Raman and x-ray investigations of ice VII to 36.0 GPa

Abstract

Raman data for ice VII to 30 GPa and x-ray lattice parameters to 36 GPa, all at room temperature, are presented and discussed. Both the Raman OH-stretching peak frequency (Δv̄) and the edge distance of the body-centered cubic unit cell (a) decrease at a decreasing rate with pressure rise. The OH peak frequency is found to be virtually linear in the nearest-neighbor O–O distance (rOO) between about 2.5 and 2.9 Å. However, the data can also be fitted with a slight cubic dependence, which when extrapolated, gives rise to a minimum in Δv̄ at rOO=2.35±0.06 Å. This is close to the value of 2.4 Å found for the symmetric O–H–O units in other materials. This minimum suggests, but does not prove, that a symmetric hydrogen bond structure O–H–O, and thus a new ice, may result at pressures of about 75±20 GPa. Treatment of the O–H⋅⋅⋅O units according to couplet oscillators using experimentally confirmed assumptions indicates that Δv̄≊A/rOH2 which when combined with present and previously published data yields, approximately, A≊2943 cm−1 Å2. Thus, the decrease observed in Δv̄ is thought to occur because the OH bond length (rOH) increases with increasing pressure as the O–O distance rOO in the corresponding hydrogen-bonded O–H⋅⋅⋅O unit decreases.