R. J. Nelmes

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.

High-pressure structures and phase transformations in elemental metals

At ambient conditions the great majority of the metallic elements have simple crystal structures, such as face-centred or body-centred cubic, or hexagonal close-packed. However, when subjected to very high pressures, many of the same elements undergo phase transitions to low-symmetry and surprisingly complex structures, an increasing number of which are being found to be incommensurate. The present critical review describes the high-pressure behaviour of each of the group 1 to 16 metallic elements in detail, summarising previous work and giving the best present understanding of the structures and transitions at ambient temperature. The principal results and emerging systematics are then summarised and discussed.

Structural Diversity of Sodium

Sodium exhibits a pronounced minimum of the melting temperature at ∼118 gigapascals and 300 kelvin. Using single-crystal high-pressure diffraction techniques, we found that the minimum of the sodium melting curve is associated with a concentration of seven different crystalline phases. Slight changes in pressure and/or temperature induce transitions between numerous structural modifications, several of which are highly complex. The complexity of the phase behavior above 100 gigapascals suggests extraordinary liquid and solid states of sodium at extreme conditions and has implications for other seemingly simple metals.

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.

The crystal structure of YBa2Cu4O8 as a function of pressure up to 5 GPa

Abstract The crystal structure of YBa2Cu4O8 (1–2–4) has been determined as a function of pressure up to 5 GPa at room temperature, using X-ray diffraction techniques and a single-crystal sample. The principal changes with pressure are in the (fractional) z- coordinates of Ba, Cu(2), O(1) and O(4), the shortening of the basal-plane copper to apical oxygen distance Cu(2)−O(1), the reduction in the pucker of the barium-apical oxygen layer, and the increase of the orthorhombic distortion of the unit cell. There are also a number of other CuO distances that change significantly in fractional rather than absolute terms, and the compressibility of different parts of the structure varies considerably. The rates of change are generally smaller than those reported in an earlier neutron-diffraction study of 1–2–4 at 1 GPa. It is now found that the rate of change of Cu(2)−O(1) with Tc under pressure in 1–2–4 is the same within 20% as has been reported for Tc variation with oxidation in YBa2Cu3Ox. This striking similarity, in two different materials, gives new support to the idea that Cu(2)−O(1) and Tc are closely related. There is some marginal evidence that the structural changes follow the non-linear pressure dependence observed for Tc above 4 GPa, but this is not yet certain.

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.

Structure of sodium above 100 GPa by single-crystal x-ray diffraction

At pressures above a megabar (100 GPa), sodium crystallizes in a number of complex crystal structures with unusually low melting temperatures, reaching as low as 300 K at 118 GPa. We have utilized this unique behavior at extreme pressures to grow a single crystal of sodium at 108 GPa, and have investigated the complex crystal structure at this pressure using high-intensity x-rays from the new Diamond synchrotron source, in combination with a pressure cell with wide angular apertures. We confirm that, at 108 GPa, sodium is isostructural with the cI16 phase of lithium, and we have refined the full crystal structure of this phase. The results demonstrate the extension of single-crystal structure refinement beyond 100 GPa and raise the prospect of successfully determining the structures of yet more complex phases reported in sodium and other elements at extreme pressures.

An imaging plate system for high‐pressure powder diffraction: The data processing side

The use of an imaging plate as a two‐dimensional (2‐D) detector removes many of the difficulties that arise in performing angle‐dispersive powder diffraction at high pressures in a diamond‐anvil cell. Due to the 2‐D nature of the imaging plate, a substantial part of each Debye Scherrer ring is intercepted and recorded. The averaging of the intensities around a ring so as to create a conventional one‐dimensional (1‐D) powder pattern results in a significant improvement in counting statistics and powder averaging, both severe problems in high‐pressure diffraction due to the very small sample volumes involved. For an accurately known plate geometry the 2‐D to 1‐D conversion is straightforward; however, considerable complications arise when inaccuracies in plate to sample distance, plate orientations, poor powder averaging/preferred orientation, and the presence of diamond Bragg spots are considered. The current status of the software used to analyze the imaging plate data is presented along with test data to illustrate its use.The use of an imaging plate as a two‐dimensional (2‐D) detector removes many of the difficulties that arise in performing angle‐dispersive powder diffraction at high pressures in a diamond‐anvil cell. Due to the 2‐D nature of the imaging plate, a substantial part of each Debye Scherrer ring is intercepted and recorded. The averaging of the intensities around a ring so as to create a conventional one‐dimensional (1‐D) powder pattern results in a significant improvement in counting statistics and powder averaging, both severe problems in high‐pressure diffraction due to the very small sample volumes involved. For an accurately known plate geometry the 2‐D to 1‐D conversion is straightforward; however, considerable complications arise when inaccuracies in plate to sample distance, plate orientations, poor powder averaging/preferred orientation, and the presence of diamond Bragg spots are considered. The current status of the software used to analyze the imaging plate data is presented along with test data to...

Recrystallisation of HDA ice under pressure by in-situ neutron diffraction to 3.9 GPa

Abstract We have studied by in-situ neutron diffraction the recrystallisation behaviour of HDA ice in the pressure range 0.3–3.9 GPa, i.e. the entire stability range of HDA. We report quantitative and detailed structural information on the various high pressure ice phases formed metastably at low temperatures. We find that between ~0.4–0.7 GPa, HDA transforms at 175 K to mainly phases IV and V, and XII, and at 1–1.2 GPa to a mixture of ice VI and XII. On isothermal compression at 100 K, HDA recrystallises to an ice VII-like structure which is either partially ordered or mixed with ice VIII. Full structural data obtained by Rietveld refinements are reported for all these phases at low temperatures. Phases IV, V and XII are fully hydrogen disordered when obtained from HDA. The transformation behaviour in the 0.5–1.2 GPa range is in good agreement with the picture reported from quenched-recovered samples, although some differences persist in the recrystallisation to IV/XII mixtures. The recrystallisation behaviour of HDA over the entire pressure range appears to follow closely that of liquid water under pressure.