Direct structural measurements of relaxation processes during transformations in amorphous ice

Sequential structure-factor data have been collected during the transformation from relaxed high-density to low-density amorphous ice at temperatures above 105 K. Trends in the measured structure factor and in particular the height of the first sharp diffraction peak as a function of its position show that, at ambient pressure, this transformation is clearly divided into two distinct structural relaxation trends. The corresponding radial-distribution functions show that the first of these relaxational processes may be described by a continuous depletion of the "interstitial oxygen" located in the 3.6 A region, i.e., between the first- and second-coordination shells of the low-density amorphous form, and a corresponding sharpening of oxygen-oxygen correlations with characteristic lengths of 2.75 and 4.5 A. At the end of this relaxation process, the basic structural features of the low-density amorphous form have been established and are described by well-defined first- and second-coordination shells. This is followed by a second, distinctly different, relaxation process that is characterized by much smaller changes at 2.75 and 3.6 A and a smooth expansion at longer length scales.