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

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 E_a/k_B=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.