The low-and high-temperature structures of La2CoMnO6 and La2NiMnO6 have been refined using powder neutron diffraction. At low temperatures the materials adopt a charge-ordered monoclinic structure which transforms to a rhombohedral structure at elevated temperature without loss of charge ordering. This charge ordering scheme allows us for the first time to rationalize the relationships between the physical and structural properties of these materials.

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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.

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.

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A combination of in situ high-pressure neutron diffraction at pressures up to 17.5(5) GPa and molecular dynamics simulations employing a many-body interatomic potential model is used to investigate the structure of cold-compressed silica glass. The simulations give a good account of the neutron diffraction results and of existing x-ray diffraction results at pressures up to ~60  GPa. On the basis of the molecular dynamics results, an atomistic model for densification is proposed in which rings are "zipped" by a pairing of five- and/or sixfold coordinated Si sites. The model gives an accurate description for the dependence of the mean primitive ring size ⟨n⟩ on the mean Si-O coordination number, thereby linking a parameter that is sensitive to ordering on multiple length scales to a readily measurable parameter that describes the local coordination environment.

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The method of in situ high-pressure neutron diffraction is used to investigate the structure of B 2 O 3 glass on compression in the range from ambient to 17.5(5) GPa. The experimental results are supplemented by molecular dynamics simulations made using a newly developed aspherical ion model. The results tie together those obtained from other experimental techniques to reveal three densification regimes. In the first, BO 3 triangles are the predominant structural motifs as the pressure is increased from ambient to 6.3(5) GPa, but there is an alteration to the intermediate range order which is associated with the dissolution of boroxol rings. In the second, BO 4 motifs replace BO 3 triangles at pressures beyond 6.3 GPa and the dissolution of boroxol rings continues until it is completed at 11-14 GPa. In the third, the B-O coordination number continues to increase with pressure to give a predominantly tetrahedral glass, a process that is completed at a pressure in excess of 22.5 GPa. On recovery of the glass to ambient from a pressure of 8.2 GPa, triangular BO 3 motifs are recovered but, relative to the uncompressed material, there is a change to the intermediate range order. The comparison between experiment and simulation shows that the aspherical ion model is able to provide results of unprecedented accuracy at pressures up to at least 10 GPa.