From a one-dimensional crystal to a one-dimensional liquid: A comprehensive dynamical study of C60 peapods

We report an inelastic neutron-scattering investigation of the longitudinal acoustic modes of C-60 chains confined inside single walled carbon nanotubes. We take advantage of the orientations of the chains within the plane of the pellet sample to isolate their scattering signatures in the (Q,omega) space, which we follow as a function of temperature from 260 K up to 1100 K. The results show the progressive evolution of the confined chain from a one-dimensional (1D) crystal to a linear liquid, the transition occurring within a temperature range of similar to 150 K centered around 600 K. The comparison of the data obtained on monomer and polymer peapods allows extracting the speed of sound in the monomer crystalline chains (v(mono) = 3.5 km s(-1), v(poly)/v(mono) = 1.7). We find that the sound velocity is further reduced by half in the liquid state which reveals that the melting is not only due to harmonic additive thermal fluctuations, but that anharmonic terms in the intermolecular potential play an important role at high temperatures.