Temperature and time dependence of phase formation of HgBa₂Ca_n−1Cu_nO_2(n+1)+δ superconductors

Abstract

We systematically studied the temperature and time dependence of phase formation of HgBa 2 Ca n21 Cu n O 2(n11)1d [Hg-12(n 2 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 2 1)n was examined by both x-ray diffraction (XRD) analysis and dc susceptibility measurement. A single-phase HgBa 2 CuO 41d (Hg-1201) was obtained in a relatively wide temperature range (600-850 ± C), while single-phase HgBa 2 CaCu 2 O 618 (Hg-1212) and HgBa 2 Ca 2 Cu 3 O 81d (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 HgBa 2 Ca 3 Cu 4 O 101d (Hg-1234) phase was obtained as a mixture with Hg-1223.

Chemistry and structure of Hg-based superconducting Cu mixed oxides

“…(b) Synthesis of the higher (n = 2, 3) member of the series occurs through the formation of the lower (n − 1) member at the initial step of the reaction, and this process also requires a longer annealing time. This feature was found in numerous studies, for instance, formation of Hg-1223 occurs according to the following reaction [16]: (c) The equilibrium Hg partial pressure over Hg-1212 (and over Hg-1223) determined in [17,18] is significantly higher in comparison with that for Hg-1201. For instance, Knizek et al [17] determined that P(Hg) for Hg-1212 equals 18 bar at 800 • C, while for Hg-1201 it is 2 bar (P(O 2 ) = 0.5 P(Hg) was assumed).…”

“…Particularly, Tsuchiya and Fueki [21] have prepared pure Hg-1212 samples at a relatively low temperature (625 • C during 24 h) with an excess (0.16 mol%) of Hg in starting mixture to compensate its evaporation. Similar successful synthesis of Hg-1223 described in [16] was conducted at 710 • C for 49 h, and for preparation of Hg-1212 these authors performed annealing at 665 • C for 49 h. It is important to note that the elongation annealing time in the latter studies resulted in a partial decomposition of Hg-1212 and Hg-1223, and this means that the obtained reaction products do not correspond to equilibrium conditions.…”

“…The best samples seem to have been obtained from mixtures of simple oxides [16,20,21,30,31] when relatively low temperature was used. Particularly, Tsuchiya and Fueki [21] have prepared pure Hg-1212 samples at a relatively low temperature (625 • C during 24 h) with an excess (0.16 mol%) of Hg in starting mixture to compensate its evaporation.…”

Photoemission study of electronic states inHgBa2CuO4+δwith various doping levels

“…The detailed procedure of synthesis was described in Ref. 10. The doping level was controlled by the temperature and atmosphere of the postannealing.…”

Synthesis of single-phase HgBa2Ca2Cu3O8+ high-Tcsuperconducting films using the multistep electrolytic process

“…The most successful processes consist of few steps, because the precursor BaCaCu requires larger annealing temperature (∼800 • C) and longer time due to more complicated diffusion paths. Synthesis of higher (n = 2, 3) members of the series occurs through the formation of the lower (n − 1) member at the initial step of the reaction, and this process also requires longer annealing time [8].…”