Polymorphism in vapor deposited amorphous solid water

Abstract

The physical properties of vapor deposited amorphous solid water, (H2O)as, depend strongly on a parameter not investigated before: the state of the vapor prior to deposition on a cryoplate. Supersonic flow conditions give (H2O)as desposits with a very rugged surface, whereas breakup of supersonic flow by a baffle gives transparent, glass-like deposits. These two types of (H2O)as deposits devitrify in the presence of adsorbed gas quite differently: (H2O)as prepared by baffling supersonic flow gives according to DTA on heating only one exothermic peak as expected, whereas (H2O)as prepared by condensing water vapor with supersonic flow characteristics devitrifies at higher temperatures in several steps, the prominent peaks occurring at 160, 164 and 177 K. We have investigated the influence of parameters controlling supersonic flow and condensation in the beam on the devitrification pattern of (H2O)as and conclude that supersonic flow conditions cause the formation of water clusters already in the gas phase. We further suggest that condensation of gas phase clusters on the cryoplate gives a highly porous solid with a network of pores or voids of various sizes, whereas condensation of monomers from baffled flow gives a nonporous solid. The role of adsorbed gas on the devitrification of (H2O)as can be interpreted by assuming first that surface-induced nucleation is delayed in voids or pores by adsorbed gas, and second that macroscopic fractions of the amorphous samples have different void sizes and therefore gas occupancies. In addition, we have found evidence for preferential adsorption of O2. Since (H2O)as is usually prepared by condensation of water vapor flowing out of capillaries, supersonic flow and cluster formation might have been a general feature of (H2O)as preparations. Implications are discussed with respect to reports on properties of (H2O)as. Preliminary experiments with other vapor deposited amorphous molecular solids show that the effect of stepwise devitrification is not restricted to (H2O)as. To relate the state of the vapor to the structure of the amorphous solid, we suggest as a working hypothesis that the structure of (H2O)as prepared by condensation of supersonic cluster beams should better be described in terms of a discontinuous model such as the amorphous cluster model, whereas the structure of (H2O)as prepared by condensation of monomers is as usual described best by the continuous random network model. Intermediate stages between these two extremes depending on the preparative details can be envisaged easily. This hypothesis would imply polymorphism for (H2O)as, and is discussed with respect to other amorphous solids prepared by vapor deposition.