Neutron scattering study of the methyl-group reorientations and the low temperature phase transition in (CH3)2 SnF2

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Abstract
The reorientation of the methyl groups in solid (CH3)2SnF2 has been investigated by neutron quasielastic scattering. It is found that a phase transition occurs in the solid at about 70 K, which might be of second order. The shape of the measured quasielastic lines can be described by instantaneous threefold jumps of the methyl groups. From the derived residence times as a function of temperature below the phase transition, an activation energy Ea/kB=250 K and a residence time at infinite temperatures τ0=0.10 ps have been determined. Above the phase transition the activation energy is at least a factor 2 smaller.