Universal inverse deuterium isotope eect on the T c of BEDT-TTF-based molecular superconductors J.A. Schlueter a, * , A.M. Kini a , B.H. Ward a , U. Geiser a , H.H. Wang a , J. Mohtasham b , R.W. Winter b , G.L. Gard b a Materials Science Division, Argonne National Laboratory, 9700 South Cass Avenue, Building 200, Argonne, IL 60439-4831, USA b Department of Chemistry, Portland State University, Portland, OR 97207-0751, USA Received 22 May 2000; received in revised form 1 September 2000; accepted 11 September 2000 Abstract Deuterium substitution of the ethylene end groups of the ET [ET: BEDT-TTF or bisethylenedithio)tetra- thiafulvalene] electron-donor molecule results in a 0.25 K increase in the superconducting transition temperature of three molecular superconductors derived from this molecule. Simplistically speaking, this change in T c is in con- tradiction to that predicted by the electron±phonon coupling mechanism of the BCS theory. We suggest that the slight shortening of the C±D bond relative to the C±H bond, coupled with the recent ®ndings of a large, negative uniaxial pressure derivative of T c in j-ET) 2 CuSCN) 2 and b 00 -ET) 2 SF 5 CH 2 CF 2 SO 3 , can explain this unique eect. Herein we report the ®rst study of the eect of deuterium substitution on the superconducting transition temperature in a mo- lecular-based superconductor in which the electron-donor molecules are packed in a b 00 motif, viz., b 00 -ET) 2 SF 5 CH 2 - CF 2 SO 3 . This compound is ideally suited for this study because it contains discrete non-polymeric) anions, has a completely ordered structure, is inde®nitely stable in air at room temperature, and is free from possible magnetic impurities. Substitution of the eight hydrogen atoms of the ET molecule by deuterium causes the T c of b 00 -ET) 2 SF 5 - CH 2 CF 2 SO 3 to increase from 4:34 0:05 to 4:61 0:03 K. These values were determined by measuring several rep- resentative crystals from various parallel electrocrystallization experiments containing h 8 - or d 8 -ET that was prepared in parallel syntheses. This is the ®rst example which demonstrates that the inverse positive) isotope eect previously observed in j-phase salts is also present in a b 00 -phase superconductor. Ó 2001 Elsevier Science B.V. All rights re- served. Keywords: Organic superconductors; Superconducting transitions T c ; Bisethylenedithio)tetrathiafulvalene; Penta¯uoroalkylsulfonate anion; Isotope eect 1. Introduction The number of known molecular-based super- conductors is rapidly approaching 100. Over half of these superconductors contain the ET [BEDT- TTF, or bisethylenedithio)tetrathiafulvalene] elec- tron-donor molecule see Fig. 1 for an illustration Physica C 351 2001) 261±273 www.elsevier.nl/locate/physc * Corresponding author. Tel.: +1-630-252-3588; fax: +1-630- 252-9151. E-mail address: jaschlueter@anl.gov J.A. Schlueter). 0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0921-453400)01620-8