Study of the phase transitions of (DI-DCNQI)(2)M (M=Ag, Li,Cu) through the analysis of the temperature-dependent vibronic and vibrational infrared absorptions

The temperature-dependent phase transitions of 2,5 diiodo-dicyanoquinondiimmine (2,5-DI-DCNQI) title compounds are analyzed by looking at the bond order wave (BOW) and at the charge density wave (CDW) which characterizes the transitions. The analysis is done by studying the temperature-dependent powder infrared spectra which allow one to have a separate evaluation of both types of contribution to the phase transition of these molecular metals. A model for the spectroscopic infrared features related to the transitions is reported and used for the analysis. It is found that all the title compounds develop a BOW structure at low temperatures with characteristics which recall an instability toward a tetramerized 2kF periodicity particularly for the Ag and Li compounds. An appreciable CDW amplitude contribution to the transitions is not found in the spectra.