CRYSTAL-STRUCTURE, ESR AND CONDUCTIVITY STUDIES OF BIS(ETHYLENEDITHIO) TETRATHIAFULVALENE (ET) ORGANIC CONDUCTOR (H-8-ET)(2)[HG(SCN)(2)BR] AND ITS DEUTERATED ANALOG (D-8-ET)(2)[HG(SCN)(2)BR]

Protonated (I) and deuterated (II) crystals of (ET)(2)[Hg(SCN)(2)Br] (ET = bis(ethylenedithio) tetrathiafulvalene) are isostructural with B2/b space group and Z = 4. Crystal structure consists of (ET)(2)(+) dimers forming a K-type motif and arranged in layers parallel to the bc-plane which are separated by anionic sheets of polymeric anions. A two-positional disorder exists in ethylene groups of deuterated ET molecules, while such disorder appears only in one of the ethylene groups in the hydrogenated salt. The crystals exhibit semimetallic behaviour at high temperatures with a transition to non-metallic state at 100 K for salt I and 60 K for salt II as is shown from d.c. conductivity measurements. Electron spin resonance (ESR) g factor and linewidth and their temperature behaviour are extensively discussed on the basis of existing theories and experimental data for low-dimensional organic conductors. The linewidth is determined by spin-phonon scattering driven relaxation processes which are mediated by Peierls fluctuations in the conducting phase and by spin-lattice relaxation processes below the critical temperature. The first-order phase transition in hydrogenated salt at 100 K is associated with a localization of electrons on ET dimers, whereas the transition of semiconductor-semiconductor type with possible magnetic ordering is suggested for deuterated salt. A considerable isotope effect in (ET)(2)[Hg(SCN)(2)Br] is considered to be related to a weak hydrogen bond between the terminal ethylene group of ET(+) and the bromine atom of the anion in hydrogenated salt. A discussion of the nature of the phase transitions is given.