LiF addition to (Cu,C) Ba 2 Ca 3 Cu 4 O y superconductor P. Badica a,b , A. Crisan a,b , M. Hirai a,c , A. Iyo a,d, * , H. Kito a,d , G. Aldica b , Y. Tanaka a,d a NeRI, AIST, Central 2, Tsukuba, Ibaraki 305-8568, Japan b Nat. Inst. Mat. Phys., POB MG-7, Bucharest, Romania c Science University of Tokyo, 2641 Yamazaki, Noda, Chiba, Japan d CREST, JST, Kawaguchi, Saitama 332-0012, Japan Abstract Samples of (Cu 0:6 C 0:4 )Ba 2 Ca 3 Cu 4 O y ((Cu,C)-1234) with addition of 0.1 mol LiF have been synthesized by high pressure technique. It was found that LiF promotes the formation of superconducting phases with n higher than 4 of the (Cu,C)Ba 2 Ca n1 Cu n O y series. In order to obtain single-phase samples, for the same synthesis conditions, the oxygen content in the starting mixture (supplied by AgO) should be increased from 0.45 to 0.73 mol AgO, when using LiF. Samples with LiF have shown a certain morphology suggesting an enhanced 2D and suppressed 3D type of growth assisted by a liquid phase. T c decreased from 116.5 to 113 K for the sample with LiF, but at the same time the transition width also decreased and J c has shown a fish-tail effect. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: (Cu 0:6 C 0:4 )Ba 2 Ca 3 Cu 4 O y ; LiF; Morphology; Critical current density; Critical temperature 1. Introduction Several features of (Cu 1x C x )Ba 2 Ca 3 Cu 4 O y ((Cu,C)- 1234) [1] make this material interesting for applications; T c is close to 117 K even in the over-doped region [2], H irr versus (1  T =T c ) is relatively high [3,4] and (Cu,C)-1234 does not contain any toxic or volatile element. These advantages are shadowed by the fact that synthesis is difficult (usually under high pressure) and the absolute J c values are not superior to other HTS (Tl-, Hg-, Y-, Bi- based systems) suggesting that in (Cu,C)-1234 there are less pinning centers [4]. Therefore, search for additions or substitutions en- hancing synthesis properties and/or J c is of interest. In this paper we present our data on synthesis and char- acterization of (Cu,C)-1234 with addition of LiF. 2. Experimental Samples with starting composition (Cu 0:6 C 0:4 )Ba 2 - Ca 3 Cu 4 O y with and without addition of LiF have been prepared by high pressure method. We have used a precursor powder with composition Ba 2 Ca 2:7 Cu 4:6 C x O y prepared by solid-state route (890 °C/24 h/O 2 -flow) from Ba and Ca carbonates and CuO. The carbon content x in the precursor powder determined by FT-IR was 0.1. One mol of precursor was mixed with 0.3 mol CaCO 3 . For the samples without LiF, addition of 0.45 mol AgO with the role of oxidizing agent was necessary in order to obtain single phase while for the samples with 0.1 mol LiF the optimum amount of AgO was found to be 0.73 mol. After sealing in the Au-capsules, all samples were treated at 980 °C for 2 h under a pressure of 3.5 GPa. * Corresponding author. Address: NeRI, AIST, Central 2, Tsukuba, Ibaraki 305-8568, Japan. Tel.: +81-298-61-5435; fax: +81-298-61-5447. E-mail address: iyo-akira@aist.go.jp (A. Iyo). 0921-4534/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-4534(02)02535-2 Physica C 388–389 (2003) 395–396 www.elsevier.com/locate/physc