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.

Generation of methane in the Earth's mantle: In situ high pressure–temperature measurements of carbonate reduction

“…The hydrocarbon was clearly identified as methane based on the sharp band at 2,972 cm Ϫ1 , matching the position of bulk methane at this pressure (20). The broad feature near 3,200 cm Ϫ1 is caused by O-H stretching in ice VII (21). This sample exhibited considerable heterogeneity; Raman measurements of laser-heated experiments were typically challenging because after heating the samples were finegrained and fluorescent, and a layer of opaque material across the diamond surface often obscured the interior of the sample chamber from optical measurements.…”

Phase diagram of ice in the VII–VIII–X domain. Vibrational and structural data for strongly compressed ice VIII

“…Because of the convenience of working at room temperature, there have been numerous investigations of ice VII by Raman scattering, 6,9,27,41 infrared absorption, 7,24,25,46 -49 xray diffraction 9,41,42,50 -54 and neutron diffraction up to 26 GPa. 55,56 Owing to the pressure-temperature shape of the stability domain of ice VIII, very high-pressure investigations must be performed at least down to 100 K. With Raman scattering, apart from the investigation performed in the Geophysical Laboratory (Carnegie Institute) for determining the VII-X transition, where the general behaviour of the various modes of ice VIII was given to 62 GPa at 20 K, 23 currently only partial data are available.…”

Ab initiostudies on high pressure phases of ice

“…Despite considerable discussion in the literature [4][5][6][7][8][9], there is to date no compelling direct evidence for the existence of ice X. Experimentally, Raman spectra of ice VIII up to 50 GPa at 100 K demonstrate the appearance of a new band above 40 GPa [4] which could reflect such a transition; and Brillouin scattering studies up to 67 GPa at 300 K also give an indication of a transition at 44 GPa from an anomaly in the behavior of the longitudinal sound velocity [5]. Earlier work based on extrapolation of Raman spectra from lower pressures at room temperature also suggested that a symmetric hydrogen-bond structure might form in ice VII at pressures of about 75 ± 20 GPa [6].…”