HIGH-PRESSURE SYNTHESIS OF HG-1234 AND STRONGLY-OVERDOPED HG-1223 PHASES

The Hg-1223 and Hg-1234 phases were synthesized using the program-cont rolled high pressure chamber. The variation of the oxygen content in t he starting oxides mixture 'HgBa2Ca3Cu4Ox', obtained by changing BaO/B aO2 and Cu2O/CuO ratios, influences significantly the phase formation. By varying the pressure (P-s), temperature (T-s), time (t(s)) and oxy gen content (x), the optimum synthesis conditions of the Hg-1234 phase were determined with P-s = 2 GPa, T-s = 900 degrees C, t(s) = 2 h and x = 10.2. Using the initial mixture with the high oxygen content (x = 10.6) only Hg-1223 with the lattice constants a = 3.8474(9) Angstrom, c = 15.79(1) Angstrom was obtained. Hg-1223 thus prepared is strongly overdoped and exhibits superconductivity at 118 K, while the treatmen t in oxygen flow enhanced T-c up to 134 K and cell parameters to a = 3 .852(1) Angstrom, c = 15.84(1) Angstrom. Dependence of T-c vs. a-param eter (and consequently, oxygen content delta) for the HgBa2Ca2Cu3O8+de lta superconductor has a cupola-shaped character, as found previously for the lower members of the homologous series. (C) 1997 Elsevier Scie nce B.V.