Effects of carbon or carbonate impurity in the starting material BaO on the sintering temperature and the superconductivity for HgBa2Can-1CunO2(n+1)+d (Hg-12(n-1)n, n = 1, 2 and 3) were investigated. In the solid state reaction of HgO, BaO, CaO and CuO in the sealed quartz-tube, the upper sintering temperatures to obtain single-phase of Hg-12(n-1)n (n = 1, 2 and 3) increased with decreasing the carbon content in the BaO powder. According to our recent study, the carbon content was determined from the correlation with the lattice parameter. This suggests that an existence of carbon or carbonate impurity significantly influenced the phase formation. The optimal sintering temperatures starting from the BaO with almost minimized carbon content (approximately 0.15 mol%) were 800 °C, 725 °C and 775 °C, for the Hg-1201, -1212 and -1223, respectively. An increase in the sintering temperature led to the larger grain and better crystallinity, resulting in the excellent superconductivity. Improvements in superconducting volume-fraction, Tc and pinning properties were clearly observed in the Hg-1201 prepared from the high-purity BaO. This work indicates that the control of carbon content in the starting materials is one of the key factors to obtain high-quality Hg-based superconducting oxides.
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