Communications 54 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim,2000 0935-9648/00/0101-0054 $ 17.50+.50/0 Adv. Mater. 2000, 12, No. 1 Halides of BET-TTF: Novel Hydrated Molecular Metals** By Elena Laukhina, Elisabet Ribera, JosØ Vidal-Gancedo, Salavat Khasanov , Leokadiya Zorina, Rimma Shibaeva, Enric Canadell, Vladimir Laukhin, Markus Honold, Moon-Sun Nam, John Singleton, Jaume Veciana, and Concepció Rovira* The monatomic halide anions Cl ± , Br ± , and I ± are prom- ising counterions for the preparation of novel molecular metals and superconductors based on cation-radical salts of organic donors. Indeed, some organic metals containing halide anions were among the first to demonstrate the ex- pected potential of low-dimensional metals. For instance, the (TSeT) 2 Cl and (TSeT) 2 Br (where TSeT is tetraseleno- tetracene) salts were found to be the first quasi-one-di- mensional organic metals with very high conductivities at room temperature (s > 2500 S cm ±1 ); they undergo a first- order phase transition under pressures of 5 kbar and 2 kbar, respectively, to new phases with metallic behavior down to 1.3 K and residual resistivities of about 10 ±5 W cm. [1±4] The salt (BEDT-TTF) 3 Cl 2 ×2H 2 O, where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene, was the first organic metal with a crystallized solvent (water) to be discovered that exhibits a superconducting transi- tion below 3.5 K under hydrostatic pressures between 10.2 kbar and 13.5 kbar. [5±7] It was recently shown that all chlorides of BEDT-TTF and its analogues such as bis(ethylenedioxo) tetrathiafulvalene (BEDO-TTF) and bis(butylenedithio) tetrathiafulvalene (BBDT-TTF) form a family of hydrated radical cation salts. [8] The anions of the known chloride salts of BEDT-TTF or BEDO-TTF or BBDT-TTF are not monatomic units but coordinated complexes, including Cl ± anions, water molecules, and/or hydroxonium cations. [8] In such cases, the structure of the anion component of the crystal is of crystallochem- ical interest. These precedents raise the question of whether it is possible to obtain crystalline radical cation salts based on BEDT-TTF or other TTF derivatives with bare chloride (or bromide) counterions. In previous investigations, we prepared hydrated mixed- valence BET-TTF (bisethylenethio-tetrathiafulvalene) bro- mides possessing a constant spin susceptibility down to 100 K, but the poor quality of the samples precluded trans- port measurements. [9] Nevertheless, because of the promis- ing Pauli-like behavior of the susceptibility, indicating me- tallic properties, we decided to pursue the study of this family of radical cation salts and prepare BET-TTF radical salts with Br ± or Cl ± anions. In order to clarify the effect of crystallized solvent on the electrical properties of the BET-TTF halides, we synthe- sized these salts by electrochemical oxidation of BET-TTF in chlorobenzene (PhCl) with traces of H 2 O, MeOH, or EtOH. The conditions for electrocrystallization were similar to those used for other salts derived from BET-TTF. [10] The features of the synthesis of the BET-TTF halides and their electrical properties are summarized in Table 1. As can be seen, BET-TTF bromides were obtained as black needles and/or semitransparent brown plates. In contrast, electro- crystallization of BET-TTF with chloride ions gave only semitransparent brown plates. BET-TTF bromide needles and plates show the same electron spin resonance (ESR) pa- rameters at room temperature, which indicates that they are the same phase in spite of their different morphologies. Furthermore, the same cell parameters were obtained for BET-TTF bromide crystals grown with traces of water, methanol, or ethanol and with different morphologies. As explained below, the X-ray crystal structures of both BET- TTF bromide and chloride salts have been determined, giv- ing the same composition for both, i.e., (BET-TTF) 2 X× 3H 2 O (X = Br, Cl). This suggests that these salts form the same crystalline phase, which includes water in the struc- ture, regardless of the presence of other guest solvents. Moreover, bromide crystals grown in pure PhCl gave the same phase, as evidenced by conductivity, ESR, and visible to near-infrared (vis-NIR) data, but the quality of the crys- tals was very poor. The electronic spectra of both bromide and chloride salts show a very wide ªAº band centered around 4346 nm (Fig. 1, Table 2), which indicates that they are mixed valence salts. The similarity of the two NIR spec- tra (Fig. 1) is in agreement with the isostructural character of the two salts (see below). Therefore, the BET-TTF donor forms hydrated cation radical salts with monatomic halide anions, a rare instance amongst organic donors. [8] Single crystals suitable for X-ray analysis were obtained for both bromide and chloride salts. According to our data [11] the salts are isostructural, as suggested by their electronic and transport properties (see below). The struc- tural features will be described for the bromide salt. The crystal structure (see Fig. 2a) is characterized by layers of the BET-TTF radical cation alternating along the a-direc- tion with honeycomb-like polymeric anion networks. There are two donor molecules per unit cell. The BET-TTF radi- ± [*] Dr. C. Rovira, Dr. E. Laukhina, Dr. E. Ribera, Dr. J. Vidal-Gancedo, Prof. E. Canadell, Dr. V. Laukhin,Prof. J. Veciana Institut de Cincia de Materials de Barcelona, UAB Campus UAB, E-08193 Bellaterra (Spain) Dr. E. Laukhina, Dr. V. Laukhin Institute of Problems of Chemical Physics, RAS 142432 Chernogolovka (Russian Federation) Dr. S. Khasanov, L. Zorina, Dr. R. Shibaeva Institute of Solid State Physics, RAS 142432 Chernogolovka (Russian Federation) M. Honold, M.-S. Nam, Dr. J. Singleton Clarendon Laboratory, Department of Physics, University of Oxford Oxford OX1 3PU (UK) [**] This work was supported by DGES-Spain (project nos. PB96-0872-01 and PB96-0859), Generalitat de Catalunya (project nos. 1997 SGR-24 and 1998 SGR-0106), the Russian Foundation for Basic Research (project no. 97-03-32828 a ), the Russian National Program ªPhysics of quantum and wave processesº, and NATO (project no. CRG.LG.974316). V.L. is grateful to Foundation BBV for a Fellowship at ICMAB and E.R. thanks CIRIT for a Fellowship.