Raman spectroscopy study of the slow order-order transformation of deuterium atoms: Ice XIX decay and ice XV formation.

The nature of the hydrogen substructure of a deuterated and deuterium chloride (DCI)-doped ice VI sample after cooling at 1.8 GPa has been a topic of recent interest-especially because the novel ice polymorph ice XIX was discovered in the course of such studies. We here investigate deuterated samples containing 5% HO using Raman spectroscopy to probe for transitions associated with rearrangement of D-atoms in ice XIX. The protocol involving heating at subambient pressure (10 mbar) in this study follows closely the one used in our earlier neutron diffraction study. Heating of ice XIX induces a complex cascade of processes involving both ordering and disordering of D atoms. Our Raman spectra demonstrate that the transition sequence is ice XIX → ice VI → ice XV, in accordance with our earlier neutron diffraction result. First signs for ice XIX decay are evident at 100 K, while ice XV build-up is seen only at 108 K and above. Between 100 and 108 K, a transiently disordered D-substructure appears, where at 108 K, ice VI forms from ice XIX and simultaneously decays to produce ice XV-thereby establishing a dynamic equilibrium. Using isothermal, time-resolved Raman spectroscopy in real time, we here determine rate constants, Avrami exponents, and activation energies for both slow processes, ice XIX decay and ice XV build-up. The first transition in this sequence, ice XIX decay, is faster than the second transition, ice XV build-up, so that ice VI accumulates. On the basis of the Johnson-Mehl-Avrami-Kolmogorov data obtained from the isothermal Raman experiment, we additionally report kinetic models for the development of fractions of ices XIX, XV, and VI in non-isothermal heating experiments at different heating rates. These models consider the two coupled first-order transitions as separated processes, where the phase fractions are calculated for incrementally small temperature (or time) steps. These models compare favorably with our previous observations for slowly or rapidly heated ice XIX samples, such as in calorimetry or neutron diffraction experiments.