Recent theoretical and experimental studies have produced several unusual and interesting results on dense lithium, the first metal in the periodic table. These include the deviation from simple metal behaviour, superconductivity at 17 K, and a metal to semiconductor transition 1-5 . Despite these efforts, at present there is no agreement on the location of the highpressure solid phases and melting curve of Li, and there is no clear picture of its phase diagram above 50 GPa (refs 4-7). Using powder and single-crystal high-pressure diffraction techniques, we have mapped out the lithium phase diagram up to 130 GPa over a wide temperature range between 77 and 300 K. Whereas the melting temperatures of materials usually rise under pressure, and even the lightest condensed gases, hydrogen and helium, melt at temperatures of the order of 10 3 K at 50 GPa (refs 8,9), we find that at these pressures lithium remains a liquid at temperatures as low as 190 K, by far the lowest melting temperature observed for any material at such pressure. We also find that in its solid state above 60 GPa, lithium adopts three novel and complex crystal structures not previously observed in any element. Estimates of the zeropoint energy suggest that quantum effects play a significant role in shaping the lithium phase diagram.The familiar properties and states of matter can be markedly modified by applying pressure and temperature. Besides those encountered in daily life (gas, liquid and solid), some exotic states, for example superfluids or superconductors, can be observed. Quantum effects, the energies of which are very small on an everyday scale, are responsible for the formation of these unusual forms of matter. To create any of these states, low temperatures are needed to decrease the energy of the system to the level where the quantum effects become dominant. Conversely, by applying pressure, and thereby bringing the atoms closer to each other, it is possible to increase the kinetic energy (that is, the zero-point energy) of the system. If the other energy terms that make up the total energy increase more slowly with pressure than the zero-point energy, it might be possible to reach a compression at which the quantum effects play the dominant role 10 . One of the obvious consequences of the zero-point energy being comparable to or in excess of differences in characteristic structural energies per atom would be melting of the solid under compression (cold melting) 10,11 . For light elements, such as hydrogen, melting influenced by the zero-point energy is expected to happen even at T = 0 (at compressions which are at present beyond the capabilities of experimental techniques), leading to a metallic liquid ground state with exotic properties 12 .Is it then possible to create a metallic liquid ground state in systems other than dense hydrogen? Most metallic elements with strong interatomic interactions are solids under normal conditions,

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.

The long-unknown crystal structure of Bi-III has been solved. It comprises a body-centered-tetragonal (bct) "host" and a bct "guest" component made up of chains that lie in channels in the host; the guest is incommensurate with the host along the tetragonal c axis. Diffraction data for Sb-II reveal that it too can be fitted with the same composite structure. The structures of these two high-pressure phases of Bi and Sb are similar to those reported recently in the alkaline-earth metals Ba and Sr.

No abstract

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. 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.

Of the simple diatomic molecules, oxygen is the only one to carry a magnetic moment. This makes solid oxygen particularly interesting: it is considered a 'spin-controlled' crystal that displays unusual magnetic order. At very high pressures, solid oxygen changes from an insulating to a metallic state; at very low temperatures, it even transforms to a superconducting state. Structural investigations of solid oxygen began in the 1920s and at present, six distinct crystallographic phases are established unambiguously. Of these, the epsilon phase of solid oxygen is particularly intriguing: it exhibits a dark-red colour, very strong infrared absorption, and a magnetic collapse. It is also stable over a very large pressure domain and has been the subject of numerous X-ray diffraction, spectroscopic and theoretical studies. But although epsilon-oxygen has been shown to have a monoclinic C2/m symmetry and its infrared absorption behaviour attributed to the association of oxygen molecules into larger units, its exact structure remains unknown. Here we use single-crystal X-ray diffraction data collected between 13 and 18 GPa to determine the structure of epsilon-oxygen. We find that epsilon-oxygen is characterized by the association of four O2 molecules into a rhombohedral molecular unit, held together by what are probably weak chemical bonds. This structure is consistent with existing spectroscopic data, and further validated by the observation of a newly predicted Raman stretching mode.

No abstract

No abstract