Study of the phase transitions of (DI-DCNQI)(2)X using vibronic and vibrational spectra

A model for the vibrational and vibronic features of the infrared spectra o f a quarter-filled system is reported and shown to be able to characterise the BOT Sr and CDW ground state of these compounds. The infrared spectra of (DI-DCNQI)(2)Ag and of the corresponding compounds with Li and Cu are anal ysed. In particular for the Ag compound one finds that a 4k(F) BOW begins t o develop from room temperature and that a CDW develops below 200 K. The un derstanding of the periodicity of the low temperature BCDW necessitates fur ther investigations.