Phase transition in metal hexammine complexes—i the I.R. spectra of Ni(NO3)26NH31

Far i.r. spectra of Ni(NO₃)₂6NH₃ were recorded above and below the order-disorder phase transition at T_c = 243 K. The results are compared to the spectra of the isomorphous crystal NiCl₂6NH₃, which has no transition in the same temperature range. The antisymmetric stretching vibration of the ammonias shows a broken degeneracy below T_c for Ni(NO₃)₂6NH₃. No broken degeneracy was observed in the nickel-nitrogen vibration of the |Ni(NH₃)₆|²⁺ clusters. The discussion of our results lends support to the assumption that the phase transition is due to the collective freezing of the degrees of rotation of the ammonias.