ELSEVIER Physica C 341 348 (2000) 793-794 PHYSICA www.elsevier.nl/Iocale/physc Suppression of the metal to semiconductor transition in bismuth cobaltates: Can cobaltates superconduct ? *S. M. Loureiro ~, D. P. Young ~, R. Jinb, Y. Liub, P. Bordet c, Y. Qind, H. Zandbergena, M. Godinho", M. Nfifiez-Regueiro f, B. Batloggg and R. J. Cava ~ "Department of Chemistry and Materials Institute, Princeton University. Princeton, NJ 08544, USA bDepartment of Physics. The Pennsylvania State University, University Park, P,4 16802, USA CLaboratoire de Cristallographie, CNRS. BP 166,38042 Grenoble Cedex 09, France aDelft University of Technology, Laboratory for Material Science, 2628 ,4l Delft, The Netherlands eDepartamento de Fisica. FCUL, Ed. CI, Campo Grande, 1700 Lisboa, Portugal fCRTBT, BPI 66, 38042 Grenoble Cedex 09, France ZBell Laboratories, Lucent Technologies, Murray Hill, NJ 07974, USA Single crystals of Bi2M~Co2Oy phases with M=Ca, Sr, and Ba were synthesized by the flux method. These compounds are analogous to Bi-2212 superconducting systems with cobalt replacing copper. Due to its strong chemical and structural similarity with the Bi-2212 superconductors, this system provides a unique opportunity for determining which electronic features are essential for superconductivity in the cuprates. The undoped Ca- and St-analogues are semiconducting while the Ba-analogue has a metal to semiconductor transition at -80K. In order to change the carrier concentration ofthes¢ phases, new Pb-substituted single crystals were prepared by a similar method. Resistivity measurements have shown that Pb-doping induces a clear increase in the metallic character of the samples for the Sr- and Ba-analogues. The metal to semiconductor transition in Bi2Ba3Co2Oycan be completely suppressed by Pb-doping, and (Bi,PbhBa3Co2Oy is metallic down to 30inK. The magnetic susceptibility data did not show any evidence of ordering, and the magnetic moment/Co atom was found to be =Ip.B. Resistivity measurements carried out up to 20 GPa have shown that the samples become more semiconducting with the increase of pressure. 1. Introduction In order to investigate whether superconductivity can occur in other non-copper based compounds having half-filled eg level d- bands, we have studied compounds of the Bi2M3Co2Oy type. They are analogous to the cuprate superconductor BieSreCaCu2Oy with (Bi202) layers separated by alternating rock-salt (MO) layers (M, being an alkali-earth cation) and (C002) planes. The Bi202 double layers act as a charge reservoir in the Bi-2212 superconducting cuprates. The main difference between these compounds resides in the fact that (C002) planes now replace the (CuO2) planes. The bismuth cobaltates present a unique opportunity for comparing local environments of transition metal oxide planes in layered cobalt and copper oxides and may provide some insight as to why cuprates superconduct. 2. Results and Discussion The normal-state resistance data for St- and Ba- analogues show clear increases of metallic character with increasing ionic radius of the alkali-earth cation, in agreement with results reported by Tarascon et al. t Undoped, these compounds were either semiconducting (Ca and Sr) or displayed a metal to semiconductor transition near 80K (Ba). Pb-substitution in the Sr- and Ba-analogues induces a clear change in the transport properties. The Sr-analogue becomes metallic down to 80K [SrPbl/3]. Increasing Pb- substitution levels shift the transition further down, to 30K. For Ba, the effect is even more dramatic: sample BaPbl/3 becomes metallic down to 7K while the heavily doped sample BaPbl/1 is metallic down to 30inK (Fig. 1). "Corresponding Author 0921-4534/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. Pll S0921-4534(00)00696-1