Hydrogen is the most common element in the universe and is an insulating gas.  Hydrogen is also the simplest of the elements: it contains one proton and one neutron electron.

But at high pressures hydrogen may turn into a superconductor and scientists at the Carnegie Institution in Washington D.C.,  say they have discovered a hydrogen-based compound that could be helpful in the search for metallic and superconducting forms of hydrogen.

Because hydrogen is so light, quantum theory says that it will have a significant energy even when it is cooled to very low temperatures. This is why hydrogen only solidifies at just 14 degrees above absolute zero. 

SiH4 metallic hydrogen
The hydrogen based compound SiH4(H2)2 may be a useful system in which to explore metallic hydrogen.   Credit: Image copyright American Physical Society [Illustration: Alan Stonebraker after T. Strobel et al.]

Scientists predicted that it should be possible to form a metal from hydrogen, but the pressure that would be required to do so – some 4 million atmospheres – exceeds that at the center of the earth.

By forming compounds of hydrogen with another element like Silicon (Si) it is possible to make fairly dense forms of hydrogen that do become metals at more experimentally accessible pressures. In fact, SiH4 becomes a metal at about one tenth the pressure needed to make pure hydrogen metallic, and a superconductor at about 1 million atmospheres.

In their paper, Timothy Strobel, Maddury Somayazulu, and Russell Hemley present extensive high-pressure experiments on a mixture of SiH4 and H2. At pressures of only ~ 7.5 GPa, they discovered a new compound – SiH4(H2)2 – in which the hydrogen bonds are unusually weak and which may become a metal at higher pressures.

The ultimate goal of such studies is to generate conditions under which hydrogen effectively becomes metallic, and hopefully superconducting, at pressures lower than those required for pure solid hydrogen.

The results are reported in Physical Review Letters.