Physica C 212 (1993) 271-273 North-Holland PHYSICA Increase of Tc in HgBa2CuOy by reducing treatment Makoto Itoh, Ayako Tokiwa-Yamamoto, Seiji Adachi and H. Yamauchi Superconductivity Research Laboratory, lnternationaISuperconductivity Technology Center, 10-13Shinonome l-chome, Koto-ku. Tokyo 135, Japan Received 13 May 1993 The superconductivity of HgBa2CuOr annealed in various reducing as well as oxidizing atmospheres was investigated. The as- sintered HgBa2CuOespecimens prepared by solid state reaction starting with mono-oxides of Hg, Ba and Cu exhibited supercon- ductivity at 90 K. It was found that the superconductivity critical temperature (To) increased up to 96 K after a reducing treat- ment of the as-sintered specimen, whereas annealing of the specimen in high-pressure oxygen gas resulted in a decrease of To. 1. Introduction Since the discovery of high-To superconducting cuprates [ 1 ], many studies on a variety of related compounds have been reported. It has been revealed that the two-dimensional CuO2 sheet is essential for high-Tc superconductivity, and Tc's are dependent on various factors such as the number of CuO2 sheets between the charge reservoir blocks (e.g. (Bi-O)2, T1-O, Cu-O chain), the carrier concentration in the sheet and so on. One of the most important factors is the coordination number of the Cu ion in the sheet. The values for Tc of superconducting cuprates which contain CuO6 octahedra sheets are ~40 K, except for that of TIzBa2CuO6, i.e. To~90 K [2,3]. Putilin et al. [ 4 ] synthesized the Hg-based "1212" cuprates, HgBa2LnCu207 (Ln = La, Nd, Eu, Gd, Dy, Y) and demonstrated the utility of a Hg-O mono- layer as a new charge reservoir block. Recently, they found that a Hg-"1201" cuprate, HgBa2CuOy, showed superconductivity at 94 K [ 5 ]. This value of T~ is as high as that of TIzBazCuO6 compared with those of other cuprate superconductors which con- tain CuO6 octahedra sheets. They investigated its crystal structure by X-ray powder diffraction, and oxygen content, y, by iodometric titration. The Hg ion in HgBazCuOy was found to have a "stick"-type coordination, i.e. coordination number=2, and the oxygen content of the sample, y, was determined to be ~4.08. They speculated that the excess oxygen ions of 0.08 per formula unit were located in the Hg layer and introduced holes into the CuO2 sheets for superconductivity to occur. However, Putilin et al. did not report superconducting properties of other samples with different amounts of oxygen. As a pre- liminary study, we investigated the post-annealing effect on the superconducting properties of Hg- BazCuO.v samples in various atmospheres. 2. Experimental Samples with a stoichiometric nominal composi- tion of HgBa2CuOy were prepared by a conventional solid state reaction method using powders of HgO (yellow, 99.99%), BaO (99%) and CuO (99.99%). Mixing was performed in a dry box to prevent the powders from absorbing CO2 and moisture. Pelle- tized samples were placed in a gold tube and then encapsulated in a quartz tube evacuated to a pres- sure of approx. 10 -4 Torr. The samples were heated at 700-750°C for 15-60 min. Subsequently, the samples were annealed for 6 h in flowing Ar gas or in high-pressure oxygen gas of 270 arm. The prod- ucts were examined by X-ray powder diffraction (XRD) with Cu Kct radiation, and the unit cell di- mensions (for the resultant Hg-" 1201" phase) were determined by a least squares method. Weight changes of the samples before and after the anneal- ing were checked. The DC magnetic susceptibility 0921-4534/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.