A NEUTRON-SCATTERING STUDY OF THE QUASI-ONE-DIMENSIONAL CONDUCTOR (TASE4)(2)I

The Peierls phase transition in the quasi-one-dimensional conductor (T aSe4)(2)I is investigated by means of elastic and inelastic neutron sc attering. The effective critical exponent beta, extracted from the tem perature dependence of the integrated intensity from the CDW satellite reflections, is anomalously low, suggesting that the phase transition may be of first order. The intensity distribution among symmetry-rela ted satellite reflections indicates a domain structure with slowly flu ctuating domain populations. Correlation lengths associated with the d iverging 'central peak' are determined and are found to be nearly isot ropic, at variance with results obtained on other quasi-one-dimensiona l compounds, such as platinum chains (KCP) or blue bronze, K0.3MoO3. D oping with 1.2% Nb has a severe effect on the modulated state. The low -temperature satellites are replaced by a diffuse scattering distribut ion elongated along c. The absence of a phonon soft mode and the pres ence of a diverging central peak at the phase transition is interprete d within the framework of strong electron-phonon coupling. Finally, we propose a Ginzburg-Landau phenomenological model, where the interplay between the electronically coupled optical-like order parameter (Ta-a tom tetramerization along the chain axis) and the elastic deformations lies at the origin of the phase transition in (TaSe4)(2)I.