Since the late 1950's, high-pressure structural studies have become increasingly frequent, following the inception of opposed-anvil cells, development of efficient diffractometric equipment (brighter radiation sources both in laboratories and in synchrotron facilities, highly efficient area detectors) and procedures (for crystal mounting, centring, pressure calibration, collecting and correcting data). Consequently, during the last decades, high-pressure crystallography has evolved into a powerful technique which can be routinely applied in laboratories and dedicated synchrotron and neutron facilities. The variation of pressure adds a new thermodynamic dimension to crystal-structure analyses, and extends the understanding of the solid state and materials in general. New areas of thermodynamic exploration of phase diagrams, polymorphism, transformations between different phases and cohesion forces, structure–property relations, and a deeper understanding of matter at the atomic scale in general are accessible with the high-pressure techniques in hand. A brief history, guidelines and requirements for performing high-pressure structural studies are outlined.

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High-pressure crystallography

The development of apparatus to maintain materials at high hydrostatic pressure has been an active area of research for many years. From the pioneering work of Bridgman, during the early part of this century (Bridgman 1971), until the late 1960s, massive hydraulicly driven Bridgman-anvil and piston-cylinder devices dominated high-pressure science. Although there were later improvements in design, such as multi-anvil devices, it was not until the advent of the gasketed diamond-anvil cell, in the mid 1960s, that high-pressure studies were possible in non-specialized laboratories. Diamond has remarkable properties; not only is it the hardest known material it is also highly transparent to many ranges of electromagnetic radiation. Indeed incorporating these attributes, the gasketed diamond-anvil cell has become the standard tool for the generation of high pressures over the last three decades and has been applied in a wide range of experimental studies such as Brillouin scattering (Whitfield et al. 1976), Raman spectroscopy (Sharma 1977, Sherman 1984), NMR measurements (Lee et al. 1987) and, of course X-ray diffraction. It is this utility across a large range of science through physics, earth science and lately the life sciences that makes the development of the diamond-anvil cell as significant a revolution for measurement under non-ambient conditions in the physical sciences as that of the invention of the transistor to the whole sphere of electronics and computation.



 Figure 1. 
Assembly of the gasketed diamond-anvil cell: principle of pressure generation.



As with all successful designs, the principles upon which the gasketed diamond-anvil cell (Van Valkenburg 1964) operates are elegantly simple (Fig. 1). The sample is placed in a pressure chamber created between the flat parallel faces (culets) of two opposed diamond anvils and the hole penetrating a hardened metal foil (= the gasket). A pressure calibrant is placed beside the sample and the free volume within …

High-Pressure Single-Crystal Techniques

Subtle pressure-induced structural changes in YBa2Cu3O6.93 and YBa2Cu3O6.60 have been measured by neutron powder diffraction for samples in a hydrostatic helium-gas pressure cell. Small, but significant, differences in the compression of particular Cu-O bonds (notably Cu(2)-O(4)) are observed. However, when the charges on the two copper sites are calculated, requiring overall charge conservation versus pressure, it is found that the net pressure-induced charge transfer of holes from Cu(1) to Cu(2) is essentially the same for both systems. We conclude that the much smaller value of dTc/dP for YBa2Cu3O6.93 results from the fact that, in the 90 K superconductor, the Tc has already reached its optimum value and the introduction of additional hole carriers cannot further increase Tc.

Pressure-induced charge transfer and dTc/dP in YBa2Cu3O7−x

The hydrates of the heavier noble gases Ar, Kr and Xe are easy to prepare, and they crystallize in two different structures1. The corresponding hydrates of He and Ne apparently do not exist. We now find by neutron powder diffraction experiments that, when applying He gas pressures of > 0.28 GPa, a gas hydrate with the idealized formula He · 6D20 is formed. The He content varies with the applied He pressure as expected of an ideal solution behaviour with a Langmuir constant of 0.07 (1) GPa−1. The host lattice of this first well-characterized He-clathrate is very similar to ice II. The He enclathration stabilizes the ice II structural framework and hampers the formation of ice III, V and IX.

Enclathration of helium in ice II: the first helium hydrate

Pressure up to 10 GPa is a powerful method for studying polymorphism in organic crystal structures, and this review surveys work carried out on high-pressure polymorphism in amino acids. High-pressure polymorphs have been established crystallographically for glycine, alanine, serine, cysteine and leucine. Phase transitions can be driven by the avoidance of very short intermolecular contacts or by promotion of a more stable molecular conformation. Experimental methods are also briefly surveyed, along with three methods that have proved very helpful in the analysis of high-pressure polymorphs, namely the PIXEL method for calculation of intermolecular energies, topological analysis with Voronoi–Dirichlet partitioning and Hirshfeld surfaces for gaining a graphical overview of intermolecular interactions.

High-pressure polymorphism in amino acids

An apparatus for inelastic neutron scattering experiments at low temperature is described. This pressure assembly consists of an autofrettaged aluminum 7075‐T6 alloy cell and a shield. The rupturing pressures for some cells have been measured at different temperatures; the efficiency of the shield has also been tested. These studies demonstrate the feasibility and the safety of experiments under high pressure up to 4 kilobar even at very low temperature.

C. Vettier

Papers

New high pressure cell for neutron scattering at very low temperatures