High-pressure synthesis of the ‘‘zero-charge-reservoir’’ 0223 superconductor in the Sr–Ca–Cu–O system Masako Ushiki, Teruki Motohashi, Hisao Yamauchi, Maarit Karppinen * Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan Accepted 14 November 2001 Abstract We have refined the high-pressure synthesis of the ‘‘zero-charge-reservoir’’ 0223 phase of the Sr–Ca–Cu–O system. Samples were prepared from three different starting cation compositions of Sr 2 CaCu 2 O 6þy , Sr 2 Ca 2 Cu 3 O 8þy and Sr 2 Ca 3 Cu 4 O 10þy . The amount of oxygen, y, was controlled by using different amounts of SrO 2 in the starting material mixture. With SrO 2 of commercial grade, two types of HRTEM images were observed for the a–b plane: one with a double periodicity and another with no periodicity. TEM-EELS analysis revealed that the former was due to carbonate inclusions. Using freshly synthesized SrO 2 , crystalline samples with T c  110 K were obtained. The samples showed body-centered symmetry in ED patterns, being thus of the 0 ðSrÞ 2 ðCaÞ 23 phase. The actual amount of oxygen in the obtained samples as determined by wet-chemical analysis was found considerably lower than the nominal value. However, the wet-chemical analysis revealed a much higher oxidation state for the 0 ðSrÞ 2 ðCaÞ 23 phase than generally expected for a high-T c superconductor. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 74.72.Jt Keywords: Zero-charge-reservoir superconductor; High-pressure synthesis; HRTEM 1. Introduction The ‘‘zero-charge-reservoir’’ superconductive copper oxides, (Sr,Ba) 2 Ca n1 Cu n O 2þ2n [0 ðSr;BaÞ 2 ðCaÞ  ðn  1Þn, are among the least understood high- T c superconductors, as described in general by M m A 2 Q n1 Cu n O mþ2þ2nd ½M  m ðAÞ 2 ðQÞ ðn  1Þn, be- cause of their peculiar blocking block structure and hole-doping mechanism [1]. However, these phases show T c values higher than 100 K for n P 2 [2–7]. The lack of understanding is primarily due to the difficulty in obtaining high-quality samples. The n ¼ 1 compounds, (La,Ba/Sr) 2 CuO 4 , are ob- tained under normal-pressure conditions, but among the 0 ðAÞ 2 ðQÞ ðn  1Þn phases those which do not contain halogen ions and/or trivalent A-site cations are highly oxidized [8], and have been obtained only by means of ultra-high-pressure synthesis (5 GPa) [9]. In previous works, stabi- lization of various zero phases through chlorine and fluorine substitutions has been extensively studied [10–14]. In the present study, we focus on the synthesis of the non-halogen-substituted 0 ðSrÞ 2 ðCaÞ 23 phase of the Sr–Ca–Cu–O system. Physica C 378–381 (2002) 167–172 www.elsevier.com/locate/physc * Corresponding author. Tel.: +81-45-924-5333; fax: +81-45- 924-5365. E-mail address: karppinen@rlem.titech.ac.jp (M. Karppi- nen). 0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0921-4534(02)01404-1