Lattice dynamics in solid oxygen

Lattice dynamical calculations for the bulk α, β, and γ phases of solid O2 and for the monolayer α and β phases have been made in the harmonic approximation. In the α and β phases, atom-atom 6−12 potentials are employed. In the γ phase, effective potentials are used between molecular centers and only the translational lattice vibrations are calculated. It is found that Laufer and Leroi’s potential parameters give two k=O frequencies at 42.7 and 43.6 cm−1 in the bulk α-O2, and at 40.7 cm−1 for the degenerate k=0 modes in the β phase. The observed Raman lines for α-O2 at 43 and 79 cm−1, which are both known to exhibit isotope shifts, are thus tentatively assigned to an accidentally degenerate line and a two-phonon band, respectively, In view of the possible contribution from anharmonic effects, the agreement of the calculation with experiment (48-51 cm−1) in β-O2 may be better than it seems. For the bulk γ-O2, a discrepancy is observed between the calculated elastic constants and those derived from Brillouin scattering experiments. This discrepancy may be due to the neglect of translation-rotation coupling. In the monolayer O2, Raman active modes at 28.3 and 40.6 cm−1 for the α phase, and 31.9 cm−1 for the β phase are predicted.

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