Phoney one-dimensional instability in (TaSe4)2I

We present an x-ray study of the quasi one-dimensional conductor (TaSe 4 ) 2 I. This compound exhibits a Peierls transition at 263 K. which stabilises a lattice distortion at the reduced wave vector (0.05, 0.05, 0.085) (chains run along c axis). The x-ray patterns exhibit diffuse lines running through the Bragg spots, which could be attributed to precursor of the Peierls instability. Computer simulations show that these lines correspond to uncorrelated movements of the chains in the c direction. We relate these movements to the dispersionless acoustic phonon mode, recently evidenced by neutron scattering. We discuss the remarkable fact that this mode, though occurring at wave vectors very close to those of the Peierls instability, is not coupled to the electronic degrees of freedom.