A strong Kohn anomaly in ZrTe_3 is identified in the mostly transverse
acoustic phonon branch along the modulation vector q_P with polarization along
the a* direction. This soft mode freezes to zero frequency at the transition
temperature T_P and the temperature dependence of the frequency is strongly
affected by fluctuation effects. Diffuse x-ray scattering of the incommensurate
superstructure shows a power law scaling of the intensity and the correlation
length that is compatible with an order parameter of dimension n = 2.Comment: 4 pages, 4 figures. accepted at Phys. Rev. Let