ELSEVIER Physica C 269 (1996) 354-360 PHYSICA Thermogravimetric study on HgBa2Ca n_ 1CUnO2(n+ 1)+~3 (n = 1, 2, 3) superconductors at various oxygen partial pressures Ayako Tokiwa-Yamamoto *, Atsushi Fukuoka 1, Makoto Itoh 2, Seiji Adachi, H. Yamauchi 3 Keiichi Tanabe Superconductivity Research Laboratory, International Superconductivity Technology Center, 1-10-13 Shinonome, Koto-ku, Tokyo 135, Japan Received 15 July 1996 Abstract The oxygen non-stoichiometry of HgBa2Ca ._ 1Cu,O2(,+ 1)+ 8 (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 arm 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. 1. Introduction It is well known that superconducting properties of high-T c superconducting cuprates significantly de- pend on the oxygen non-stoichiometry. In order to * Corresponding author. Fax: + 81 3 3536 5714, 5717 (inter- national)/5705 (domestic). 1Present address: Tokyo Gas Co., Ltd. Frontier Technology Research Institute, 1-7-7, Suehiro-cho, Tsurumi-ku, Yokohama, 230 Japan. 2 Sumitomo Electric Industries, Ltd., 1-1-3, Shimaya, Kono- hana-ku, Osaka-shi, Osaka, 554 Japan. 3 Tokyo Institute of Technology,Materials and Structures Lab- oratory, Nagatsuta, Midori-ku,Yokohama, 226 Japan. control the oxygen content in the sample, we need to understand the oxygen non-stochiometry as functions of temperature and partial pressure of oxygen (P(O2)). GaUagher et al. [1] reviewed the details of the oxygen non-stoichiometry as measured by ther- mogravimetric analysis (TGA) in various high-T c superconducting cuprates including BaaYCu 306 + ~ These data are essential to synthesize a sample with a desired oxygen content. Since the discovery of the HgBa2Ca(n_ 1)Cu,- 02(,+ 1)+ 8 (Hg-12(n -- 1)n) homologous series [2,3], which exhibits the highest T~ among superconduct- ing cuprates, superconducting and structural proper- ties of the Hg-12(n- 1)n phases have been exten- sively studied [2-17]. Crystal structure of Hg-12(n - 1)n samples is basically the same as the structure 0921'-4534/96/$15.00 Copyright © 1996Elsevier Science B.V. All rights reserved Pll S0921-4534(96)00470-4