THE DIELECTRIC BEHAVIOR OF VAPOR-DEPOSITED AMORPHOUS SOLID WATER AND OF ITS CRYSTALLINE FORMS

The dielectric permittivity and loss of vapor-deposited amorphous solid water (ASW) have been measured for fixed frequencies of 1 and 10 kHz from 80 K to its crystallization temperature. Similar measurements have also been made on the cubic ice formed after the crystallization of ASW and the hexagonal ice formed on heating the cubic ice. The loss tangent shows a broad sub-T(g) relaxation peak centered at about 100 K and an approach towards a plateau value which appears as a shoulder. The peak is attributed to thermally activated rotation of H2O molecules with one or two dangling OH groups on the surface of the pores of the microporous sample, and the shoulder to localized motions within the network structure. Sintering of the samples on thermal cycling between 77 and almost-equal-to 120 K in vacuo causes the broad peak to vanish. With increase in temperature, above T(g), the loss tangent shows the emergence of the expected alpha-relaxation peak of a liquid at T > T(g), whose completion is terminated by the onset of crystallization to cubic ice at about the same temperature for 1 kHz and for 10 kHz measurements. Thereafter, crystallization becomes slower, thus allowing further observation of the low-temperature part of the alpha-relaxation peak. The dielectric loss of amorphous solid water at 80 K is nearly 20 times that of the cubic ice formed on its crystallization after heating to 193 K, and nearly 3.5 times higher at T(g). The dielectric loss of the cubic ice formed on crystallization tends towards a plateau value prior to rapidly increasing with increase in temperature, an evidence for a low temperature relaxation which vanishes on conversion to hexagonal ice. This relaxation indicates a remnant topologically disordered structure of intergranular water and/or stacking faults in cubic ice. Samples sintered by thermal cycling in vacuo to almost-equal-to 120 K crystallized in one step, whereas those without prior thermal cycling crystallized in two steps to cubic ice. Annealing of the ASW at 130 K completely removes the broad peak at almost-equal-to 100 K and reduces the magnitude of the beta-relaxation process. Partial crystallization of ASW completely removes the beta-relaxation process, which indicates either preferential crystallization at the local regions of high mobility in the amorphous matrix or a change in the structure of the amorphous phase during crystallization.