Raman studies of hydrogen trapped in As₄O₆·2H₂ at high pressure and low temperature

Raman spectroscopic measurements of the arsenolite-hydrogen inclusion compound As₄O₆·2H₂ 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 As₄O₆·2H₂ 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 Q₁(1) and Q₁(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_g and T_2g symmetries dominated by H₂ stretching modes as predicted by our previous density functional theory calculations. We observed low-frequency modes of As₄O₆·2H₂ vibrations dominated by H₂ "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 As₄O₆·2H₂, 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 H₂ molecules is skewed along the crystallographic ⟨111⟩ direction. Last but not least, low-temperature synchrotron powder x-ray diffraction measurements on As₄O₆·2H₂ revealed that the bulk structure of the compound is preserved down to 5 K at 1.6 GPa.