Raman Spectra of NH₄-TCNQ-I and -II

Abstract

Polarized Raman spectra of NH₄-TCNQ-I and -II have been observed in the temperature range including phase transition temperatures T_c=301 and 215 K, respectively. The intensity of low frequency modes decreases rapidly with approaching the phase transition temperature from below. Mode assignment has been made on low frequency modes in the low temperature phase of two crystals. A comparison among Raman spectra of NH₄-TCNQ-I and alkali metal-TCNQ salts reveals that NH₄-TCNQ-I resembles K-TCNQ in the stacking arrangement of the TCNQ molecules in the chains. The intensity variation is discussed in terms of the change of Raman selection rule and Raman polarizability by the dimerization of the TCNQ molecules.