SOLID HYDROGEN AT 342 GPA - NO EVIDENCE FOR AN ALKALI-METAL

Solid hydrogen, an electrical insulator, is predicted to become an alk ali metal under extreme compression, although controversy surrounds th e pressure required to achieve this(1-3). The electrical conductivity of hydrogen as a function of pressure and temperature is of both funda mental and practical interest-metallic hydrogen may be of relevance to planetary interiors(4), and has been suggested as a potential high-te mperature superconductor(5). Calculations(1,2) suggest that depairing (destruction of the molecular bond) should occur around 340 GPa, accom panied by the formation of an alkali metal at this pressure(1), or at substantially higher pressures(2,3). Here we report that solid hydroge n does not become an alkali metal at pressures of up to 342 +/- 10 GPa , achieved using a diamond anvil cell. This pressure (which is almost comparable to that at the centre of the Earth) significantly exceeds t hose reached in earlier experiments-216 GPa (ref, 6) and 191 GPa (ref. 7)-at which hydrogen was found to be nonmetallic. The failure of soli d hydrogen to become an alkali metal at the extreme pressures reported here has implications for our current theoretical understanding of th e solid-state phase.