Critical Slowing Down of Orientational Fluctuations and Single Molecule Reorientation Rate at an Order-Disorder Transition of a Molecular Crystal
The relation between single molecule reorientations and critical orientation fluctuations, occurring near structural order-disorder phase transitions in certain molecular crystals, is studied by considering the neutron incoherent scattering function SS(\(\overrightarrow Q \),ω) for a two-site rotational jump model. An expression for SS (\(\overrightarrow Q \),ω) is derived in terms of the single molecule rotational correlation time tR, the mean residence time to of a molecule within an orientational cluster and the fraction C of molecules participating in cluster formation at any given instant. It is concluded that the critical slowing down of the relaxation of orientational clusters near the phase transition should be observable by incoherent neutron scattering via the development of a narrow quasielastic component within SS (\(\overrightarrow Q \),ω).
KeywordsCluster State Molecular Crystal Wall State Jump Rate Incoherent Neutron
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- 1.See for instance : A.J. Leadbetter and R.E. Lechner in “The Plastically Crystalline State”, Ed. J.N. Sherwood, J. Wiley and Sons (1979), p.285–320.Google Scholar