Mikhail I. Eremets

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Hydrogen at high pressures of ∼400 GPa might be in a zero-temperature liquid ground state (N. Ashcroft, J. Phys.: Condens. Matter A 12, 129 (2000), E. G. Brovrnan et al., Sov. Phys. JETP 35, 783 (1972)). If metallic hydrogen is liquid, the melting T _melt(P) line should possess a maximum. Here we report on the experimental evaluation of the melting curve of hydrogen in the megabar pressure range. The melting curve of hydrogen has been shown to reach a maximum with T _melt = 1050 ± 60 K at P = 106 GPa and the melting temperature of hydrogen decreases at higher pressures so that T _melt = 880 ± 50 K at P = 146 GPa. The data were acquired with the aid of a laser heating technique where diamond anvils were not deteriorated by the hot hydrogen. Our experimental observations are in agreement with the theoretical prediction of unusual behavior of the melted hydrogen [S. Bonev et al., Nature 481, 669 (2004)].

The metallization of hydrogen directly would require pressure in excess of 400 gigapascals (GPa), out of the reach of present experimental techniques. The dense group IVa hydrides attract considerable attention because hydrogen in these compounds is chemically precompressed and a metallic state is expected to be achievable at experimentally accessible pressures. We report the transformation of insulating molecular silane to a metal at 50 GPa, becoming superconducting at a transition temperature of T _c = 17 kelvin at 96 and 120 GPa. The metallic phase has a hexagonal close-packed structure with a high density of atomic hydrogen, creating a three-dimensional conducting network. These experimental findings support the idea of modeling metallic hydrogen with hydrogen-rich alloy.

The strong electron correlations play a crucial role in the formation of a variety of electronic and magnetic properties of the transition metal oxides. In strongly correlated electronic materials many theoretical predictions exist on pressure-induced insulator-metal transitions, which are followed by a collapse of localized magnetic moments and by structural phase transitions [1]. The high-pressure studies provide additional degree of freedom to control the structural, electronic, optical, and magnetic properties of transition metal oxides. With the development of the high-pressure diamond-anvil-cell technique the experimental studies of such transitions are now possible with the advanced synchrotron techniques. In our studies, the iron monooxide Fe_0.94O was studied under high pressures up to 200 GPa in diamond anvil cells. The single crystals enriched with Fe⁵⁷ isotopes have been prepared for nuclear resonance measurements. The results of synchrotron Mössbauer spectroscopy (nuclear forward scattering -NFS), and electro-resistivity measurements suggest a complicated scenario of magnetic interactions governed by band-broadening effects.

Nitrogen usually consists of molecules where two atoms are strongly triple-bonded. Here, we report on an allotropic form of nitrogen where all atoms are connected with single covalent bonds, similar to carbon atoms in diamond. The compound was synthesized directly from molecular nitrogen at temperatures above 2,000 K and pressures above 110 GPa using a laser-heated diamond cell. From X-ray and Raman scattering we have identified this as the long-sought-after polymeric nitrogen with the theoretically predicted cubic gauche structure (cg-N). This cubic phase has not been observed previously in any element. The phase is a stiff substance with bulk modulus >or=300 GPa, characteristic of strong covalent solids. The polymeric nitrogen is metastable, and contrasts with previously reported amorphous non-molecular nitrogen, which is most likely a mixture of small clusters of non-molecular phases. The cg-N represents a new class of single-bonded nitrogen materials with unique properties such as energy capacity: more than five times that of the most powerfully energetic materials.

Experimental aspects of Raman measurements at megabar pressures are considered. Features of diamond anvil cells working above 200 GPa are described. Problems of luminescence background at megabar pressures and the use of low‐luminescence diamonds are considered. Methods of pressure measurements at very high pressures are discussed. The pressure dependence of the Raman shift from the diamond anvil tip was used as a reliable estimate of the pressure. Copyright © 2003 John Wiley & Sons, Ltd.

The wavelength and pressure dependencies of the refractive index of diamond were determined from measurements in diamond anvil cell under pressure up to 40 GPa in the spectral range 400–800 nm. Interference spectra on diamond plate were measured. Single oscillator model with pressure dependent oscillator strength and oscillator frequency was used to fit the experimental data.

Focusing laser radiation in the center of a diamond anvil cell (DAC) allows investigations up to P≲100 GPa with the pulsed' and sustained heating to 5000 K^2,3. The use of laser radiation permits the exclusion of a heater if the sample itself strongly absorbs the radiation. Many materials are transparent for 1.06 μm YAG-laser radiation usually used for the heating. Therefore it is necessary to mix absorbing radiation powders, for example, graphite, platinum²³. The use of the powerful C0₂-laser for the heating considerably extends the scope of the materials under investigation, as the wavelength radiation Λ= 10 μm is in the range of the strong lattice absorption (absorption coefficient∼ 10³-10⁴ cm-^l) of many oxides, nitrides and so on.