M. I. McMahon

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.

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...

High-pressure structural studies of group-15 elements

Recent advances in high-pressure experimental techniques have yielded high-quality x-ray diffraction data for the high-pressure phases of the group-15 elements Bi, Sb and As, and have made it possible to solve several long-standing problems in their structures. In particular, several complex incommensurate host–guest structures have been identified. This paper reviews the present state of knowledge of the structural transition sequences for these elements at high pressure and room temperature, including a summary of previous work, a detailed presentation of the new structures, and revised equations of state.

Structural studies of III–V and group IV semiconductors at high pressure

Abstract Extensive new structural results on II–VI, III–V and group IV semiconductors under pressure have been obtained over the past two years at SRS Daresbury, using angle-dispersive techniques and an image-plate detector. In this paper, a brief overview is presented of recent work on Si, Ge, GaSb, InSb, InAs, InP and GaAs.

Incommensurate crystal structures in the elements at high pressure

Abstract Recent advances in high-pressure diffraction techniques have revealed remarkably complex crystal structures in the metallic elements at high pressure. In an increasing number of cases, these structures are found to be incommensurate, having either a host-guest composite structure, or modulations of the atomic positions. In this paper we review the structures of these phases, and discuss the insight provided by the structures into the behaviour of the elements at high pressure.

Structural studies of II-VI semiconductors at high pressure

Abstract Extensive new structural results on II–VI, III–V and group IV semiconductors under pressure have been obtained over the past two years at SRS Daresbury, using angle-dispersive techniques and an image-plate detector. In this paper, a brief overview is presented of recent work on ZnTe, CdTe, HgTe, HgS.

Angle‐dispersive powder‐diffraction techniques for crystal structure refinement at high pressure

An imaging‐plate system designed for the full Rietveld refinement of crystal structures at high pressure is described. Emphasis is given to techniques that have been developed to obtain data free from contaminating diffraction peaks. Initial results from studies of III–V semiconductors and La2CuO4 are given.

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. The existence of such patterning on the nanometre scale points at medium-range interactions that are not fully screened by the itinerant electrons in this metal. On the basis of first-principles electronic structure calculations, pseudopotential theory and an analysis of the lattice periodicities and interatomic distances, we rationalize why the Ba phases with the common densely packed crystal structures become energetically unfavourable in comparison with the complex-structured Ba-IVc phase, and what the role of the well-known pressure-induced s-d electronic transfer is.