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

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 $\mathrm{(\Delta v̄)}$ 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 ${\mathrm{(r}}_{\mathrm{OO}})$ 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 $\mathrm{\Delta v̄}$ at $r_{\mathrm{OO}}\text{=2.35±0.06\hspace{0.17em}}\mathrm{\AA }.$ 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 ${\mathrm{\Delta v̄\approxeq A/r}}_{\mathrm{OH}}^2$ which when combined with present and previously published data yields, approximately, $\text{A≊2943\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\hspace{0.17em}{\mathrm{\AA }}^2.$ Thus, the decrease observed in $\mathrm{\Delta v̄}$ is thought to occur because the OH bond length ${\mathrm{(r}}_{\mathrm{OH}})$ increases with increasing pressure as the O–O distance $r_{\mathrm{OO}}$ in the corresponding hydrogen‐bonded O–H⋅⋅⋅O unit decreases.