Diffraction pattern and structure of amorphous solid water at 10 and 77 °K

X-ray diffraction data show that there are at least two forms of amorphous solid water which differ in density and second nearest-neighbor oxygen–oxygen distribution. (a) The lower density form, made at 77 °K, has a diffraction pattern consistent with a structure that has oxygen–oxygen nearest-neighbor tetrahedral symmetry on average, and a nearest neighbor O–O separation of 2.76 Å with small dispersion. The density of this material is estimated to be 0.94 g cm−3. While it is not possible to uniquely define the structure, the data available support the notion that its fundamental characteristic is the existence of a randomized network of hydrogen bonds with O–O–O angular distribution derived from (i.e., centered about) that of ice Ih. Comparison of neutron diffraction and x-ray diffraction data suggests strongly that the first shell hydrogen bonds are nearly linear and that orientational correlations between water molecules are limited to nearest neighbors. (b) The higher density form, made at 10 °K, has a diffraction pattern similar to, yet distinctively different from, that of the high temperature deposit. The O–O nearest neighbor distance is the same, 2.76 Å, but the dispersion in this separation is larger in the low temperature form. The diffraction pattern shows an extra peak at 3.3 Å, corresponding to about 1.4 molecules, the existence of which is responsible for the estimated higher density, namely 1.1 g cm−3. The data are consistent with several models which share the feature of introducing small O–O–O angles into the structure. We discuss the relationships between our data, and inferences from the data, and the corresponding data for liquid water.