Knowledge Bank

It has been proposed that low temperature phase transformation in hydrogen-bonded solids such as ice depend on the concentration and mobility of orientational (Bjerrum L) point defects. It defect activity is necessary for the growth of crystallinc ice from other phases of water (such as liquid water or amorphous solid water), it is important to understand the behaviour of orientational defects in these systems. In this study, the isotopic scrambling of $D2O$ molecules isolated in amorphous $H2O$ ice by mobile point defects has been used as probe of defects mobility at temperatures below the glass transition temperature. The sequential passage of defects through sites in the ice lattice initially occupied by $D2O$ molecules results in the formation of spectroscopically distinguishable deuterated species in the ice lattice. From the infrared spectra of these samples, the change in concentration of these sepctroscopically distinguishable species is then followed with time and over a range of temperatures, enabling the determination of kinetic parameters relating to defect mobility. A mechanism for the isotopic scrambling process in amorphous ice below the glass transition temperature has been proposed. This mechanism involves point defect motion to explain the experimentally observed changes in concentration of deuterated species with respect to time. The isotopic exchange data seems to indicate a luck of significant molecular diffusional motion (fluidity) in amorphous ice at temperature just below the glass transition temperature. This finding is inconsistent with the recent conjecture that molecular diffusional motion plays a significant role in the glass transition which occurs in amorphous ice at approximately 130 K $\mathrm{<mrow\; data-mjx-texclass="ORD">1,2</mrow>}$