Craig L. Bull

Density-driven structural transformations in network forming glasses: a high-pressure neutron diffraction study of GeO2 glass up to 17.5 GPa

The structure of GeO(2) glass was investigated at pressures up to 17.5(5) GPa using in situ time-of-flight neutron diffraction with a Paris-Edinburgh press employing sintered diamond anvils. A new methodology and data correction procedure were developed, enabling a reliable measurement of structure factors that are largely free from diamond Bragg peaks. Calibration curves, which are important for neutron diffraction work on disordered materials, were constructed for pressure as a function of applied load for both single and double toroid anvil geometries. The diffraction data are compared to new molecular-dynamics simulations made using transferrable interaction potentials that include dipole-polarization effects. The results, when taken together with those from other experimental methods, are consistent with four densification mechanisms. The first, at pressures up to approximately equal 5 GPa, is associated with a reorganization of GeO(4) units. The second, extending over the range from approximately equal 5 to 10 GPa, corresponds to a regime where GeO(4) units are replaced predominantly by GeO(5) units. In the third, as the pressure increases beyond ~10 GPa, appreciable concentrations of GeO(6) units begin to form and there is a decrease in the rate of change of the intermediate-range order as measured by the pressure dependence of the position of the first sharp diffraction peak. In the fourth, at about 30 GPa, the transformation to a predominantly octahedral glass is achieved and further densification proceeds via compression of the Ge-O bonds. The observed changes in the measured diffraction patterns for GeO(2) occur at similar dimensionless number densities to those found for SiO(2), indicating similar densification mechanisms for both glasses. This implies a regime from about 15 to 24 GPa where SiO(4) units are replaced predominantly by SiO(5) units, and a regime beyond ~24 GPa where appreciable concentrations of SiO(6) units begin to form.

PEARL: the high pressure neutron powder diffractometer at ISIS

ABSTRACT The PEARL instrument at ISIS has been designed for, and dedicated to, in situ studies of materials at high pressure, using the Paris–Edinburgh press. In recent years, upgrades to the instrument have led to improvements in data quality and the range of achievable pressures and temperatures; currently 0.5–28 GPa and 80–1400 K. This paper describes the technical characteristics of the instrument, its current capabilities, and gives a brief overview of the science that has been performed, using representative examples.

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. The structure has 21 molecules in a rhombohedral unit cell, and is quite strongly distorted from the cubic close-packed structure of methane I, although some structural similarities remain. Full knowledge of this structure is important for modeling of methane at higher pressures, including in relation to the mineralogy of the outer solar system. We discuss interesting structural parallels with the carbon tetrahalides.

Toroidal anvils for single-crystal neutron studies

The standard design of single-toroid anvil and gasket used in Paris-Edinburgh cells has been modified to achieve greater angular access for neutron scattering. These anvils have potential uses for single-crystal studies and phonon measurements at high pressure. To date, they have been used successfully to 8 GPa.

Feasibility of in situ neutron diffraction studies of non-crystalline silicates up to pressures of 25 GPa

There is an increasing interest in the structural modifications found in liquids and amorphous systems as a function of pressure. Neutron diffraction is a key technique for determining these structures, but its application in high pressure studies remains in its infancy. Recent developments now permit in situ neutron scattering studies of amorphous materials to very high pressure conditions. Here we present new data for MgO–SiO2 and SiO2 glasses collected at up to 8.6 and 24 GPa respectively, using two distinct high pressure anvil geometries. The data collected on the MgO–SiO2 system appear to be reliable, and suggest strong changes in the chemical ordering. In contrast, the higher pressure SiO2 data highlight significant difficulties in performing appropriate corrections for pressure-dependent background and attenuation effects. These challenges are discussed, and future improvements to the technique are proposed.

Observation of ammonia dihydrate in the AMH-VI structure at room temperature – possible implications for the outer solar system

Ammonia dihydrate (ADH) is an important constituent of the outer solar system and its high-pressure behaviour is relevant to the modelling of Titan, Uranus and Neptune. Our neutron diffraction studies show that ADH can exist at room temperature in the substitutionally disordered structure of the ammonia monohydrate (AMH) phase VI. This implies that a solid solution may exist between ADH and AMH at high pressure, and this is of probable importance to models of the outer solar system.

The structure of MgO–SiO2 glasses at elevated pressure

The magnesium silicate system is an important geophysical analogue and neutron diffraction data from glasses formed in this system may also provide an initial framework for understanding the structure-dependent properties of related liquids that are important during planetary formation. Neutron diffraction data collected in situ for a single composition (38 mol% SiO(2)) magnesium silicate glass sample shows local changes in structure as pressure is increased from ambient conditions to 8.6 GPa at ambient temperature. A method for obtaining the fully corrected, total structure factor, S(Q), has been developed that allows accurate structural characterization as this weakly scattering glass sample is compressed. The measured S(Q) data indicate changes in chemical ordering with pressure and the real-space transforms show an increase in Mg-O coordination number and a distortion of the local environment around magnesium ions. We have used reverse Monte Carlo methods to compare the high pressure and ambient pressure structures and also compare the high pressure form with a more silica-poor glass (Mg(2)SiO(4)) that represents the approach to a more dense, void-free and topologically ordered structure. The Mg-O coordination number increases with pressure and we also find that the degree of continuous connectivity of Si-O bonds increases via a collapse of interstices.

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. Using this method we analysed the stress distribution on the surface and in the bulk of the double-toroidal anvils, and we conclude that the failure occurs on the surface of the anvil and that it is caused by the tensile stress. We also use the model to show possible ways of optimising the anvil design in order to extend its operational pressure range.

Time-of-flight single-crystal neutron diffraction to 10 GPa and above

Single-crystal diffraction techniques offer significant advantages over powder diffraction methods, but until recently high-resolution neutron single-crystal studies at high pressure have been restricted to pressures below 2 GPa. We present technical developments which now allow accurate single-crystal studies up to 10 GPa and at temperatures down to 10 K using time-of-flight Laue diffraction and the Paris-Edinburgh press. Prospects for work above 10 GPa are discussed, and some data from a successful test at 12 GPa are shown.

Pressure-induced dehydration and the structure of ammonia hemihydrate-II

The structure of the crystalline ammonia-bearing phase formed when ammonia monohydrate liquid is compressed to 3.5(1) GPa at ambient temperature has been solved from a combination of synchrotron x-ray single-crystal and neutron powder-diffraction studies. The solution reveals that rather than having the ammonia monohydrate (AMH) composition as had been previously thought, the structure has an ammonia hemihydrate composition. The structure is monoclinic with spacegroup P2(1)/c and lattice parameters a = 3.3584(5) Å, b = 9.215(1) Å, c = 8.933(1) Å and β = 94.331(8)° at 3.5(1) GPa. The atomic arrangement has a crowned hexagonal arrangement and is a layered structure with long N-D···N hydrogen bonds linking the layers. The existence of pressure-induced dehydration of AMH may have important consequences for the behaviour and differentiation of icy planets and satellites.