We used Raman and visible transmission spectroscopy to investigate dense hydrogen (deuterium) up to 315 (275) GPa at 300 K. At around 200 GPa, we observe the phase transformation, which we attribute to phase III, previously observed only at low temperatures. This is succeeded at 220 GPa by a reversible transformation to a new phase, IV, characterized by the simultaneous appearance of the second vibrational fundamental and new low-frequency phonon excitations and a dramatic softening and broadening of the first vibrational fundamental mode. The optical transmission spectra of phase IV show an overall increase of absorption and a closing band gap which reaches 1.8 eV at 315 GPa. Analysis of the Raman spectra suggests that phase IV is a mixture of graphenelike layers, consisting of elongated H2 dimers experiencing large pairing fluctuations, and unbound H2 molecules.

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,

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.

Ab initio random structure searching and single-crystal x-ray diffraction have been used to determine the full structures of three phases of lithium, recently discovered at low temperature above 60 GPa. A structure with C2mb symmetry, calculated to be a poor metal, is proposed for the oC88 phase (60-65 GPa). The oC40 phase (65-95 GPa) is found to have a lowest-enthalpy structure with C2cb symmetry, in excellent agreement with the x-ray data. It is calculated to be a semiconductor with a band gap of ∼1  eV at 90 GPa. oC24, stable above 95 GPa, has the space group Cmca, and refined atomic coordinates are in excellent agreement with previous calculations.

Silane (SiH 4 ) is found to (partially) decompose at pressures above 50 GPa at room temperature into pure Si and H 2 . The released hydrogen reacts with surrounding metals in the diamond anvil cell to form metal hydrides. A formation of rhenium hydride is observed after the decomposition of silane. From the data of a previous experimental report (Eremets et al., Science 319, 1506), the claimed high-pressure metallic and superconducting phase of silane is identified as platinum hydride, that forms after the decomposition of silane. These observations show the importance of taking into account possible chemical reactions that are often neglected in high-pressure experiments.

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Using in situ optical spectroscopy we have investigated the temperature stability of the mixed atomic and molecular phases IV of dense deuterium and hydrogen. Through a series of low-temperature experiments at high pressures, we observe phase III-to-IV transformation, imposing constraints on the P-T phase diagrams. The spectral features of the phase IV-III transition and differences in appearances of the isotopes Raman spectra strongly indicate the presence of proton tunneling in phase IV. No differences between isotopes were observed in absorption spectroscopic studies, resulting in identical values for the band gap. The extrapolation of the combined band gap yields 375 GPa as the minimum transition pressure to the metallic state of hydrogen (deuterium). The minute changes in optical spectra above 275 GPa might suggest the presence of a new solid modification of hydrogen (deuterium), closely related structurally to phase IV.

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.alkali metals ͉ crystal structure ͉ high pressure A t ambient pressure the alkali metals are simple metals; i.e., they can be described as nearly-free-electron metals that are characterized by a weak interaction between their single valence electron and the atomic core (1, 2). At ambient conditions the alkali metals all crystallize in the close-packed body-centered cubic (bcc) structure. However, under sufficient compression, they undergo a series of structural phase transitions. At pressures ranging from 2.2 GPa in cesium to 65 GPa in sodium, they transform from the bcc to the face-centered-cubic ( fcc) structure (3-5). Further compression leads to the formation of a wide variety of lower-symmetry and often very complex crystal structures (6, 7), which range from distorted variants of the bcc structure in lithium and sodium (5,8,9) to an incommensurate composite or ''host-guest'' crystal structure in rubidium (10, 11). The discovery of a whole series of these symmetry-lowering transitions over the last decade-not only in the alkali metals, but also in various other elements (6, 7, 9)-had been unexpected because it often involves a reduction in coordination number, which is opposite to the usual trend for pressureinduced phase transitions. The experimental discoveries have been complemented by computational studies, but the physical mechanisms that lead to the formation of the complex phases are not yet fully understood, and their physical properties not yet known in detail.Because of the low x-ray scattering power and relatively high transition pressures of sodium, it is only recently that it has become possible to investigate its high-pressure behavior. A number of high-pressure phases have been identified above 100 GPa (ref. 12 and M. Hanfland, K. Syassen, N. E. Christensen, and D. L. Novikov, unpublished data-see ref. 5), and the melting line was discovered to be very unusual: it first rises close to 1,000 K at Ϸ30 GPa and then falls to room temperature (Ϸ300 K) at 118 GPa (12) (see Fig. 1). Previous x-ray diffraction studies have shown that, with increasing pressure at room temperature, sodium transforms first from the bcc phase to fcc at 65 GP...