Structural changes associated with the phase transition in t-butyl cyanide

The structure of the low-temperature phase of t-[²H₉]butyl cyanide [(CD₃)₃CCN] has been determined and refined as a function of temperature between 5 and 232 K (T_tr = 233 K) using powder diffraction data. Although the structure may be refined using a fully ordered model, arguments are given to support an alternative and equally successful refinement involving the onset of disorder similar to that already established in the high-temperature phase. The order parameter is then determined from the unit-cell angle β and its temperature dependence is in reasonable accord with simple mean field behaviour with T_c = 245 K; the transition is shown to be of the order-disorder type. No structural changes have been found near the previously reported transition at 213 K.