J. S. Loveday

Stable methane hydrate above 2 GPa and the source of Titan's atmospheric methane

Methane hydrate is thought to have been the dominant methane-containing phase in the nebula from which Saturn, Uranus, Neptune and their major moons formed. It accordingly plays an important role in formation models of Titan, Saturns largest moon. Current understanding assumes that methane hydrate dissociates into ice and free methane in the pressure range 1–2 GPa (10–20 kbar), consistent with some theoretical and experimental studies. But such pressure-induced dissociation would have led to the early loss of methane from Titans interior to its atmosphere, where it would rapidly have been destroyed by photochemical processes. This is difficult to reconcile with the observed presence of significant amounts of methane in Titans present atmosphere. Here we report neutron and synchrotron X-ray diffraction studies that determine the thermodynamic behaviour of methane hydrate at pressures up to 10 GPa. We find structural transitions at about 1 and 2 GPa to new hydrate phases which remain stable to at least 10 GPa. This implies that the methane in the primordial core of Titan remained in stable hydrate phases throughout differentiation, eventually forming a layer of methane clathrate approximately 100 km thick within the ice mantle. This layer is a plausible source for the continuing replenishment of Titans atmospheric methane.

Neutron powder diffraction above 10 GPa

A new pressure set-up is described, which allows powder neutron diffraction measurements and full structure refinements to be performed well above 10 GPa. Examples of results are given for D2O to 8 GPa, LiD to 10 GPa and Fe to 15 GPa.

Metastable ice VII at low temperature and ambient pressure

Ice exhibits many solid-state transformations under pressure, and also displays a variety of metastable phases. Most of the high-pressure phases of ice can be recovered at ambient pressure provided that they are first cooled below about 100 K. These ice polymorphs might exist on the surfaces of several satellites of the outer planets. One of the few exceptions to this (meta)stability on quenching has been ice VII, the dominant high-pressure phase. Here we show that isothermal compression of D2O ice VI below 95 K produces pure ice VII, and that this phase can remain stable at atmospheric pressure. It remains metastable indefinitely at 77 K. Like the other recoverable ice phases, it transforms to low-density amorphous ice between about 120 and 150 K at 1 bar. Thetemperature range over which ice VII remains metastable increases markedly on compression to 6 GPa, indicating that ice VII is in fact the most robust of all the metastable ice phases.

NEUTRON POWDER DIFFRACTION AT PRESSURES BEYOND 25 GPA

Full structural studies of condensed media under high pressure by neutron powder diffraction have been limited in practice to 2–3 GPa for several decades. This range is in general too small to allow a precise determination of the pressure dependence of atomic coordinates. As a consequence, almost no direct measurements exist, for example, of the pressure dependence of the bond lengths in H2 and the planetary ices. In this letter, a technique is presented which makes it possible to pressurize samples of 35 mm3 volume up to 30 GPa and to collect neutron diffraction patterns in a few hours by time‐of‐flight techniques. This method provides data which can be treated by Rietveld profile refinement methods, as demonstrated on a sample of D2O ice VII at 26 GPa. This represents a tenfold increase of the pressure range over which refinable neutron diffraction data can be obtained and should have a number of applications in such fields as fundamental physicochemistry, and geo‐ and planetary sciences.

A large volume pressure cell for high temperatures

Abstract Studies of matter under very high pressure at synchrotron radiation sources are mostly done using pressure cells with single-crystal diamond anvils. In some cases the available volume (≤ 10−3mm3)in such cells causes problems especially at high temperature and for crystal synthesis. To ensure sufficient homogeneity of pressure and temperature, the use of cells with large sample volumes (≥ 1 mm3) is necessary. Existing devices for such measurements are compared with a novel setup which consists of a toroidal anvil arrangement and a lightweight (50 kg) press with 250 tonnes (2.5 MN) capacity. Preliminary tests of this instrument with synchrotron radiation are reported. Presented at the IUCr Workshop on ‘Synchrotron Radiation Instrumentation for HighPressure Crystallography’. Daresbury Laboratory 20-21 July 1991

The effect of diffraction by the diamonds of a diamond-anvil cell on single-crystal sample intensities

The integrated intensities measured in X-ray single-crystal high-pressure structural studies using a diamond-anvil cell are shown to be reduced substantially when the diamonds diffract at the same setting as the sample – by as much as 50% in some cases. The pressure and wavelength dependence of this process have been studied and also the effect of changing the beam divergence by the use of a synchrotron beam. The consequences for the accuracy of structural information derived from data sets collected at high pressure are considered and a data-collection strategy for detecting and avoiding the effects of diamond diffraction is proposed.

Structural studies of III–V and group IV semiconductors at high pressure

Abstract Extensive new structural results on II–VI, III–V and group IV semiconductors under pressure have been obtained over the past two years at SRS Daresbury, using angle-dispersive techniques and an image-plate detector. In this paper, a brief overview is presented of recent work on Si, Ge, GaSb, InSb, InAs, InP and GaAs.

High-pressure transitions in methane hydrate

Three recent studies of the high-pressure transformations of methane hydrate [Chem. Phys. Lett. 325 (2000) 490; Proc. Natl. Acad. Sci. 97 (2000) 13484; Nature 410 (2001) 661] have reported apparently different behaviours. Detailed comparison of our X-ray and neutron diffraction data with those obtained in earlier work shows that there is in fact consistent behaviour on isothermal compression at room temperature with a transition from the clathrate I structure hydrate to a hexagonal hydrate with an unknown structure at ∼0.9 GPa.

High pressure neutron diffraction using the paris-edinburgh cell: Experimental possibilities and future prospects

Abstract High pressure neutron diffraction using the Paris-Edinburgh cell has attracted considerable interest ever since it has been shown that full structural data can be obtained at pressures up to 10 GPa. In this paper we will focus on the current state of this technique. Specifically, we report on new experimental possibilities concerning: i) access to “ultrahigh” pressures beyond 20 GPa, ii) experiments at variable temperatures down to 100 K, and iii) experiments on single crystals in inelastic neutron scattering. Current attempts to increase the pressure and temperature range are discussed.

Techniques for neutron diffraction on solidified gases to 10 GPa and above: Applications to ND3 phase IV

Neutron powder diffraction can provide important structural information on hydrogenous compounds which are gases at ambient temperature. For high pressure studies, however, this technique has been seriously limited by the fact that it was impossible (a) to load such gases in large volume devices and (b) to compress them to elevated pressures above some 1 GPa. In this letter we show that, using a previously described pressure cell, a wide range of gaseous samples may be loaded and compressed to ∼10 GPa with standard tungsten carbide anvils. We illustrate the effectiveness of the technique with neutron powder diffraction data recently collected on deuterated ammonia ND3 phase IV, where accurate structural data were obtained after a few hours collection time.