Raman studies of hydrogen trapped in AsO·2H at high pressure and low temperature.

Raman spectroscopic measurements of the arsenolite-hydrogen inclusion compound AsO·2H were performed in diamond anvil cells at high pressure and variable temperature down to 80 K. The experimental results were complemented by ab initio molecular dynamics simulations and phonon calculations. Observation of three hydrogen vibrons in AsO·2H is reported in the entire temperature and pressure range studied (up to 24 GPa). While the experiments performed with protium and deuterium at variable temperatures allowed for the assignment of two vibrons as Q(1) and Q(0) transitions of ortho and para spin isomers of hydrogen trapped in the inclusion compound, the origin of the third vibron could not be unequivocally established. Low-temperature spectra revealed that the lowest-frequency vibron is actually composed of two overlapping bands of A and T symmetries dominated by H stretching modes as predicted by our previous density functional theory calculations. We observed low-frequency modes of AsO·2H vibrations dominated by H "librations," which were missed in a previous study. A low-temperature fine structure was observed for the J = 0 → 2 and J = 1 → 3 manifolds of hydrogen trapped in AsO·2H, indicating the lifting of degeneracy due to an anisotropic environment. A non-spherical distribution was captured by molecular dynamics simulations, which revealed that the trajectory of H molecules is skewed along the crystallographic ⟨111⟩ direction. Last but not least, low-temperature synchrotron powder x-ray diffraction measurements on AsO·2H revealed that the bulk structure of the compound is preserved down to 5 K at 1.6 GPa.