Preparation of polycrystals with various T c and single crystal growth of Ba 2 Ca 3 Cu 4 O 8 (O 1y F y ) 2 under high pressure A. Iyo a, * , M. Hirai b , K. Tokiwa c , T. Watanabe c , Y. Tanaka a,c a AIST and CREST, AIST Central 2, Tsukuba, Ibaraki 305-8568, Japan b Department of Physics, Tokyo University of Science and CREST, Chiba 278-8510, Japan c Department of Applied Electronics, Tokyo University of Science and CREST, Chiba 278-8510, Japan Received 13 November 2002; accepted 7 February 2003 Abstract Polycrystalline samples of Ba 2 Ca 3 Cu 4 O 8 (O 1y F y ) 2 (0234F) with a variety of T c (55–105 K) were synthesized under high pressure. The dynamic range of the T c 50 K in an underdoped region, has been found to be remarkably wide. We also found that plate-like single crystals of 0234F (1 mm 2 in plane area and 30 lm in thickness) grow when samples with a nominal composition of y  1:0 are heated at high temperature (1150 °C). Cleavage was observed for the crystals. The 0234F will be a convenient material for investigations of physics of the multi-layered superconductors and/ or the underdoped state that has a key for understanding high-T c superconductivity. Ó 2003 Elsevier B.V. All rights reserved. PACS: 74.25.Fy; 74.62.)c; 74.72.Jt; 74.62.Bf Keywords: High pressure synthesis; Single crystals; Ba 2 Ca 3 Cu 4 O 8 (O 1y F y ) 2 ; Multi-layered cuprate; Underdoped state 1. Introduction A series of Ba 2 Ca n1 Cu n O 2n (O 1y F y ) 2 (02(n)1) nF, n P 3) is typical multi-layered cuprates that has crystallographically inequivalent CuO 2 planes in a unit cell. As shown in Fig. 1 for 0234F, its crystal structure is simply composed of an alter- nate stacking of a superconducting block (SCB) with an infinite-layer structure and a charge res- ervoir block (CRB) with a rock-salt one along the c-direction. Recently, unique physical properties of the multi-layered cuprates that originate from inhomogeneous distribution of the carrier (hole) concentration between the inequivalent CuO 2 planes are being clarified [1]. In (Cu,C)Ba 2 Ca 3 - Cu 4 O y ((Cu,C)-1234), inequivalent CuO 2 planes seem to have own T c (two-T c ) due to the large in- homogeneous distribution [2,3]. In five-layered compound of HgBa 2 Ca 4 Cu 5 O y (Hg-1245), a co- existence of superconductivity and antiferromag- netism has been reported by the NMR and lSR studies [4,5]. The 02(n)1)nF superconductor has unique features as follows: the carrier concentration ðT c Þ is controlled by an anion ratio of O 2 to F  at apical sites, and the system strongly prefers an under- doped state [6,7]. Even in an underdoped state, it * Corresponding author. Tel.: +81-298-61-5435; fax: +81- 298-61-5447. E-mail address: iyo-akira@aist.go.jp (A. Iyo). 0921-4534/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-4534(03)01109-2 www.elsevier.com/locate/physc Physica C 392–396 (2003) 140–144