Cooperative dynamics in the disordered phase of tertiary butyl cyanide

The cooperative dynamics in the high temperature disordered phase of tertiary butyl cyanide have been investigated using coherent inelastic neutron scattering techniques. Diffuse scattering peaks have been observed near Brillouin zone centres corresponding to Bragg reflections which are systematically absent in the disordered tetragonal phase but present in the low temperature ordered monoclinic phase and show clearly that local correlations exist which are closely similar in structure to that of the monoclinic phase. The extent of correlation of the local monoclinic structure within the disordered phase is approximately 2 unit cells and the correlation time for the cooperative relaxations among the four local structures is approximately 10 -12 s. The uniaxial C 3 molecular rotations present in both phases are strongly coupled with the fluctuations in the local structure and there is a suggestion that the two motions become decoupled on approaching the transition. Transverse and longitudinal acoustic phonons propagating in the [110] and [100] directions have been measured and the initial slopes to the dispersion curves are presented. A highly overdamped soft mode, assigned to the librational motion of the molecule, has been observed across the whole of the zone.