ESR study of the phase transitions in organic conductors (H /sub 8/-ET) /sub 2/[Hg(SCN) /sub 2/Br] and (D /sub 8/-ET) /sub 2/[Hg(SCN) /sub 2/Br]

Summary form only given. Electron spin resonance g-factor, linewidth, and magnetic susceptibility of bis(ethylene dithio)tetratiafulvalene (H/sub 8/-ET) and its fully deuterated analogue (D/sub 8/-ET) with polymeric anion [Hg(SCN) /sub 2/Br]/sup -/ have been studied vs temperature and single crystal orientation. Pronounced temperature dependences of ESR parameters were observed between 4 and 300 K. A first order phase transition with the localization of unpaired electrons on ET dimers was found at about 100 K for the hydrogen-containing compound, whereas two phase transitions of semiconductor-semiconductor type and possible magnetic ordering at low temperatures were found for the deuterium salt. The drastic isotope effect in these salts is considered to be associated with a weak hydrogen bond between terminal C-H group of the cation radical and Br atom of the anion in hydrogen-containing compound. Discussion of the temperature - and orientation - dependence of the ESR parameters and the nature of the phase transition is given.