Formation of amorphous ice by careful annealing of ice XII

Thermally induced transformations of ice XII were characterized using the Raman spectra of the isolated O-D vibration of 5% HDO molecules in H₂O ice XII. Spectra were collected continuously as samples were annealed between 77--160 K. Raman spectra first show a sharp peak centered on Δν=2424cm^-1 from ice XII at 120 K. As the sample is further annealed isothermally at 120 K this peak broadens and shifts to higher frequency Δν=2439cm^-1 and full width at half maximum 65 cm^-1, indicating the production of low-density amorphous ice by decompression amorphization. Ice Ic is formed upon further annealing at 130--140 K. The local hydrogen bond strength of the resulting amorphous phase was found to be close to that of glassy water.