Pressure-induced localisation of the hydrogen-bond network in KOH-VI.

Using a combination of ab initio crystal structure prediction and neutron diffraction techniques, we have solved the full structure of KOH-VI at 7 GPa. Rather than being orthorhombic and proton-ordered as had previously be proposed, we find that this high-pressure phase of potassium hydroxide is tetragonal (space group I4/mmm) and proton disordered. It has an unusual hydrogen bond topology, where the hydroxyl groups form isolated hydrogen-bonded square planar (OH)4 units.

The crystal structure of methane B at 8 GPa--an α-Mn arrangement of molecules.

From a combination of powder and single-crystal synchrotron x-ray diffraction data we have determined the carbon substructure of phase B of methane at a pressure of ∼8 GPa. We find this substructure to be cubic with space group I4¯3m and 58 molecules in the unit cell. The unit cell has a lattice parameter a = 11.

An improved method for calibrating time-of-flight Laue single-crystal neutron diffractometers.

A robust and comprehensive method for determining the orientation matrix of a single-crystal sample using the neutron Laue time-of-flight (TOF) technique is described. The new method enables the measurement of the unit-cell parameters with an uncertainty in the range 0.015-0.

Strength analysis and optimisation of double-toroidal anvils for high-pressure research.

We used the finite element method for stress and deformation analysis of the large sample volume double-toroidal anvil and gasket assembly used with the Paris-Edinburgh press for neutron scattering, in order to investigate the failure of this assembly observed repeatedly in experiments at a load of approximately 240 tonnes. The analysis is based on a new approach to modelling an opposed anvil device working under extreme stress conditions. The method relies on use of experimental data to validate the simulation in the absence of the material property data available for high pressure conditions.

Extraordinarily complex crystal structure with mesoscopic patterning in barium at high pressure.

Elemental barium adopts a series of high-pressure phases with such complex crystal structures that some of them have eluded structure determination for many years. Using single-crystal synchrotron X-ray diffraction and new data analysis strategies, we have now solved the most complex of these crystal structures, that of phase Ba-IVc at 19 GPa. It is a commensurate host-guest structure with 768 atoms in the representative unit, where the relative alignment of the guest-atom chains can be represented as a two-dimensional pattern with interlocking S-shaped 12-chain motifs repeating regularly in one direction and repeating with constrained disorder in the other.

Note: Achieving quasi-hydrostatic conditions in large-volume toroidal anvils for neutron scattering to pressures of up to 18 GPa.

We present developments that allow neutron-scattering experiments to be performed, with both single-crystal and powder samples, under quasi-hydrostatic conditions to pressures beyond previous limits. Samples of sodium chloride and squaric acid (H(2)C(4)O(4)) have been loaded with argon as the pressure-transmitting medium in encapsulated gaskets redesigned for double-toroidal anvils, using a gas-loading method at ambient temperature. These samples have been compressed up to 18 GPa in a Paris-Edinburgh press, and no evidence of peak broadening in either the single-crystal or the powder experiments was observed.

Crystal structures of dense lithium: a metal-semiconductor-metal transition.

Ab initio random structure searching and single-crystal x-ray diffraction have been used to determine the full structures of three phases of lithium, recently discovered at low temperature above 60 GPa. A structure with C2mb symmetry, calculated to be a poor metal, is proposed for the oC88 phase (60-65 GPa). The oC40 phase (65-95 GPa) is found to have a lowest-enthalpy structure with C2cb symmetry, in excellent agreement with the x-ray data.

A rotator for single-crystal neutron diffraction at high pressure.

We present a modified Paris-Edinburgh press which allows rotation of the anvils and the sample under applied load. The device is designed to overcome the problem of having large segments of reciprocal space obscured by the tie rods of the press during single-crystal neutron-scattering experiments. The modified press features custom designed hydraulic bearings and provides controls for precision rotation and positioning.

Gas loading apparatus for the Paris-Edinburgh press.

We describe the design and operation of an apparatus for loading gases into the sample volume of the Paris-Edinburgh press at room temperature and high pressure. The system can be used for studies of samples loaded as pure or mixed gases as well as for loading gases as pressure-transmitting media in neutron-scattering experiments. The apparatus consists of a high-pressure vessel and an anvil holder with a clamp mechanism.

The distorted close-packed crystal structure of methane A.

We have determined the full crystal structure of the high-pressure phase methane A. X-ray single-crystal diffraction data were used to determine the carbon-atom arrangement, and neutron powder diffraction data from a deuterated sample allowed the deuterium atoms to be located. It was then possible to refine all the hydrogen positions from the single-crystal x-ray data.

Potassium under pressure: a pseudobinary ionic compound.

Experimentally, we have found that among the "complicated" phases of potassium at intermediate pressures is one which has the same space group as the double hexagonal-close-packed structure, although its atomic coordination is completely different. Calculations on this P6(3)/mmc (hP4) structure as a function of pressure show three isostructural transitions and three distinctive types of chemical bonding: free electron, ionic, and metallic. Interestingly, relationships between localized metallic structures and ionic compounds are found.

Two million hours of science.

After over a quarter of a century, the doors of the world's first synchrotron radiation source have closed. Its contribution to materials science in the past and the future should not be underestimated.

Structural diversity of sodium.

Sodium exhibits a pronounced minimum of the melting temperature at approximately 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.

High-pressure gas hydrates.

It has long been known that crystalline hydrates are formed by many simple gases that do not interact strongly with water, and in most cases the gas molecules or atoms occupy 'cages' formed by a framework of water molecules. The majority of these gas hydrates adopt one of two cubic cage structures and are called clathrate hydrates. Notable exceptions are hydrogen and helium which form 'exotic' hydrates with structures based on ice structures, rather than clathrate hydrates, even at low pressures.

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.

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.

Structure of dense liquid water by neutron scattering to 6.5 GPa and 670 K.

We present a neutron diffraction study of liquid water to 6.5 GPa and 670 K. From the measured structure factors we determine radial and angular distributions.

Observation of a wurtzite form of gallium arsenide.

After a pressure decrease to ambient, the high-pressure SC16 phase of GaAs is found to transform to the hexagonal wurtzite structure. This has been suggested for GaAs in calculations but never previously observed experimentally. Wurtzite-GaAs is found to be stable at ambient pressures at temperatures up to 473 K, with a structure that is only slightly distorted from ideal.

Nature of the polyamorphic transition in ice under pressure.

We present a neutron diffraction study of the transition between low-density and high-density amorphous ice (LDA and HDA, respectively) under pressure at approximately 0.3 GPa, at 130 K. All the intermediate diffraction patterns can be accurately decomposed into a linear combination of the patterns of pure LDA and HDA.

Structural complexity in gallium under high pressure: relation to alkali elements.

Ga-II, the stable phase of Ga between 2 and 10 GPa at room temperature, is shown to have a complex 104-atom orthorhombic structure. A new phase, Ga-V, is found between 10 and 14 GPa, with a rhombohedral hR6 structure. Ga-II has a modulated layer structure like those recently reported for Rb-III and Cs-III, with similar 8- and 10-atom a-b layers stacked along the c axis in the sequence 8-10-8-8-10-8-8-10-8-8-10-8.

High density amorphous ices: disordered water towards close packing.

The structure of amorphous ice under pressure has been studied by molecular dynamics at 160 K. The starting low-density phase undergoes significant changes as the density increases, and at rho=1.51 g/cm(3) our calculated g(OO)(r) is in excellent agreement with in situ neutron diffraction data obtained at 1.

Chain "melting" in the composite Rb-IV structure.

The Bragg peaks from the structure formed by the guest chains in the incommensurate composite structure of Rb-IV are all found to broaden strongly at pressures below 16.7(1) GPa. This signals a loss of the interchain correlation.

Structure of high-density amorphous ice under pressure.

We report in situ neutron diffraction studies of high-density amorphous ice (HDA) at 100 K at pressures up to 2.2 GPa. We find that the compression is achieved by a strong contraction ( approximately 20%) of the second neighbor coordination shell, so that at 2.

Structure of Rb-III: novel modulated stacking structures in alkali metals.

The crystal structure of Rb-III, stable between 13 and 17 GPa, has been determined from quasi-single-crystal x-ray diffraction data. It is orthorhombic, space group C222(1), with 52 atoms in the unit cell, and has an 8-10-8-8-10-8 stacking of 8- and 10-atom layers. The recently reported 84-atom structure of Cs-III can be understood as an 8-8-10-8-8-8-8-10-8-8 stacking of the same layers.

Complex crystal structure of cesium-III.

The structure of Cs-III, stable between 4.2 and 4.3 GPa at room temperature, has been determined from single-crystal x-ray diffraction data.