Inelastic incoherent neutron scattering study of the pressure dependence of ice VII and VIII

The structural pressure dependence of the high-pressure ices VII and VIII has been measured up to 20 GPa by neutron powder diffraction. Measurements of the atomic thermal motion reveal that, while it is strongly reduced by pressure in both phases, the stepwise change in the oxygen thermal motion at the VIII/VII transition is pressure independent within the precision of the measurements. Also, the O-D bondlength in ice VIII changes by less 0.002 •ð to 20 GPa. These results suggest that the site-separation of the disordered oxygen atoms in ice VII and the direction of the disorder displacement remain closely similar to those found at 5 GPa.

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“…scattering measurements of the density of states[4]. The steps in Uiso(O) and Uperp(D) thus provide the first direct,…”

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confidence: 96%

Structures of Ice VII and Ice VIII to 20GPa.

Nelmes

Loveday

Marshall

et al. 1998

THE REVIEW OF HIGH PRESSURE SCIENCE AND TECHNOLOGY

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“…2). Finally, inelastic incoherent neutron scattering experiments on ice VIII show that lattice translational frequencies are 20% lower at ambient pressure [7,8] than at 2.2 GPa, implying an apparent pressure coefficient of 20 cm 21 GPa 21 , in sharp contrast with its wellestablished value of 7 cm 21 GPa 21 for pressures above 3 GPa [1][2][3]. In principle, this problem might have been resolved by optical studies (Raman infrared) between 0 and 3 GPa at low temperature.…”

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confidence: 99%

Structural Instability in Ice VIII under Pressure

Besson¹,

Klotz²,

Hamel³

et al. 1997

Phys. Rev. Lett.

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Below 125 K, tetragonal ice VIII remains in a metastable form at ambient pressure. This form has long been considered to be identical to ice VIII, although unexplained discrepancies exist between their vibrational spectra. Neutron diffraction studies reveal an isostructural phase transformation below 2 GPa related to a weakening of the bonds between the two D 2 O sublattices of ice VIII. In this new phase, ice VIII 0 , the tetragonal distortion is 20% larger than in ice VIII, which accounts for the differences between the vibrational frequencies of the two phases. [S0031-9007 (97)02963-3] PACS numbers: 62.50. + p, 61.12.Ex, 63.20. -e, 64.70.KbThe pressure-temperature phase diagram of water ices is extremely complex. The various solid forms of H 2 O and D 2 O have been and still are the subject of intense activity in experimental and theoretical physiochemistry. These studies are also of relevance to planetary science since water ices under a few GPa are major constituents of the larger ice moons such as Titan and Ganymede. Modeling of these bodies requires, among other things, a precise knowledge of their structural properties which govern their mechanical and thermodynamic behavior.A remarkable feature of water ices is the large number of metastable modifications which can be retrieved at ambient pressure and low temperature. In addition to ice I h , at least eight solid phases can be generated by pressurizing to 2-3 GPa and pressure quenching at low temperature. One of these is ice VIII, which is the ordered form of the higher-pressure ices. It is stable above 2 GPa at 265 K and can be retrieved at ambient pressures at temperatures below ϳ125 K. Hence, ice VIII has been studied outside of the high-pressure apparatus at ambient pressure, and the properties of such samples have been generally considered as representative of the stable highpressure modification.It has nevertheless long been noted [1] that between 2 GPa and ambient, strong nonlinearities occur in various properties. Raman-active vibron frequencies, for instance, measured at 3 GPa and above [1-3] do not extrapolate down to the ambient pressure values obtained from retrieved samples [4] (Fig. 1). The same is true for the phonon modes [1,2]: An extrapolation of the infrared active translational frequencies from high-pressure values down to ambient [3,5] shows a ϳ20% discrepancy with respect to the retrieved sample values [6] (Fig. 2). Finally, inelastic incoherent neutron scattering experiments on ice VIII show that lattice translational frequencies are 20% lower at ambient pressure [7,8] than at 2.2 GPa, implying an apparent pressure coefficient of 20 cm 21 GPa 21 , in sharp contrast with its wellestablished value of 7 cm 21 GPa 21 for pressures above 3 GPa [1-3]. In principle, this problem might have been resolved by optical studies (Raman infrared) between 0 and 3 GPa at low temperature. However, this has not been possible below ϳ1 GPa because devices such as diamond anvil cells do not allow full decompression of a sample at low temperature: Even w...

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“…Below 125 K, this phase can be quenched to ambient pressure, and the corresponding solid was believed to be identical to D2O VIII [2]. However, the extrapolation to zero pressure of the phonon and vibron modes measured above 2 GPa [3] does not coincide with the frequencies observed in metastable ice VIII, indicating significant structural changes in the pressure range 0-2 GPa at low temperatures. Measurements in this P-T domain require gasous helium to drive the piston of the Paris-Edinburgh cell equipped with a Bridgman unsupported area seal as described in ref.…”

Section: Resultsmentioning

confidence: 97%

High Pressure Neutron Studies to 25GPa: Some Recent Results.

Klotz

Besson

Hamel³

et al. 1998

THE REVIEW OF HIGH PRESSURE SCIENCE AND TECHNOLOGY

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We review some results recently obtained using the Paris-Edinburgh cell for high pressure neutron studies up to 25 GPa, both structural and lattice dynamical studies: Low temperature (85 K) structural studies on recovered D2O ice VIII in the pressure range 0-7 GPa, experiments to 23 GPa to resolve the high pressure structure of YbD2, and inelastic neutron scattering experiments on single cristalline Ge and Zn to study their transverse acoustic phonon frequencies to 10 GPa.[neutron diffraction, inelastic neutron scattering, Paris-Edinburgh cell]

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Inelastic incoherent neutron scattering study of the pressure dependence of ice VII and VIII

Cited by 18 publications

References 28 publications

Structures of Ice VII and Ice VIII to 20GPa.

Structures of Ice VII and Ice VIII to 20GPa.

Structural Instability in Ice VIII under Pressure

High Pressure Neutron Studies to 25GPa: Some Recent Results.

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