Calorimetric study of proton ordering in hexagonal ice catalyzed by hydrogen fluoride

Heat capacities of hexagonal ices doped with 2.6, 26 and 260 m mol dm^?3 HF were measured with an adiabatic calorimeter. The HF doping accelerated proton ordering which has been known to take place sluggishly around 100 K. The ice containing 26 m mol dm^?3 HF showed the largest excess entropy ((0.102±0.01) J K^?1 mol^?1) and the shortest relaxation time. The relaxation time at 90 K was about $\text{1}\text{30}$ of that of the pure ice I_h at the same temperature. The activation enthalpies obtained were the same for all of the doped ices, (23.5±2.0) J mol^?1, which is approximately equal to the activation energy of the mobility of the Bjerrum L-defect.