Megabar pressures as a scientific tool. Superconductivity in elements and polymeric nitrogen
Dr. Mikhail Eremets (MPI für Chemie, Mainz)

Under pressure, interatomic distances in materials decrease and the electronic bandwidths increase, eventually leading to metallization of all materials at sufficiently strong compression. We will present experiments on transformation of some elements: boron, lithium, oxygen, xenon and related compounds to metals and superconductors at megabar pressures up to 300 GPa (3 millions atmospheres). Hydrogen, the simplest of the elements, might be high temperature superconductor (Tc~200 K), if it’s compressed into a very dense solid form at pressures of ~400 GPa. However it remains insulator up to accessible pressures of 250 GPa. To circumvent the problem of achieving ultimate pressures we compressed hydrogen-rich molecules (hydrides) and reached the necessary density of hydrogen at much lower pressure due to “chemical precompression”. We found super-conductivity in silane SiH4.
Nitrogen prior metallization represents a unique way of transformation – from the molecular triple bonded N N solid to a polymeric structure in which each atom is bonded to three nearest neighbours by single covalent bonds. This is a high energy density material (HEDM), because nitrogen exhibits a uniquely large difference in energy between the 1/3 triple and the single bond. Mailhiot et al proposed that the covalent polymeric lattice should have an unusual cubic gauche (cg-N) structure. We have synthesized atomic single-bonded nitrogen with the cg-N structure at high pressure ~110 GPa and high temperature ~2000 K at the laser heating. This energetic material might be also superhard substance.
We will give a short introduction to the high pressure technique with diamond anvil cells and a brief presentation of available optical, X-ray diffraction, electrical techniques, and also some properties of diamond - the strongest material – at megabar pressures.