An infrared spectroscopy study of the phase transition in solid ammonia

The infrared spectra of solid ammonia at different phases and the existence of a metastable phase have been in controversy for the last fifty years. In order to address this problem, we studied the infrared spectra of solid ammonia in an ultrahigh vacuum chamber at distinct temperatures. Having prepared amorphous ammonia at 10 K, we observed a transition from the amorphous phase to the cubic crystalline phase at 57 K; successive re-cooling from 85 K back to 10 K confirms the presence of crystalline ammonia. No metastable phase has been detected.