Phase transitions of (1:1) alkaline salts of TCNQ as studied by vibronic intensity enhancement in the infrared spectra

The results of a study of the temperature dependence of the infrared absorption spectra of M+TCNQ− (M+=Na+, K+, Rb+, Cs+, and NH4+) salts are reported and exemplified in detail for the case of triclinic RbTCNQ(II). On lowering the temperature, the infared absorptions which have already been recognized as due to vibronic activation of the totally symmetric modes of the TCNQ anion display a striking intensity enhancement. The plot of the vibronic band intensities against temperature displays an abrupt variation at nearly the same transition temperatures reported in the literature and determined by magnetic susceptibility and dc conductivity measurements. This result suggests that the vibronic intensity enhancement follows the distortion of the regular TCNQ anion stacks in the high temperature phases to alternating stacks in the low temperature ones.