X-ray diffraction study of the quasi-one-dimensional conductors (MSe4)2I (M=Ta and Nb)

Resistivity and X-ray diffraction measurements have been made for the quasi-one-dimensional conductors (MSe 4 ) 2 I (M=Ta and Nb). (TaSe 4 ) 2 I is known to undergo a phase transition at T c approximately=260K. A phase transition of (NbSe 4 ) 2 I was found to take place at T c approximately=210K. Non-linear conduction was observed below T c . Superlattice reflections due to the formation of a charge-density wave (CDW) were observed at eight points around fundamental Bragg spots in the reciprocal space. The CDW is known to be formed along only one direction; therefore the appearance of eight peaks instead of two is caused by the formation of domains. The condensed phonon was revealed by the measurements on (TaSe 4 ) 2 I to be a transverse acoustic mode that polarizes in the c plane. Diffuse scattering shows that these compounds are not in the class of systems with extreme anisotropy.