oxygen moves first up and then down because the oxygen insertion has two effects of opposite sign on the Ba polyhedron. For Hg-1223 high pressure induces an appreciable increase of T-c (133 K at ambient pressure and 164 K at 40 GPa). The structural determination at high pressure revealed a large shortening of the apical Cu-O distance. The da/adP, and dc/cdP coefficients are -0.0013/GPa, and -0.0042/GPa, respectively, showing that the compressibility is very anisotropic. Since the apical Cu-O distance lies entirely along the c axis, the large anisotropy is mainly due to the shortening of this distance which goes from 2.75 Angstrom at ambient pressure to 2.48 Angstrom at 11 GPa. Also in this case, the increase of T-c with pressure is linked to the position of the apical oxygen.