X-ray study of phase transitions in alkali tetracyanoquinodimethanides (TCNQ)

The phase transitions in Na-, K-, and Rb-tetracyanoquinodimethanide (TCNQ) single crystals have been investigated by the x-ray critical scattering technique. The intensities of superlattice reflections, which appeared below the phase-transition temperature T_c, decrease with increasing temperature toward T_c and the diffuse scattering intensities observed above T_c increase with decreasing temperature. The intensity distribution of x-ray diffuse scattering in the momentum space shows a very strong anisotropy belonging to the a^* planes, which comes from the one-dimensional crystal instability. These results are discussed in terms of the one-dimensional electron-phonon interaction.