SYNTHESIS AND STRUCTURE OF ZETA-(BEDT-TTF)2(I3)(I5) AND (BEDT-TTF)2(I3)(TIL4) - COMPARISON OF THE ELECTRICAL-PROPERTIES OF ORGANIC CONDUCTORS DERIVED FROM CHEMICAL OXIDATION VS ELECTROCRYSTALLIZATION

Chemical oxidation of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or ET, C10S8H8) in benzonitrile solution by slow diffusion of iodine vapor produces a mixture of black lustrous crystals. These include the well-known a- and 0-(ET)2I3 phases, δ-(ET)I3, and ε-(ET)2(I3)(I8)0.5, as well as a new phase, ζ-(ET)2(I3)(I5). Single-crystal X-ray diffraction studies show that this latter salt is monoclinic, P21/a, with a = 15.113 (4) A, b = 15.993 (6) A, c = 18.159 (9) A, β =100.99 (3)°, Vc = 4308 (2) A3, and Z = 4. The crystal structure consists of two-dimensional sheets containing both ET donor molecules and I3-anions, and these mixed ET-I3-sheets are separated by layers of V-shaped I5- anions. This ζ-phase is structurally very similar to ε-(ET)2(I3)(I8)05, Four-probe resistivity measurements of single crystals of ε-(ET)2(I3)(I8)0.5and ζ-(ET)2(I3)(I5) both prepared by the present chemical oxidation method show that, although these materials are metallic to temperatures as low as 1.7 K, they do not undergo transitions to the superconducting state. These findings were confirmed by radio-frequency (rf) field penetration depth measurements of the same samples, which failed to detect superconductivity down to temperatures of 0.5 K, Band electronic structure calculations based on the crystal structure parameters of ε-(ET)2(I3)(I8)0.5and ζ-(ET)2(I3)(I5) predict semiconducting properties for stoichiometric crystals. Therefore, it is proposed that the observed metallic properties result from the presence of anion vacancies producing slightly nonstoichiometric crystals. A new salt (ET)2(I3)(T1I4) was prepared by electrochemical oxidation, and a crystal structure study reveals that it is monoclinic, P21/n, with a = 8.348 (3) A, b = 15.294 (8) A, c = 34.217 (16) Å, β = 93.61(3)°, Vc = 4360 (3) A3, and Z = 4. As in the case of ε-(ET)2(I3)(I8)0.5and δ-(ET)2(I3)(I5), (ET)2(I3)(T1I4) consists of mixed two-dimensional sheets containing both ET donor molecules and I3-anions, which are separated by layers of tetrahedral T1I4-ions. The salt is a semiconductor, in agreement with our band structure calculations. © 1987, American Chemical Society. All rights reserved.