I. A. Trojan

Transparent dense sodium

Under pressure, metals exhibit increasingly shorter interatomic distances. Intuitively, this response is expected to be accompanied by an increase in the widths of the valence and conduction bands and hence a more pronounced free-electron-like behaviour. But at the densities that can now be achieved experimentally, compression can be so substantial that core electrons overlap. This effect dramatically alters electronic properties from those typically associated with simple free-electron metals such as lithium (Li; refs 1–3) and sodium (Na; refs 4, 5), leading in turn to structurally complex phases and superconductivity with a high critical temperature. But the most intriguing prediction—that the seemingly simple metals Li (ref. 1) and Na (ref. 4) will transform under pressure into insulating states, owing to pairing of alkali atoms—has yet to be experimentally confirmed. Here we report experimental observations of a pressure-induced transformation of Na into an optically transparent phase at ∼200 GPa (corresponding to ∼5.0-fold compression). Experimental and computational data identify the new phase as a wide bandgap dielectric with a six-coordinated, highly distorted double-hexagonal close-packed structure. We attribute the emergence of this dense insulating state not to atom pairing, but to p–d hybridizations of valence electrons and their repulsion by core electrons into the lattice interstices. We expect that such insulating states may also form in other elements and compounds when compression is sufficiently strong that atomic cores start to overlap strongly.

Superconductivity in Hydrogen Dominant Materials : Silane

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 Tc = 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.

Structural transformation of molecular nitrogen to a single-bonded atomic state at high pressures.

The transformation of molecular nitrogen to a single-bonded atomic nitrogen is of significant interest from a fundamental stand point and because it is the most energetic non-nuclear material predicted. We performed an x-ray diffraction of nitrogen at pressures up to 170 GPa. At 60 GPa, we found a transition from the rhombohedral (R3c) epsilon-N(2) phase to the zeta-N(2) phase, which we identified as orthorhombic with space group P222(1) and with four molecules per unit cell. This transition is accompanied by increasing intramolecular and decreasing intermolecular distances. The major transformation of this diatomic phase into the single-bonded (polymeric) phase, recently determined to have the cubic gauche structure (cg-N), proceeds as a first-order transition with a volume change of 22%.

The strength of diamond

Diamond is the hardest known material; however, its mechanical stability, in particular the yield strength σy, is not known. We report reproducible generation of plastic deformation of diamond at room and low temperatures. We determined σy=130–140GPa by measuring the pressure distribution over the diamond anvils and by using the theory of elasticity. Yielding in diamond is accompanied by a strong luminescence with a characteristic spectrum. Our findings may be used to extend the static pressure limit beyond the current value of about 300GPa(3Megabars).

Single-crystalline polymeric nitrogen

The authors synthesized polymeric nitrogen at pressures above 110GPa with laser heating above 2000K. To prove its cubic gauche (cg) structure, the authors have grown single crystals which produced strong reflections at large diffraction angles. The authors found nine peaks in the 2Θ=35° range with a special diamond anvil cell. All of the peaks agree with the cg-N structure. The authors checked various combinations of gasket materials and heaters to study possible contamination of the polymeric nitrogen by products of the reaction of the hot nitrogen with these materials or diamond anvils, and found that this is not the case.

Exotic magnetism in the alkali sesquioxides Rb4O6 and Cs4O6

Among the various alkali oxides the sesquioxides

The mechanism of suppression of strong electron correlations in FeBO3 at high pressures

{\text{Rb}}_{4}{\text{O}}_{6}

Synthesis of High-Nitrogen Energetic Material

and

High-Pressure Magnetic Properties and P - T Phase Diagram of Iron Borate ¶

{\text{Cs}}_{4}{\text{O}}_{6}

Equation of state and high-pressure irreversible amorphization in Y3Fe5O12

are of special interest. Electronic-structure calculations using the local spin-density approximation predicted that