Hydrogen at high pressure

Experimental and theoretical work on solid hydrogen under high pressure is reviewed with special emphasis on three aspects of the field. The first concerns the equation of state of hydrogen and the properties of its molecular phase. Both experimental and theoretical studies show that hydrogen has a rich variety of unusual properties even in its molecular phase, as the formation of many highly anisotropic crystal structures with little energy difference exemplifies. The second aspect is the insulator-metal transition which, while customarily associated with atomization, i.e., with the dissociation of hydrogen molecules, is also possible in the molecular phase according to recent theoretical studies. In discussing the metallic phase, finally, the existence of a metastable phase at normal pressure and prospects for the high superconducting transition temperatures in metallic hydrogen are considered.