Thermogravimetric study on HgBa₂Ca_n-1Cu_nO_2(n+1)+δ (n = 1, 2, 3) superconductors at various oxygen partial pressures

The oxygen non-stoichiometry of HgBa₂Ca_n-1Cu_nO_2(n+1)+δ (Hg-12(n - 1)n, n = 1, 2, 3) superconductors was studied by thermogravimetric analysis (TGA) in a flowing gas atmosphere with various partial pressures of oxygen ranging from 10^-5 to 10 atm. In 1 atm oxygen, irreversible reactions, probably corresponding to vaporization of Hg, were observed above about 420, 480 and 510°C for Hg-1201, 1212 and 1223 samples, respectively. Reversible reactions with oxygen absorption and desorption were confirmed below these temperatures. By controlling the oxygen partial pressure during the heating process up to 300°C and a subsequent cooling process, we obtained Hg-12(n - 1)n samples with various oxygen contents. The superconducting properties of these samples were also studied. It was found that the oxygen non-stoichiometry of Hg-12(n - 1)n superconductors is very sensitive to the oxidizing atmosphere when they are annealed, even below 300°C.