Temperature and time dependence of phase formation of HgBa₂Ca_n-1Cu_nO₂(n+1)+δ superconductors

We systematically studied the temperature and time dependence of phase formation of HgBa₂Ca_n-1Cu_nO_2(n+1)+δ [Hg-12(n-1)n, n = 1, 2, 3, 4] superconductors by a single step firing method. All samples were synthesized from high-purity HgO, BaO, CaO, and CuO in a sealed quartz tube. The phase formation of Hg-12(n - 1)n was examined by both x-ray diffraction (XRD) analysis and dc susceptibility measurement. A single-phase HgBa₂CuO_4+δ (Hg-1201) was obtained in a relatively wide temperature range (600-850 °C), while single-phase HgBa₂CaCu₂O₆₊₈ (Hg-1212) and HgBa₂Ca₂Cu₃O_8+δ (Hg-1223) were obtained only in narrow temperature and time ranges for the synthesis. The optimum temperatures for Hg-1212 and Hg-1223 were found to be 665 and 710 °C, respectively, when the sintering time was fixed for about 50 h. The HgBa₂Ca₃Cu₄O_10+δ (Hg-1234) phase was obtained as a mixture with Hg-1223.