Critical Slowing Down of Orientational Fluctuations in a Plastic Crystal

For the first time a critical slowing down of orientational fluctuations on approaching the order-disorder transition of a plastic crystal has been detected. The observed inverse relaxation rates Γ^-1 in solid CD₄ range from 10^-11 sec (11^∘{} above T₀=27 K) to 2.5×{}10^-10 sec (close to T₀). The temperature dependence of Γ can be fitted by a power law with an exponent ρ=1.13±0.13. For the q dependence of Γ an exponent z=1.90±0.14 is found close to T₀.