Longitudinal lattice modes in linear mixed valence compounds

Mixed valence compounds of platinum or palladium have a crystal structure that consists of long, linear chains. The frequencies of the three collective (phonon) longitudinal k ≈ 0 optical modes bear little relevance to those of longitudinal modes of the independent molecules, being different in general and not in a one to one correspondence with them. Therefore, they have to be taken into account as such when the observed intensity peaks in resonance Raman or infrared spectra of these compounds are analyzed.