Metallization of hydrogen under pressure
Though often placed at the top of the alkali metal column in the periodic table, hydrogen does not, under ordinary conditions, exhibit the properties of an alkali metal. Instead, it forms diatomic H2 molecules, analogous to halogens and non-metals in the second row of the periodic table, such as nitrogen and oxygen. Diatomic hydrogen is a gas that, at atmospheric pressure, liquefies and solidifies only at very low temperature (20 degrees and 14 degrees above absolute zero, respectively). Eugene Wigner and Hillard Bell Huntington predicted that under an immense pressure of around 25 GPa (250,000 atm; 3,600,000 psi) hydrogen would display metallic properties: instead of discrete H2 molecules (which consist of two electrons bound between two protons), a bulk phase would form with a solid lattice of protons and the electrons delocalized throughout. Since then, producing metallic hydrogen in the laboratory has been described as "...the holy grail of high-pressure physics."
The initial prediction about the amount of pressure needed was eventually shown to be too low. Since the first work by Wigner and Huntington, the more modern theoretical calculations point towards higher but nonetheless potentially accessible metallization pressures of around 400 GPa.
Liquid metallic hydrogen
Helium-4 is a liquid at normal pressure near absolute zero, a consequence of its high zero-point energy (ZPE). The ZPE of protons in a dense state is also high, and a decline in the ordering energy (relative to the ZPE) is expected at high pressures. Arguments have been advanced by Neil Ashcroft and others that there is a melting point maximum in compressed hydrogen, but also that there might be a range of densities, at pressures around 400 GPa (3,900,000 atm), where hydrogen would be a liquid metal, even at low temperatures.
In 1968, Neil Ashcroft suggested that metallic hydrogen might be a superconductor, up to room temperature (290 K or 17 °C), far higher than any other known candidate material. This hypothesis is based on an expected strong coupling between conduction electrons and lattice vibrations.
Possibility of novel types of quantum fluid
Presently known "super" states of matter are superconductors, superfluid liquids and gases, and supersolids.
Egor Babaev predicted that if hydrogen and deuterium have liquid metallic states, they might have quantum ordered states that cannot be classified as superconducting or superfluid in the usual sense. Instead, they might represent two possible novel types of quantum fluids: superconducting superfluids and metallic superfluids. Such fluids were predicted to have highly unusual reactions to external magnetic fields and rotations, which might provide a means for experimental verification of Babaev's predictions. It has also been suggested that, under the influence of a magnetic field, hydrogen might exhibit phase transitions from superconductivity to superfluidity and vice versa.
Lithium alloying reduces requisite pressure
In 2009, Zurek et al. predicted that the alloy LiH6 would be a stable metal at only one quarter of the pressure required to metallize hydrogen, and that similar effects should hold for alloys of type LiHn and possibly "other alkali high-hydride systems", i.e. alloys of type XHn where X is an alkali metal.