Optical properties of organic conductor (ET) ₄Hg ₃I ₈ : alternative type of electron-vibrational interaction

(Received 6 December 1993, revised 1 June 1994, accepted 7 July 1994)

Abstract
The reflectivity spectra of single crystals of (ET) ₄Hg ₃I ₈ and (d ₈-ET) ₄Hg ₃I ₈ were studied. In contrant with earlier studied clorine- and brominemercurate compounds of the series (ET) ₄Hg - X ₈ (X = Cl, Br, I), the spectra of iodinemercurate compounds exhibit low reflectivity without well-defined plasma edge and without electron-vibrational bands in the region 1 000 - 1 400 cm ^-1. The latter fact may be due to electron-vibrational interaction with non-totally symmetric modes which may originate from peculiarities of crystal structure of iodinemercurate compounds. A phase transition of the first type is observed both for (ET) ₄Hg ₃I ₈ and (d ₈-ET) ₄Hg ₃I ₈. At the transition temperature a jump in conductivity and reflectivity is observed.