Under different temperature and pressure conditions, ice can form with a variety of different lattice structures. Although we freeze it in cubes, what we can find in our home freezers is a hexagonal form of ice, usually called ice Ih. However, for a long time, scientists have been trying to form a phase of ice at ambient pressure whose structure is actually cubic. This crystalline form of ice, named Ic, whose presence in the atmosphere was inferred by some experimental observations, had, until recently, never been obtained as a pure sample via a synthesis performed in the laboratory.
The ice Ic synthesis route and its crystalline structure were determined in recent work mainly based on neutron diffraction measurements on HRPD beamline and on the D20 diffractometer at ILL. Now, once again, neutrons prove to be an essential tool to the researchers who want to learn more about these ice phases. This time, the group used TOSCA to study the vibrational dynamics of the water molecules arranged in the cubic structure Ic.
In their study, published in the Journal of Physical Chemistry C, they heated a powder sample of ice XVII, an emptied C0 hydrate that is metastable at ambient pressure, to 150 K, to transform it into pure cubic ice Ic. The large amount of sample made available by this synthesis route, and the high-level capability of the TOSCA spectrometer, allowed the researchers to take an accurate measurement of the inelastic spectrum in the energy transfer range 0-130 meV.
They measured both ice Ic and ice Ih, where the latter was directly obtained by warming the same cubic ice sample up to 268 K. By a rigorous data treatment, the team was able to extract the H-projected density of phonon states (H-DOPS) for both ice forms, and to quantitatively compare them with advanced quantum simulations, known as centroid molecular dynamics (CMD).
The combined effect of the high resolution in the experimental data and the reliability of the CMD computational method have also allowed them to perform a clear assignment of the spectral bands and to determine some of the phonon dispersion curves for both ice I phases.
“This work represents one of the most accurate attempts to investigate phonons in ice I and opens the door for further studies concerning not only the dynamics of proton-disordered cubic ice, but also that of its postulated ordered form," explains Leonardo del Rosso, from the Consiglio Nazionale delle Ricerche.
Further information
Related publication: “Density of Phonon States in Cubic Ice Ic" Journal of Physical Chemistry C, just Published, (2021), DOI: 10.1021/acs.jpcc.1c07647
